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DOD-HDBK-791 (AM) 17 MARCH 1988 MILITARY HANDBOOK MAINTAINABILITY DESIGN TECHNIQUES METRIC NO DELIVERABLE DATA REQUIRED AREA MISC BY THIS DOCUMENT DISTRIBUTION STATEMENT A. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED. Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.

Transcript of MAINTAINABILITY DESIGN TECHNIQUES METRIC · 2014. 11. 18. · in maximum maintainability. The...

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DOD-HDBK-791 (AM)17 MARCH 1988

MILITARY HANDBOOK

MAINTAINABILITY DESIGNTECHNIQUES

METRIC

NO DELIVERABLE DATA REQUIRED AREA MISC

BY THIS DOCUMENT

DISTRIBUTION STATEMENT A. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED.

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DOD-HDBK-791(AM)

DEPARTMENT OF DEFENSEWASHINGTON, DC 20301

Maintainability Design Techniques

1. This standardization handbook was developed by the School of Engineering & Logistics with theassistance of other organizations within the Department of the Army and industry.

2. This document supplements departmental manuals, directives, military standards, etc., and providesbasic information on the design of equipment so that the equipment can be (a) serviced efficiently andeffectively if servicing is required and (b) repaired efficiently and effectively if it should fail. Thehandbook -by text. illustrations, tables, and examples embraces information on the extent and nature ofthe maintenance problem as it exists today and the principles and techniques. if included in future designs.will reduce the problem. Designers will find this handbook an invaluable aid in applying the principles ofmaintainability engineering.

3. Beneficial comments (recommendations, additions. deletions) and any pertinent data that may beused in improving this document should be addressed to the Commandant. School of Engineering andLogistics, AI-TN: AMXMC-SEL-E, Red River Army Depot. Texarkana, TX 75507-5000.

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DOD-HDBK-791(AM)

FOREWORD

The purpose of this handbook is to provide Armydesign engineers with guidelines to assist them in in-corporating maintainability into Army materiel early inresearch and development. Information collected frommaintenance records provides practical examples—goodand bad—that illustrate the design principles that resultin maximum maintainability. The designer can use theseprinciples to build maintainability into materiel andthereby contribute substantially to solving the Army’smaintenance problem.

Chapter 1 is an introduction to the principle of main-tainability, its importance, and methods of achieving it.The following 10 chapters simplification, standardiza-

tion and interchangeability, accessibility, modulariza-tion, identification and labeling, testability and diagnos-tic techniques, preventive maintenance, human factors,and environmental factors---describe in detail their rolein achieving the maintainability principle.

This handbook was developed under the auspices ofthe Army Materiel Command’s Engineering DesignHandbook Program, under the direction of the USArmy Management Training Activity. The handbookwas prepared under the direction of and edited by theResearch Triangle Institute under Contract No.DAAG-34-73-C-0051 .

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DOD-HDBK-791(AM)

CONTENTS

1-11-11-31-3.11-3.21-41-4.11-4.21-4.3

1-5

2-12-22-2.12-2.22-2.32-2.42-2.52-2.62-2.6.12-2.6.22-2.6.32-2.72-32-3.12-3.22-2.32-3.42-4

3-13-13-23-2.13-2.23-33-3.13-3.2

3-3.33-3.4

LIST OF ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .LIST OF ABBREVIATIONS AND ACRONYMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 1INTRODUCTION

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MAINTAINABILITY VS MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MEASURES OF MAINTAINABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .QUANTITATIVE MAINTAINABILITY INDICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

MAINTAINABILITY PROGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MAINTAINABILITY PLAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .VERIFICATION, DE MONSTRATION, AND EVALUATION OF MAINTAINABILITY

REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .FEATURES TO FACILITATE MA INTA INABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .R E F E R E N C E S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 2SIMPLIFICATION

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DESIGN TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COORINATION OF EQUIPMENT AND JOB DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .REDUCTION OF PARTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .VALUE EN ENGEERING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .CONSOLIDATION OF FUNCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .IMPROVED ACCESS TO PARTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .STREAMLINED MAINTENANCE PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Part Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Scheduling of Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Simplified Diagnostic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SOFTWARE MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .EXAMPLES OF COMPLEX MODULES REDESIGNED FOR SIMPLICITY . . . . . . . . . . . . . . . . . . . . . . . . .

UH-60A HELICOPTER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .155-mm HOWITZER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .M230 GUN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INFRARED SUPPRESSOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SIMPLIFICATION CHECK LIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 3STANDARDIZATION AND INTERCHANGEABILITY

LIST OF SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .STANDARDIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DESIGN GOALS AND PRINCIPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ADVANTAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

INTERCHANGEABILIY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .RATIONALIZATION, STANDARDIZATION, AND INTEROPERABILITY(RSP) PROGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DESIGN PRINCIPLES . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ADVANTAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pagexi. . .

xiiixiv

1-1l-l1-21-21-41-51-51-6

1-71-71-81-8

2- 12-l2-22-22-22-22-32-32-32-42-42-42-42-42-52-52-52-52-72-7

3-13-13-13-23-23-23-2

3-33-33-3

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DOD-HOBK-791(AM)

CONTENTS (cont’d)Paragraph Page

4-7

5-15-25-35-3.15-3.25-3.35-45-55-5.15-5.1.15-5.1.25-5.1.35-5.25-5.2.15-5.2.25-5.2.35-5.2.45-5.2.55-5.2.65-5.35-65-6.15-6.25-6.35-6.3.15-6.3.25-6.3.35-6.45-6.4.15-6.4.25-5.55-7

6- I6-26-36-3.16-3.26-3.36-3.46-3.56-46-4.16-4.26-4.36-4.4

ACCESSIBILITY CHECKLIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 5MODULARIZATION

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ADVANTAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .THROWAWAY MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ADVANTAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .HAZARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..

SOFTWARE MODULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DESIGN CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

MODULARIZATION VERSUS PIECE-PART DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .General “Module Versus Piece-Part” Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .General Module Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Throwaway (Discard-at-Failure) Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FUNCTIONAL GROUPING FOR MODULARIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Logical Flow Grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Circuit Grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Component Grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Standard Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Frequency Grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Evaluation of Grouping Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TYPICAL DESIGN GUIDELINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .

ELECTRONIC AND ELECTRICAL EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MISSILES AND ROCKETS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TANK-AUTOMOTIVE EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fuel Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Engine Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Total Vehicle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

MODULARIZATION IN ARMAMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Feed and Eject Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Complete Gun Modularization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

HELlCOPTER ENGINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MODULARIZATION CHECKLIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 6IDENTIFICATION AND LABELING

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BASIC CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .T Y P E S A N D U S E S O F I D E N T I F I C A T I O N. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .EQUIPMENT IDENTIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .EQUIPMENT FUNCTIONAL AND/OR INSTRUCTIONAL MARKINGS . . . . . . . . . . . . . . . . . .HAZARDOUS CONDITION MARKINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .PART IDENTIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

METHODS OF LABELING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INK STAMPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .STEEL STAMPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ENGRAVING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MOLDING-IN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-284-314-31

5-15-15-25-25-25-35-35-35-35-45-45-45-45-45-55-55-55-55-55-65-65-65-95-95-95-95-95-95-135-135-135-165-175-17

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6-4.56-4.66-4.76-4.8

6-4.96-4.106-4.116-4.126-56-5.16-5.26-5.36-5.3.16-5.3.26-5.3.2.16-5.3.2.26-5.3.2.36-5.3.2.46-5.3.2.56-5.3.2.66-5.3.2.76-5.46-6

6-7

7-17-2

7-2.17-2.27-2.37-2.47-37-3.17-3.27-3.37-3.47-3.4.17-3.4.27-3.57-3.5.17-3.5.27-3.5.2.17-3.5.2.27-3.5.2.37-3.5.37-47-4.17-4.1.17-4.1.27-4.1.37-4.27-5

DECALCOMANIA TRANSFER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .STENCILING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .PHOTOETCHING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .METAL PLATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TAGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PHOTOCONTACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SCREEN PRINTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ADHESIVE-BACKED LABELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LABELING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .LABEL CONTENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .LABEL PLACEMENT AND POSITIONING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .LABEL DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Color Contrast and Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Letter and Numeral Size and Font . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Type Style . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Letter or Numeral Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Character Stroke Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Stroke Continuity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Character Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Word and Line Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Character Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

METHOD OF APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .COLORS FOR LABELS AND SIGNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .IDENTIFICATION CHECKLISTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 7TESTABILITY AND DIAGNOSTIC TECHNIQUES

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TESTABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..TESTABILITY ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INTERFACE RELATIONSHIPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .CHARACTERISTICS EXTERNAL TO BIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DIAGNOSTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DESIGN NEEDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DEVELOPMENT AND DEMONSTRATION TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DIAGNOSTICS FOR MECHANICAL SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Condition Monitoring of Mechanical Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nondestructive Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DIAGNOSTIC TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Importance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Types of Diagnostic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Logic Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Automatic Test Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Built-In Test Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Artificial Intelligence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .FUNCTIONAL TESTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Fault Localization, Isolation, and Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Verification Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Testing for Hidden Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PERCENTAGE OF FAILURES DETECTABLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DIAGNOSTIC EQUIPMENT DESIGN CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page

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7-5.17-5.27-5.37-5.47-5.57-5.67-5.77-5.87-5.97-5.107-67-6.17-6.27-6.37-7

8-08-18-28-2.18-2.28-2.38-2.48-38-3.18-3.28-48-4.18-4.1.18-4.1.28-4.28-4.2.18-4.2.28-4.2.38-4.38-4.3.18-4.3.28-4.48-4.58-5

9-19-29-2.19-2.29-39-49-4.19-4.29-4.3

CLASSIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SNEAK CIRCUITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BUILT-IN TEST EQUIPMENT VERSUS AUTOMATIC TEST EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . .DIAGNOSTIC SOFTWARE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TEST POINT IDENTIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TEST SEQUENCE CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SELECTION OF TRANSDUCER OR SENSOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .STIMULI SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .OUTPUT FORMAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....LIKELIHOOD ANALYSIS COMPUTER PROGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ADVANCED ATTACK HELICOPTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .T700 GAS TURBINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHECKLISTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 8PREVENTIVE MAINTENANCE

LIST OF SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .RELIABILITY CENTERED MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .OBJECTIVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .RELIABILITY CENTERED MAINTENANCE LOGIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .DISPOSITION OF RELIABILITY CENTERED MAINTENANCE ANALYSIS . . . . . . . . . . . . . . . . . . . . .

PREVENTIVE MAINTENANCE EVALUATION PARAMETERS AND TRADE-OFFS . . . . . . . . . .AVAILABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .COST OF OWNERSHIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DESIGN CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .LUBRICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Oiling and Greasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Army Oil Analysis Program (AOAP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FILLING AND DRAINING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Filling Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Draining Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CLEANING AND PRESERVING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Preserving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

MOISTURE PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ADJUSTMENT AND ALIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PREVENTIVE MAINTENANCE AND SERVICING DESIGN CHECKLIST . . . . . . . . . . . . . . .REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 9HUMAN FACTORS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ANTHROPOMETRY (BODY MEASUREMENTS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SOURCES OF INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

HUMAN STRENGTH AND HANDLING CAPACITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .HUMAN SENSORY CAPABILITIES . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SIGHT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .HEARING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TOUCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-117-117-117-127-127-137-167-167-167-177-177-177-177-207-207-27

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PSYCHOLOGICAL FACTORS . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MOTIVATION AND TRAINING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .CAPABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .HUMAN ERROR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ..Causes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Contributing Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PHYSICAL FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 1OENVIRONMENTAL FACTORS

10-1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2 ENVIRONMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2.1 NATURAL, INDUCED, AND COMBINED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2.2 WARTIME ENVIRONMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2.2.1 Chemical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2.2.2 Nuclear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2.2.3 Electromagnetic Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2.2.4 Enemy and Friendly Fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3 EFFECTS OF ENVIRONMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1 EFFECTS ON PERSONNEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.1 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.1.1 Effective Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.1.2 Wet-Bulb Global Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.1.3 Windchill Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.1.4 Guidelines for Maintenance Operations in Extreme Climates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.2 Other Climatic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.3 Whole-Body Vibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.1.4 Terrain Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2 EFFECTS ON EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.1 Moisture Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.2 Desert Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.3 Arctic Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.4 Vibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.5 Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.6 Acceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.7 Nuclear Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.1.8 Electromagnetic Radiation Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3.2.2 Summary of Environmental Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9-139-139-139-149-149-149-149-159-159-179-17

10-110-210-210-210-310-310-310-510-510-510-510-610-610-610-610-1010-1010-1210-1210-1310-1310-1410-1410-1510-1510-1510-1510-1710-1810-26G-1l-l

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DOD-HDBK-791(AM)

LIST OF ILLUSTRATIONS

Figure No. Title Pagel-11-21-32-12-23-13-23-33-44-14-24-34-44-54-64-74-84-94-104-114-12

4-134-144-154-164-174-184-194-204-214-224-234-244-254-264-274-284-294-304-314-324-334-344-354-365-15-25-35-45-55-65-75-85-95-106-1

Distribution of Maintenance Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Maintenance Time Distribution and Maintainability Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Maintainability Level of Effort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Comparison of the M188 20-mm Gattling Gun to the M230 30-mm Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Simple Self-Powered Feed System in M230 Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Typical Maintenance Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example for Selection of Parts for Program Parts Selection List (PPSL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .NSN Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TM DE Selection Flow Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Types of Covers and Accesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Spring Loaded Sliding and Hinged Access Doors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sliding Access Door With Handle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Rollout, Rotatable, and Slide-Out Drawers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Access to Permit Easy Removal of Component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Methods of Securing Hinged Covers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Avoidance of Structural Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Component Mounting Positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Clearance for Nuts and Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Common Orientation of Tube Sockets to Facilitate Tube Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Simultaneous Maintenance Actions on a Modern Helicopter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Frequency With Which Hand Tools Are Used at Least Once in 427 Maintenance Tasks onElectronic Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

One-Hand Access Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Access-Opening Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Gloved-Hand Dimensions for 95th Percentile Man . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Foldout Construction for Electronic Chassis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Hinged Assemblies Braced in “Outw Position Leave Both Hands Free . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Stands for Maintenance as a Part of Chassis Will Prevent Damage to Parts . . . . . . . . . . . . . . . . . . . . . . . . .Side-Alignment Brackets Facilitate Correct Mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Handle Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Handles Facilitate Removal of Covers and Carrying of Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Cases Should Lift Off Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Additional Use of Handles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .U-Type Lugs Facilitate Repairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Spacing Wire Leads Facilitates Repairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Terminals Should Be Long Enough to Prevent Damage to Insulation During Repairs . . . . . . . . . . . . . . . . . . .Route Cables So They Are Not Likely to Be Walked On . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Route Cabling to Avoid Sharp Bends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Methods for Recoiling Service Loops in Sliding Chassis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Connectors Should Be Arranged So That They Can Be Grasped Firmly for Disconnection . . . . . . . . . . . . .Fastener Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mating Surface of Component Trailers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Physical Stability for Maintenance Stands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Corner Stress Cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Provide Access Door for Each Fuel Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mount Stator and Rotor Blades to Facilitate Removal of Individual BladesDesign for Functional Utilization That Corresponds to Modularization

. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Modules in Target Acquisition and Designation System (TADS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TADS Turret Assembly—Day Side Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .M1 Turbine Engine Modules for Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Fittings and Rigging for Rear Engine Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Modular Vehicle Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Data Output Balance Sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .CHAIN GUN Bolt Assembly and Drive Train Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.56-mm M231 Submachine Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Modules for 5.56-mm M231 Submachine Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Identification Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-31-41-52-52-63-53-73-93-114-34-44-54-54-54-64-64-74-84-84-9

4-114-124-134-154-164-164-164-164-174-184-184-184-184-184-184-194-194-194-204-214-224-224-244-254-275-55-75-85-105-115-125-135-145-155-156-3

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DOD-HDBK-791(AM)

LIST OF ILLUSTRATIONS (cont’d)

Figure No. Title6-26-36-46-56-66-76-86-96-106-116-126-136-146-156-166-176-18

6-196-206-216-227-17-27-37-47-57-68-18-28-38-48-58-68-78-88-99-19-29-39-4

9-510-110-210-310-410-510-610-7

Example of Warranty Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Placement of Labels for Hazardous Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Informative Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Label Brevity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Step-by-Step Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Horizontal Labels to Facilitate Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Preferred Arrow Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Consistent Positioning of Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Examples of Method of Positioning Labels to Avoid Masking by Control Handles . . . . . . . . . . . . . . . . . . . . . . . .Example of Label Placement Which Can Result in Upside-Down Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of a Poor Placement of Label . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Label Location for a Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of Placement of Label on Vertical Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Labels for Valve Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of a Caution Sign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of a Danger Sign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Character Height Criteria for Instruments, Panels, and Equipment Viewed in Close Proximity

Under Various Illumination Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Character Height for Viewing Signs at Extended Distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Label Size-Hierarchy Example for Equipment, Panel, Subpanel, and Component Identification . . . . .Radio Frequency Radiation Hazard Warning Symbol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nuclear Radiation Hazard Warning Symbol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Liquid Coolant Subsystem Showing Transducers and Parameters of Interest . . . . . . . . . . . . . . . . . . . . . . .Test Sequence for Liquid Coolant Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Logic Test Sequence for Track Radar Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Data Analysis Methodology for Vibration Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Design Approach Fault Detection and Location Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Redundant FBW Stabilator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Relation of Equipment Status on Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .RCM Analysis Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Logistical Support Analysis Record (LSAR) Data Record B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Typical Lubrication Fittings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bearing Lubrication Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Example of a Lubrication Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Oil Level Sight Plugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Drain Plug Heads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Electromotive Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .US Army Basic Trainees (1966): Breadth & Circumference Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Arm, Hand, and Thumb-Finger Strength (5th Percentile Man) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Visual Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Permissible Distance Between Speaker and Listener for Specified Voice Levels and Ambient Noise

Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Easily Recognizable Knob Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Error Increase Due to Rise in Effective Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Deriving Effective Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summer and Winter Comfort Zones and Thermal Tolerances for inhabited Compartments . . . . . . . . . . . .Windchill Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Vibration Criteria for Longitudinal and Transverse Directions With Respect to Body Axes . . . . . . . . . . . .90% Motion Sickness Protection Limits for Human Exposure to Very Low Frequency Vibration . . .Reduction in Insulation Resistance of Typical Electronic Components Exposed for Five Months inTropical Jungle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page6-36-56-56-86-86-96-96-96-106-106-106-106-116-116-116-11

6-136-146-156-166-177-127-147-157-187-197-208-28-48-58-88-98-118-138-138-149-39-59-7

9-129-11

10-610-710-810-910-1110-I2

10-13

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DOD-HOBK-791(AM)

LIST OF TABLES

Table No. Title Page

l-l2-l3-13-24-l4-24-34-44-5

4-6

4-75-16-16-26-36-47-l7-27-37-47-57-67-77-88-l8-28-38-49-l9-29-39-49-59-69-79-8

10-110-210-310-410-510-610-710-810-910-1010-1110-12

Application Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Simplification Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Standardization Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Interchangeability Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Recommended Equipment Accesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Aperture Dimensions for Regularly Clothed Technician . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .List of lop Ten Replacement Parts by Frequency of Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .List of Top Ten Replacement Parts by Man-Hours to Replace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reciprocating Engine Accessories That Should Be Accessible for Inspection, Cleaning,Adjustment, Removal, and Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Turbine Engine Accessories That Should Be Accessible for Inspection, Cleaning, Adjusting,Removal, and Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Accessibility Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Modularization Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Close Equivalent Typefaces Suitable for Labeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Best Color Combinations in Descending Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Recognizable Colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Identification Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Typical Fault Insertion Test Results Versus Field Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Typical Test Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Diagnostic Portion of Operator’s Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Summary of Diagnostic Effectiveness for Gas Turbine Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Typical FMEA Backup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Elements for Fault Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Testability Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Diagnostic Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Color Codes for Filler Caps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Drain Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Galvanic Series in Sea Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Preventive Maintenance and Servicing Design Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sources of Anthropometric Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Maximum Weight Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Illumination Requirement for Representative Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Specific Task Illumination Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Functional Evaluation of Audio Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .When to Use Auditory or Visual Form of Presentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sound Intensity Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Human Error Rate Estimate Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Relationship of Environmental Factors to Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Major Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Combination of Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sound Levels Associated With Weapon Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Human Reaction to Windchill Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Temperature Extremes for Electronic Equipment Operating in a Desert Environment . . . . . . . . . . . . . . . . . . . . .Arctic Temperature Extremes for Electronic Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Vibration-Induced Damage to Electrical and Electronic Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Effect of Acceleration on Military Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Environmental Requirements for Unsheltered Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Summary of Major Environmental Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Failure Modes of Electronic Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-62-73-123-124-44-124-234-24

4-26

4-274-295-166-126-156-156-177-67-137-187-217-227-247-257-268-138-138-148-179-29-69-79-89-109-119-129-16

10-110-310-410-510-1010-1410-1410-1610-1710-1810-1910-22

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DOD-HDBK-791(AM)

This page intentionally left blank.

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DOD-HDBK-791(AM)

LIST OF ABBREVIATIONS AND ACRONYMS

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DOD-HDBK-791(AM)

LIST OF ABBREVIATIONS AND ACRONYMS (cont'd)

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DOD-HDBK-791(AM)

CHAPTER 1INTRODUCTION

The reasons for maintainability, i.e., reduced support costs and improved operational readiness—arepresented. Maintainability and maintenance are defined, and quantitative measures of maintainability areintroduced. The maintainability program encompassing its objectives, plan, and verification is discussed.Features that facilitate maintainability are listed. The importance of introducing these features as early aspossible into the design process is stressed.

1-1 INTRODUCTIONMaintainability and reliability are the two major sys-

tem characteristics which combine to form the commonlyused effectiveness index availability. Although reliabil-iy and maintainability share co-importance, maintain-ability merits special consideration because of its in-fluence on system maintenance activities, i.e., the expen-diture of man-hours and material, which represent signif-icant budgetary costs over the life of the system.Maintenance activities also reduce the operational readi-ness of a system.

With the introduction of modern, complex materielresulting from sophisticated technology and the impor-tance of’ keeping the materiel combat ready and itspotential for higher failure rates and attendant increasedmaintenance actions, repairs could no longer be basedsolely on individual judgment and subjective analysis. Itbecame evident that "how much time is required toreplace or repair an item” was not the sole criterion,rather "how much time and skill are required to determinewhich item to replace or repair” and how to reduce theneed for maintenance or the simplification of the actionbecame equally important. The consideration of main-tainability in designing a system is not a new conceptsystems were always designed to have “good”, “maxi-mum”, or “optimum’’ maintainability. Unfortunately, theuse of these qualitative adjectives resulted in an “un-known” maintainability. New techniques, however, per-mit the conversion of these subjective qualitative judg-ments into an area of quantitative measurements. Thethreshold adopted by the US Army Materiel Command(AMC) is the operational requirement. Current AMCpolicy is to develop operational requirements in consider-ation of mission need, technical feasibility, and operatingand support costs; and to document the requirement inthe Decision Coordinating Paper (DCP) and SystemConcept Paper (SCP).

Maintainability is a risk area not because the require-ments are not technically available: rather, it is a risk areabecause of the reluctance of the technical community tochange from its traditional emphasis on performance asopposed to maintainability.

In summary, maintainability has emerged as an impor-

tant factor of the design process and an inherent designcharacteristic that is truly quantitative in nature and.therefore, lends itself to specification. demonstration, andtrade-off analysis with such characteristics as reliabilityand logistic support. The implementation of this philos-ophy seeks the goals and objectives presented in par.1-4.1. For the maintainability engineer this means that theoptimum degree of maintainability must be incorporatedin system design. beginning as early as the concept phase.If the maintainability engineer, working with the designer,fails to accomplish this, he fails to achieve his objective—i.e., the provision of operational availability. A systemthat fails to perform at times cannot safely be planned,which renders it useless for combat operations.

1-2 MAINTAINABILITY VSMAINTENANCE

Maintainability is a characteristic of design and instal-lation. This characteristic is the measure of the ability ofan item to be retained in or restored to a specified condi-tion when maintenance is performed by personnel havingspecified skill levels and using prescribed procedures andresources at each prescribed level of repair (Ref. l).

Maintenance is essentially the response to the main-tainability program, i.e., the series of actions necessaryfor retaining materiel in or restoring it to a serviceablecondition. Maintenance actions are of two types, i.e.,

1. Corrective Maintenance. An action required whenequipment fails or malfunctions

2. Preventive Maintenance. An action required tomaintain equipment in an operable condition throughperiodic servicing and/or replacement of components atspecified intervals. Preventive maintenance can, andshould, be conveniently scheduled to avoid interferencewith operating schedules. A detailed discussion of preven-tive maintenance is presented in Chapter 8.

Erroneously, corrective maintenance is referred to asunscheduled maintenance, and preventive maintenance isreferred to as scheduled maintenance. From a practicalstandpoint military personnel perform maintenance—both corrective and preventive-–whenever a window ofopportunity exists. The specific maintenance tasks are afunction of the reliability, availability maintainability,

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and durability (RAM-D) of the equipment and the opera-tional environment. The calendar time, i.e., when themaintenance action was performed, makes that actionscheduled or unscheduled.

The unscheduled interruption of a planned operation isalways undesirable and usually costly; in the extreme caseit could be catastrophic. Although unreliability is usuallythe primary case of failure and thus governs the frequencywith which maintenance actions are necessary, the ease ofmaintenance and the skill of maintenance personnel gov-ern the duration of the action. The easier it is to maintainan item of equipment, the fewer will be the demands onboth the skill and number of personnel and, in general.the greater the reduction of equipment downtime. Accord-ingly, since the time required for maintenance actions is afunction of the maintainability characteristics of theequipment, effectiveness of built-in testing and physicaldesign features that affect the speed and ease with whichmaintenance can be performed should be addressed.Design features are discussed in par. 2-5.

In addition to physical design features. personnel andhuman factor considerations are of prime importance.These considerations include the experience of the techni-cian, training required. skill level, supervision required,supervision available, techniques used, physical coordi-nation and strength and number of technicians, andteamwork requirements. Personnel and human factorsare emphasized because the Army---as well as the otherServices—is imposing a strength cap on the number ofmilitary personnel and restricting the availability of fundsfor development and training, and procurement of train-ing aids. Additionally, the scenario under which the newArmy counters threats requires the deployment of lightinfantry. Thus the impact on engineering design must bethat of ease of maintenance, adequate man machineinterface, minimal maintenance, and maximum surviv-ability. In no single area of weapon engineering are thepotential rewards as great as those which could beachieved by simplifying the human functions needed tomaintain the weapon system.

This brief introduction highlights the distinctionbetween maintainability and maintenance. In summary.maintainability is a design characteristic that makes pos-sible the accomplishment of operational objectives withminimal expenditure of support effort and resources andis a prime responsibility of the maintainability engineerworking in cooperation with the designer: maintenance isthe actions necessary for retaining materiel in or restoringit to a serviceable condition.

1-3 MEASURES OF MAINTAINABILlTY1-3.1 GENERAL

In par. l-1 it was pointed out that the maintainabilitycharacteristic had to be expressed quantitatively to bemeaningful. This characteristic is expressed as the proba-

1-2

bility that an item will be retained in, or restored to, aspecified condition within a given time period if pre-scribed procedures and resources are followed. There areseveral measurable parameters that can be used to quan-tify the maintainability characteristic, ease of mainte-nance. Ease of maintenance characterizes the maintain-ability designed into an equipment and can be measuredby the elapsed time in which the maintenance can beperformed. Thus the maintenance time required to cor-rect equipment performance deviations, such as failure ordegradation, is is a good measure of how well the equipmenthas been designed for maintainability.

When maintenance time as a design parameter is mea-sured, active time only should be considered. The empha-sis is on the word "active” since there are administrativeand logistic delays—e.g., absence of proper instructionsand waiting for a repair part that bear no relationship toequipment design.

Active-type maintenance time for corrective mainte-nance actions usually consists of three sequential steps,i.e.,

1. Time to locate the parts requiring repair2. Time to perform the repair3. Time to verify that the repair has been performed

successful).For preventive-type maintenance, the first step if elimi-nated because the equipment maintenance area ispredetermined.

Attributes of the equipment that cause variations inrepair time result from the physical characteristics of thefailed parts, their location and mounting arrangements inthe equipment, and thus their accessibility and replace-ability. Variations in the diagnostic time result from trou-bleshooting procedures, location of test points and kindof test equipment, and the sequence in which trouble-shooting is performed. Also technicians’ skills, theirdegree of familiarity with the equipment, and the environ-ment in which maintenance is being performed affectboth diagnostic time and repair time. Even identicalmaintenance actions, caused by identical failures in iden-tical equipment and performed by the same repairman orrepair crew, will have varying maintenance timed. A largenumber of measurements probably would yield a contin-uous distribution of maintenance time for this mainte-nance action even if it were performed by the same techni-cian or the same repair crew. The distribution of mainte-nance times for the same maintentince action with adifferent repair crew would also be different. Fig. l-lillustrates this phenomenon for two different crews.

Crew 1 performs the maintenance action with a meantime m 1 ; Crew 2 takes a consideraibly longer mean timem 2. Also the variance of maintenance time with whichCrew 2 works is substantially greater than that of Crew 1.Assume that both crews performed the identical mainte-nance operation under identical environmental condi-

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Figure 1-1. Distribution of Maintenance Time

tions, at the same time of day, and are refreshed. It isreasonable to conclude from Fig. l-l that Crew 1 is moreskilled and perhaps more disciplined than Crew 2.

Obviously, the time required for a given maintenanceaction is not a fixed value. This variability would be evenmore evident in sophisticated materiel where many differ-ent types of failures may occur, and where the necessarymaintenance actions have their own time distributionsand usually occur at a different rate.

The previous discussion introduced terms distribu-tion, mean. and variance used by the statistician. Thissuggests that maintenance time can be defined and ana-lyzed by rigorous statistical techniques. To employ thesetechniques, specific elements describing maintenancetime are necessary - a probability density function g(t)that shows the frequencies with which maintenance timesof different duration occur and a cumulative probabilitydistribution M(t) that shows the probability that mainte-nance time is equal to or shorter than a fixed time con-straint. Fig. 1-2 represents the case for which the repairtime appears to be normally distributed. The probabilitydensity function presented in Fig. 1-2(A) shows the fre-quency of maintenance occurrence versus a maintenancetime; the cumulative probability distribution, Fig. 1-2(B),shows the probability of accomplishing maintenance ver-sus maintenance time.

Some points of interest on the time axis of Fig. 1-2should be noted. M represents the arithmetic mean of the

maintenance time distribution and, in this special case, italso represents the median of the distribution since aGaussian distribution is indicated. Mmax is the 95th per-centile and represents the time in which at least 95% of allmaintenance actions can be completed.

M(t) is obtained by the integration of g(f). This is seenin Fig. 1-2 where the probability M(t) on the maintain-ability axis of Fig. 1-2(B) corresponds to the area M(T)under the probability density curve of Fig. 1-2(A). M(T) isthe probability that maintenance can be performed in atime T or less. Ref. 2 provides a detailed discussion ofthese relationships and the statistical concepts. Since M(t)is a probability, it can assume only positive values, i.e., 0< probability < 1. A probability of zero means impossi-bility; a probability of one means certainty.

Because of its simplicity, the Gaussian case was used toillustrate the meaning of the statistical terms distribution,mean, and variance. In practice, however, a lognormaldistribution frequently is assumed to describe the distri-bution of maintenance times. If tractability is desired, anexponential distribution may be assumed. Techniquessuch as the Kolmogorov-Smirnov test, which tests thehypothesis that the maintenance time was drawn from apopulation having a probability density function g(t),should be used to determine the best-fit distributionbefore proceeding to compute mean times to repair. Ref.3 describes the lognormal distribution and the Kolmo-gorov-Smirnov test.

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1-3.2 QUANTITATIVE MAINTAINABILITYINDICES

There are many mathematical indices used to quantifymaintainability. It is important to remember, however,that these relationships merely categorize data derivedfrom planned testing. For maintainability, the test plan-ning phase is equal in importance to the assessment phase.Testing that does not adequately demonstrate the effect ofthe physical features and personnel and support aspects(described in par. 1-2) actually provides data that effec-tively conceal the impact of these critical elements.

Indices used to support maintainability analysis must1. Be composed of measurable quantities2. Provide effectiveness-oriented data3. Be readily obtainable from operational and appli-

cable development testing.These kinds of indices provide system designers, users,and evaluators with data operational readiness, missionsuccess, maintenance manpower costs, and logistic sup-

port costs -that can be used to evaluate candidate sys-tems more precisely.

Since maintainability programs must be tailored to thespecific system, the terms used to define the various mea-sures should be similarly selected: however, the termsshould be standardized for similar major systems. It isalso important that the numerical values associated withthe terms describing system maintainability)’ must beoperational values and not inherent values (Ref. 4).Because of their importance, the terms are defined:

1. Operational Value. A measure of maintainabilitywhich includes the combined effects of item design. qual-ity, installation, environment. operation, maintenance,and repair

2. Inherent Value. A measure of maintainability)which includes only the effects of item design and itsapplication, and assumes an ideal operation and supportenvironment.

Some common, accepted indices of maintainability are1. Mean Time to Repair2. Maximum Time to Repair

1-4

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3.4.5.6.7.8.9.

10.11.12.13.

Mean Maintenance TimeEquipment Repair TimeGeometric Mean Time to RepairMaintenance Man-HoursRepair RatesMaintenance RatesProbability of Fault DetectionProportion of Faults IsolatableAutomatic Fault Isolation CapabilityPercent of False AlarmsPercent of False Removals.

These terms are defined in the Glossary. Equations forcalculating these indices, together with examples, arefound in Refs. 3, 5, and 6.

1-4 MAINTAINABILITY PROGRAM

1-4.1 OBJECTIVESDoD Directive 5000.1 states, “Improved readiness and

sustainability are primary objectives of the acquisitionprocess. Resources to achieve readiness will receive thesame emphasis as those required to achieve schedule orperformance objectives. As a management precept, oper-ational suitability of deployed weapon systems is anobjective of equal importance with operational effective-ness.”. Operational suitability as defined by Ref. 7 is “Thedegree to which a system can be satisfactorily placed infield use, with consideration being given to...maintain-ability, safety, human factors, manpower supportability,logistic supportability, and training requirements.”. Today,when large masses of troops and materiel can be deployedanywhere in the world in a matter of days or even hours,system readiness demands the utmost attention.

The importance of a maintainability program to thesystem acquisition process is well established by the pre-vious discussion. Therefore, to accomplish its stated pur-

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pose, the maintainability program must contribute toachieving the following objectives (Refs. 8 and 9):

1. Improved operational readiness2. Reduced maintenance manpower needs and assur-

ance that the fielded system can be operated and main-tained with skills and training expected to be available tothe Army

3. Reduced life cycle costs, i.e., economically oper-ated and maintained

4. Assurance that maintainability requirements areproperly demonstrated and assessed during developmentand operational test and evaluation. Fig. 1-3 illustratesthe time phasing for maintainability verification, demon-stration, and evaluation.

5. Provision of data essential for management, i.e.,assurance that the results of maintainability assessmentare presented in a form suitable to support the decision-making process.

It must be recognized that these objectives are interre-lated and affect both operational effectiveness and owner-ship costs. Accordingly, they must be applied skillfully toavoid being duplicative or contradictory, and at each levelat which maintenance is to be performed. These samegoals also may apply to related programs, e.g., reliability,and may require the same kind of tasks and analysis todemonstrate and verify them. To avoid duplication ofeffort, performance of such tasks or analysis will be coor-dinated and whenever possible combined with similartasks called for under other program elements (Ref. 9).Coordination may require trade-offs. However, in thetrade-off process care must be exercised to insure thatmaintainability demonstration and evaluation are notsacrificed.

Cost is an item of consideration at each phase in theacquisition process—concept exploration, demonstrationand validation, full-scale development, and productionand deployment. Since maintainability is dedicated to

Figure 1-3. Maintainability Level of Effort

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cost reduction over the life of a system, maintainabilityengineering input is an essential clement in the evolutionof a system.

1-4.2 MAINTAINABILITY PLANThe maintainability program will include the tasks

necessary to achieve and demonstrate the goals indicatedin par. 1-4. The various tasks and their relationships to theacquisition process are presented in MIL-STD-470 (Ref.9). A matrix illustrating these relationships is shown inTable 1-1. The degree of maintainability achieved willdepend on the imposed requirements and management’semphasis on maintainability. The tasks are not to beapplied indiscriminately: All maintainability requirementsdo not apply to all systems. Instead, they are to be tailoredas required by their users as appropriate to particular

systems or equipment program type, magnitude, andneed (Ref. 9). Tailoring (Ref. 9) is the process by whichindividual requirements of the selected specificaitions,standards, and related documents are evaluated to deter-mine the extent to which they are most suitable for aspecific system and equipment acquisition, and the modi-fication of these requirements to insure that each achievesan optimal balance between operational needs and costs.The tailoring process, however, must conform to provi-sions of existing regulations governing rnaintainabilityprograms and take care not to exclude those requirementsthat are determined essential to meeting minimum opera-tional needs.

The elements that comprise the maintainability planare explicitly detailed in MIL-STD-470 (Ref. 9) andshould be used as a guide in plan preparation. MIL-STD-

TABLE 1-1. APPLICATION MATRIX (Ref. 9)

Code Definitions for Table: (1) Requires considerable interpretation of intent to be cost-effective.MIL-STD-470 is not the primary implementation document.Other MIL-STDs or Statement of Work requirements must beincluded to define or rescind the requirements. For example, MIL-STD-471 must be imposed to describe maintainability) demonstra-tion details and methods.Appropriate for those task elements suitable to definition duringphaseDepends on physical complexity of the system unit being procured,its packaging. and its overall maintenance policy.

S =G =

C =

N/A =ACC =ENG =MGT =FSD =

PROD =

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Selectively applicable (2)Generally applicableGenerally applicable to designchanges onlyNot applicableMaintainability Accounting (3)Maintainability EngineeringManagement (4)Full-scale developmentProduction

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470 (Ref. 9) is very specific about the importance ofallocating and predicting maintainability. Thc basis forthe process must be linked closely to the equipment sup-port concept, available maintenance manpower, andoptimum design for repair as a function of survivability,redundancy, and battle damage. To obtain a maintain-ability-friendly design, a partnership between industryand Government is necessary there must be an enhancedmanagement awareness of the maintainability require-ment by both industry and Government managers duringthe acquisition process. Contractors will respond to DoDpriorities.

1-4.3 VERIFICATION, DEMONSTRATION,AND EVALUATION OFMAINTAINABILITY REQUIREMENTS

The preparation of a plan that includes the elementsthat insure the system will have the characteristic of main-tainability designed into it is an important first step.However, to assess system maintainability quantitatively,the provisions of the maintainability plan must be veri-fied, demonstrated, and evaluated. Unless this is done, thequalitative descriptors good, maximum, optimum --described in par. l-l apply.

MIL-STD-471 (Ref. 5) provides procedures and testmethods for the verification, demonstration, and evalua-tion of qualitative and quantitative maintainability require-ments. ‘These actions are performed in accordance withthe maintainability test plan described in par. 1-4.2. Thetime phasing of these actions relative to the acquisitionprocess is illustrated in Fig. 1-3. MIL-STD-471 also pro-vides for qualitative assessment of various integratedlogistic support factors related to and impacting theachievement of maintainability parameters and itemdowntime e.g., technical manuals, personnel, tools andtest equipment, maintenance concepts, and provisioning.

AR 702-3 (Ref. 8) provides additional information ontesting the various maintainability parameters; responsi-bilities for conducting the test; conduct of the tests; scor-ing of the tests; and evaluation of the maintainabilityfeatures; and evaluation of the maintainability features toidentify needed changes in equipment design or equip-ment maintenance—e.g., maintenance allocation chart—and revisions, as appropriate, to the maintenance plan/support concept.

To be effective, the results of the evaluation analysesmust provide information to designers and managers sothat the analyses are actually causing change-—if required—in the design rather than recording data on the design thatexists. The timing and credibility of these analyses mustbe such that they are accepted as part of the design evalua-tion. A common flaw in many maintainability programsis to have the analyses separated from design activity bytime, distance, or organization. The role of the maintain-ability analyses can often be an indicator of the impor-tance placed upon these tasks by the design team.

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1-5 FEATURES TO FACILITATEMAINTAINABILITY

Maintainability requirements, and the resulting main-tenance actions, must be supported by system design.Qualitative and quantitative requirements must be estab-lished to provide system design guidelines during systemdevelopment; it is mandatory that a system maintainabil-ity concept be formulated prior to detailed design. Identi-fication of maintenance requirements is a key contributorto cost-effectiveness. When life cycle cost is minimized, amajor factor is ease of maintenance.

Features that facilitate maintainability include the ele-ments of physical attributes, diagnostics, simplification,testability, inspectability, accessibility, and maintainabil-ity time/cost design criteria commensurate with the main-tainability profile of the system. Each of these elements isconsidered by the maintainability engineer in his devel-opment of concepts, criteria, and technical requirementsto assure timely, adequate, and cost-effective support ofthe design and operational needs of the system. Physicalfeatures and pertinent questions that affect maintainabil-ity follow:

1. Accessibility. Can the item be reached easily forrepair or adjustment?

2. Visibility. Can the item being worked on be seen?3. Testability. Can system faults be detected readily

and isolated to the faulty replaceable assembly level?4. Complexity. How many subsystems are in the

system? How many parts are used? Are the parts standardor special purpose?

5. lnterchangeabiliy. Can the failed or malfunction-ing unit be readily replaced with an identical unit with norequirement for alteration and calibration?

6. Identification and Labeling. Are componentsuniquely identified? Are the labels permanent, or are theyeasily erased or obliterated by operation or maintenanceactions? Are labels positioned to be easily read?

7. Verification. Can it be easily verified that therepaired item is functioning correctly?

8. Simplicity. Is the design as simple as possible? Arestandard parts and tools used? Are functions and partsconsolidated?

Pertinent questions to be asked concerning the ratio-nale to be considered in assessing the previous designfeatures are

1. Are features compatible with maintainabilityresource developments?

2. Is the system friendly to maintenance in providingaccessibility, fault isolation capability, and packaging forreliability?

3. Can calibration and precision measurements beeasily and readily accommodated or the need totallyeliminated?

4. Is the system packaged for fault isolation and

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repairability as required by the maintainability concept, ments are not sacrificed for expediency or budgetarymanpower, and skill restraints? considerations.

5. Are design trade-offs considered with maintain- The means for implementing these design features areability requirements? Exercise care so that the require- discussed in the chapters that follow.

REFERENCES1. MIL-STD-721 C, Definitions of Effectiveness Terms 6. T. F. Pleska, F. L. Jew, and J. E. Angus, Maintain-

for Reliability, Maintainability, Human Factors, and ability Prediction and Analysis Report, Hughes Air-Safety, 12 June 1981. craft Company Report, RADC-TR-78-169, Rome

2. MIL-HDBK-472, Maintainability Prediction, 12 Air Development Center, Air Force Systems Com-

January 1984. mand, Griffiss Air Force Base, NY, July 1978.

3. AMCP 706-133, Engineering Design Handbook, 7. DOD Directive No. 5000.1, Major System Acquisi-

Maintainability Engineering Theory and Practice, tions, 29 March 1981.January 1976. 8. AR 702-3, Army Materiel Systems Reliability, A vail-

4. DOD Directive No. 5000.40, Reliability and Main- ability, and Maintainability (RAM), 1 May 1982.trainability, 24 February 1982. 9. MIL- STD-470A, Maintainability Program Require-

5. MlL-STD-471 A, Maintainability Demonstration, 8 ments (For Systems and Equipments), 3 JanuaryDecember 1978. 1983.

BIBLIOGRAPHYMIL-STD-2165, Testability Program for Electronic Sys- MIL-STD-2077, Test Program Sets, General Require-

tems and Equipments, 26 January 1985. ments for, 9 March 1978.

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CHAPTER 2SIMPLIFICATION

Simplification is defined and the importance of it as a factor in maintainability is emphasized. Fivetechniques for achieving simplification are discussed:(1) Coordination of Equipment and Job Design, (2) PartReduction, (3) Function Consolidation, (4) Access Improvement, and(5) Maintenance Procedure Streamlin-ing. Part reduction and function consolidation are described as functions of system-level trade-offs and valueengineering. Features to enhance an approved support plan are presented. Examples of complex modulesredesigned to achieve simplicity are included, and a simplification design checklist is provided.

2-1 INTRODUCTIONGeneral Maxwell D. Taylor, while chairman of the

Joint Chiefs of Staff, commented on the modern Army asfollows: “our Army must be able to disperse and hide,and converge and fight. It must be able to shoot, move,and communicate. If we are to attain this concept ofmobility, we must reduce our requirements for logisticalsupport.”. Despite the fact that this statement was made20 yr ago, it is still relevant today the “shoot and scoot”tactic still applies.

For many years roughly 11 cents of every dollar in thedefense budget was expended for maintenance; the sup-port costs of some systems exceeded their acquisitioncosts many fold, and the number of maintenance techni-cians required for support exceeded the number of oper-ating personnel. The continued development and deploy-ment of highly complex systems such as guided missiles,communication networks, computers, and reconnaissancedevices—require a mass of on-site support equipmentwhich in turn demands massive logistic support.

The common denominator of all support costs appearsto be complexity --not solely system complexity but alsocomplexity of operation and maintenance. If this is true,then design for both simplicity of operation and mainte-nance is the area that offers the most promise for reduc-tion of support costs. A certain amount of equipmentcomplexity is necessary, and the designer should try toachieve ease of maintainability in spite of complexity. Themajor support costs could be greatly reduced if thedesigner working in conjunction with the maintainabilityengineer were constantly aware of the limitations of oper-ator and maintenance personnel and of the adverse envi-ronment in which such personnel must perform - personalfatigue, blackout conditions, cold, rain, mud, dust, etc. Amaintainability feature that reduces the number of main-tenance personnel, however, does not tell the completestory. This feature may also result in the elimination of anitem of test equipment which in turn eliminates therequirement for manuals to operate and repair the testitem and for backup facilities to repair the item—i.e., asnowball effect. This same scenario could be applied tothe elimination of a part.

Simplification is easy to mandate but is probably the

most difficult maintainability characteristic to achieve.The rewards are great because the results are a significantdriving factor in the reduction of life cycle costs. Simplic-ity is worth the effort invested to achieve it and should bethe constant goal of every design engineer.

2-2 DESIGN TECHNIQUESThere is a general tendency on the part of designers of

equipment to produce an overly complex product. Inmany cases the equipment uses too many parts, has tooclose operating tolerances, is too expensive to build, andis difficult and expensive to maintain. The resolution ofthese factors, to develop a simple design, is the result ofcompromises and trade-offs among the user, designer,and maintainability engineer but never at the expense ofsystem availability or effectiveness. For example, if for agiven system the desired degree of availability cannoteconomically be achieved by the incorporation of reliabil-ity in its design, then it can be achieved only by increasedemphasis on maintainability characteristics that willreduce downtime. Maintainability, however, should notbe used as a crutch for reliability. Trade-offs in maintain-ability should encompass reliability, support, cost, andstate-of-the-art design for testability using built-in testequipment and automatic test equipment.

Design techniques for achieving simplification include

Reduction in number of partsValue engineeringConsolidation of functionsImproved access to partsStreamlined maintenance proceduresSoftware maintenance.

1. Coordination of equipment and job design2.3. .4.5.6.7.

In applying these techniques, the necessity for trade-offsand compromises previously discussed must be consid-ered. Equally important is the impact of these techniqueson the logistical support plan. The resultant equipmentdesign should represent the simplest configuration possi-ble consistent with functional requirements and expectedservice and performance conditions.

Each of these techniques is discussed in the paragraphsthat follow.

2-1

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2-2.1 COORDINATION OF EQUIPMENTAND JOB DESIGN

The design of new equipment is also the design of newjobs for both the operator and the maintenance techni-cian. The more human factors engineering is consideredin the design of new equipment, the better these two jobsof operating and maintaining can be done. In the designof equipment, effort should be directed toward simplify-ing the operator’s and maintenance technician’s tasks;this can be accomplished by the recognition of the impor-tance of integrating job design and equipment designduring the early stage of equipment design. The compara-tive simplicity or difficulty of the maintenance task is builtinto the equipment. Thus the engineer is unconsciouslydesigning a job when he designs a piece of equipment;accordingly, he should be aware of the capabilities andlimitations of the human being on whom effective main-tenance depends. Therefore, specialists in human factorsengineering and in personnel training should assist inplanning the equipment. Even an equipment that per-forms complex operations can be designed so that it iscomparatively easy to operate and maintain – a telephoneis an example—by providing standard and interchange-able parts, by using a simple method of testing and diag-nostics, and by providing ready access to defective parts.These factors are touched upon in the paragraphs thatfollow and covered in detail in subsequent chapters. Thcdesign engineer must remember that regardless of theexcellence of his design, it will be no better than themaintenance of it, and the maintenance will be performedby personnel of average ability.

2-2.2 REDUCTION OF PARTSReduction in the number of parts in an end-item should

lead to a lower number of maintenance actions and thusto improved end-item availability. However, part reduc-tion is subject to interface agreements between maintain-ability and other dsign disciplines; these other disciplines

by external monitoring and or test equipment, thesefuses should be located in the external test equipment, notin both the adaption kit and the tcst set. The test set is thelogical location because of easy access for replacement.Also the inclusion of unnecessary parts in circuits reducesthe reliability of the circuits out of proportion to theirconvenience and may be the source of sneak circuits.

Modularization dissussed in detail in Chapter 5contributes to simplicity by reducing the number ofexposed parts that may require replacement. If themodule is discarded or destined for repair lit the depotlevel, there is a serendipitous effect it reduces the needfor addressing the contained parts in manuals and repairparts lists. reduces the required skill level of technicians,and reduces the inventory of repair parts. Despite thebenefit, modularizaition has the potential for increasingthe cost of repair parts and complexity of repair at thedepot level.

2-2.3 VALUE ENGINEERING

Value engineering may also be employed to bring aboutsimplification. Value engineering is a questioning type oftechnique; the type of approach can be characterized bythese questions:

1. What is it?2. What does it do?3. What does it cost?4. What else will do the job?5. What does that cost?

Cost must obviously be interpreter to include the logisti-cal cost. In pursuing this approach it is important that theessential product performance, reliability, and maintain-ability are locked into the value engineered item. Like anygood problem-solving technique, value engineering is achallenging and searching methodology. It is forever fore-ing the value engineering practitioner to dig for funda-mentals—i.e., to determine what the part or function isr ea l l y i n t ended to ach i eve and whe the r t he pa r t i snecessary.

Part reduction need not be limited to end-item design.Life cycle cost can be reduced by critically examining theneed for every piece of support equipment—stands, tools,testers, and transport devices. this is especially applicableto the apparent need for new support equipment whenexist ing s tandard equipment can be made compatiblewith new end-items. also the end-item may be altered,without loss of function, to be compatible with existingsupport equipment.

often benefit from increased numbers of parts. reliabil-ity, safety, and survivability specialists often promoteredundancy of parts to avoid mission aborts, accidents,and combat loss. the maintainability specialist mightalso want to add parts to increase testability and self-healing aspects of the end-item. thus, the important con-sideration in part reduction is that it must involve com-bined efforts and trade-offs which optimize systemcost-effectiveness.

Part reduction often can be achieved by examininginterfaces between different work assignments. Forexample, the fuel system designers for the end-item andthe engine suppliers may both have supplied check valvesto perform an identical function. In special weapon adap-tion kits, test interfaces should be examined. If electricalfuses are required to protect the safing and arming circuitsfrom excessive currents or voltage accidentally imposed

2-2.4 CONSOLIDATION OF FUNCTIONS

Consolidation of functions is probably the most impor-tant design technique for simplification. In the abstractthis can be illustrated by the simple example of multiply-ing a series of numbers by a common factor and summingthe result—i.e., multiply the elements b, c, d, and e by a

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common factor a and sum the result. This operation canbe performed by

The end results are identical; however, the first operationrequired four multiplications and one addition. and thesecond operation required only one multiplication andone addition. An analysis of the task to be performedindicates the procedure to be adopted to simplify theoperation. An analysis of the various functions to beperformed by components of weapon systems and thetypes of available hardware to do the required task oftencan lead to overall simplification. An operational orhardware approach can be employed to achieve simplifica-tion.

In the operational approach, hardware performingsimilarfunctions could be conveniently grouped to facili-tate operator performance. For example, if a systemrequired a number of readouts to determine its operabil-ity, the readouts should be grouped onto a single panel tofacilitate observation. Also any adjustment required tobring a meter reading into line should be easily observableby the operator making the adjustment.

In the hardware approach, multiple functions can beincorporated into a single item of hardware or controlledby or from a centralized location or operaition. For exam-ple, turning on the power switch of a personalized compu-ter not only provides power but also activates the “power-on self-test” ( POST) that initiates software programswhich check for the presence of peripherals, clear statusflags so that the computer can be set up according to aspecified use condition. check the presence and conditionof hard disks and other subsystems, and determine theamount of access memory available and test to insure nofailures.

A good example of the consolidation of relatedmechanical or electrical functions into a single activatoror control—lever, switch, component- to produce sim-plification is the automobile, with which everyone isfamiliar, i.e.,

1. Control of windshield wiper and washer by asingle lever

2. Consolidation of ignition and starter switch into asingle assembly

3. Use of braking systems that feature a self-adjust-ment capability

4. Use of engine assemblies that feature a valve lashcompensator

5. Use of a notched serpentine belt. with powertakeoff from crankshaft, to wrap around pulleys for everypower accessory i.e.. one pulley to perform manyfunctions.

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Logic must prevail in the practice of consolidation offunctions because, carried to the extreme, it could resultin a more complex system.

2-2.5 IMPROVED ACCESS TO PARTSAccessibility is defined as a design feature that affects

the ease of admission to an area for the performance ofvisual and manipulative actions. Thus, as a prime factorin relation to maintainability, accessibility relates to theconfiguration of hardware rather than to the physical andother limitations of personnel. Considerations of accessi-bility are, and in their turn exert influence on, virtually allother maintainability factors. For simplicity in mainte-nance, accessibility must satisfy two needs, i.e.,

1. Access to an item for inspection and testing, pro-viding ample room to attach test equipment

2. Space in which to adjust, repair, or replace thefailed item.

Configuration, or packaging, enhances accessibility byplacing items that are expected to require maintenancemost frequently where they will be most accessible. If thefailure rate for a particular item is high, accessibility ismandatory.

A detailed discussion of accessibility and methods ofachieving it are presented in Chapter 4.

2-2.6 STREAMLINED MAINTENANCEPROCEDURES

The ultimate goals of maintainability design are reduc-tion to a minimum of the system support requirementsafter the system has been released to the user and thefacilitation of whatever maintenance work the system willrequire. The integrated logistical support plan, initiatedwhen the performance requirements of the system werebeing formulated, dictates how the system is to be main-tained, e.g.,

1. Malfunction isolation shall be positive to thethrowaway level and shall not require human decision orreference to manuals.

2. All throwaway items shall be replaceable within“x” minutes.

3. No maintenance adjustments shall be required.The task of maintenance simplification does not end

with the establishment of the support plan. Derivativeactions, which may be regarded as a subset of the basicplan, can be taken by both the designer and user to insurethe success of the support plan, e.g.,

1. Part configuration2. Maintenance scheduling3. Simplified diagnostic techniques.

2-2.6.1 Part ConfigurationMaintainability simplification can be compared to a

successful “do-it-yourself’’ kit. A good design is so simpli-

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fied that a diagram of the item immediately suggests themethod of assembly or disassembly; however, until atechnician is thoroughly familiar with the operation, heshould be encouraged to study the documented procedurethe “read instructions when everything else fails” is adangerous philosophy. In a design of this nature, assem-bly is obvious by means of component parts that fittogether in a unique manner because of their externalconfiguration. Also since familiarity and use of a com-mon part are important considerations in part design andselection, maximum effort should be made to promotethe use of standard parts and components.

The assembly of parts can be simplified by part designor by support equipment designed to position parts sothat they cannot be incorrectly aligned or mounted. Acommon method is to provide locating holes for the inser-tion of dowel pins to position parts in the correct orienta-tion for mounting or for ease of insertion. Supportequipment, by proper design, can permit ease of position-ing for installation of attachments—e.g., a carrier for aweapon pod could be designed with a wing-to-carrierguidebar that could align the pod for quick attachment.

Removable fasteners, e.g., screws and bolts, should beof the same size wherever possible and of a standard size.and only in the number required to secure an item prop-erly. This simple design feature will reduce the partsinventory and lessen the requirement for special tools.

2-2.6.2 Scheduling of MaintenanceScheduled and deferrable maintenance functions can

often be performed during the same time period, whichresults in reduced equipment downtime. i.e., increasedavailability. For example, a modification work order(MWO) that does not affect system safety or functioningcan be deferred until the equipment undergoes routinescheduled maintenance, or a clutch adjustment could bedeferred until an oil change was required. Also the neces-sary special tools, repair parts, and manuals can beassembled in preparation for the deferred maintenance;this will reduce administrative downtime.

2-2.6.3 Simplified Diagnostic TechniquesBecause the preponderance of repair time required for

any item, subsystem, or system normally is attributable tofault isolation, it is imperative that provision be made forthe most effective diagnostic (troubleshooting) routines.Manual techniques—basically trial-and-error efforts byskilled technicians using meters, oscilloscopes, and othertest devices, as well as detailed schematics, to isolate amalfunctioning component by progressively eliminatingthose that are still functioning—should be eliminated atall levels of maintenance except depot. Semiautomaticand/or automatic techniques must be prescribed for unitlevel maintenance because unit maintenance is structuredfor quick turnaround based on repair by replacement and

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minor repair—check, adjust, clean, lubricate, and tightenaddition of fuel, and maintenance of liquid levels. Testequipment, if neccssary at the unit level, should be of the"go" “no go” type, which requires no interpretation ofsignal data. The vacuum tube testers, located in drugstores before the introduction of solid-state electronics.are examples.

2-2.7 SOFTWARE MAINTENANCEEase of maintenance usually is associated with good

documentation, a small number of statements per pro-gram, and the level of language used to write the pro-grams. Programs should be prepared in high level (com-piler) language, whereby a single statement translatesseveral into many machine-language instructions. Forexample, consider three variables, A, B, and C related bythe statement

VARA = VARB + VARCwhich translates into

1. Load VARB2. Add VARC3. Store VARA.

In most languages each variable must be uniquely declared(defined) and typed integer, character, logic, number ofbytes, and manner in which stored prior to use in anyprogramming instruction (module). Examples of higherlevel languages are ADA, PASCAL, and C. ADA is thepreferred language for Army software and, therefore,should be used.

Block structuring a technique allowing programsegmentation into blocks of information or subroutinesof a total program should be used because when prop-erly applied in a top-down arrangement, they are essen-tially self-documenting. This technique permits the pro-gram to be read and understood by someone who did notwrite the program; thus it lends itself to being easilychanged or maintained. In applying this technique.backward referencing (looping) should be avoided.

2-3 EXAMPLES OF COMPLEXMODULES REDESIGNEDFOR SIMPLICITY

This paragraph describes five complex hardware andsoftware modules that have been redesigned by applying arange of simplification principles.

2-3.1 UH-60A HELICOPTERThe UH-60A BLACK HAWK utility helicopter re-

quires only 2.5 to 3 man-hours of maintenance per flighthour at the unit level (Ref. 1). This compare to 12 to 20 hfor the family of helicopters it replaces. This increase inmaintainability can be attributed largely to simplificationof the main rotor head design. Unlike previous designs.which had upper and lower plates tied together withvertical and horiziontal hinges, the BLACK HAWK main

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rotor head is forged from a single piece of titanium. anelastomeric bearing, which is completey dry and requiresno lubrication and is said to have an average life of 3000 h,eliminates the need for hinges. In earlier designs, thehydraulic reservoir seals have been known to fail andcause loss of lubrication and wear to the main rotor head,which necessitates overhaul. Typically, mean time betweenremoval (MTBR) for older designs of such componentswas less than 500 h. Simplification has resulted indynamic component MTBR requirements of at least 1500h.

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automatic, externally powered gun, M230, has only 149parts, including barrel, drive motor, feed string, andrecoil adapters. This compares favorably to 300 parts inthe Model 188 Gattling gun. Fig. 2-1 contrasts a Gattlinggun design with that of the M230. Fig. 2-2 illustrates thesell-powered feed design that makes part reductionpossible.

Further, the new 30-mm gun has been designed forquick disassembly without the need for supplementarytools or equipment. The tools required for disassemble}are provided with the gun mechanism. Disassembly andreassembly are accomplished in three minutes or less. Thegear boxes are sealed for the life of the unit (Ret. 2).2-3.2 155-mm HOWITZER

For many years, the muzzles of the 155-mm guns andhowitzers were proteccted by mechanical muzzle plugs;these plugs consisted of a metal dish that was locked inplace by turning a hand wheel which took up on athreaded link and thus spread a three-legged expandingtoggle inside the gun barrel. This assembly cost $64 andrquired maintenance. A molded neoprene plug thatrequires no maintenance was substituted at a cost of $6.This simplification also has improved safety: forgettingto remove the plug before firing no longer has disastrousresults.

2-3.4 INFRARED SUPPRESSORTo reduce the threat from infrared homing missiles, the

gas turbines used in tanks and helicopters must be cooled.This cooling was achieved with large fans which weredifficult to remove and replace. A new approach using the“ocarina” IR suppressor named after a simple musicalinstrument breaks up the hot gas plume and is said todraw in more cool air than the engine is generating as hotexhaust. There are no moving parts in this simplifieddesign (Ref. 3).

2-3.3 M230 GUNIt is imperative that any gun used in a combat vehicle be

easily maintainable. the current trend is toward exter-nally powered guns, which based upon their design, haslead to a reduction in replaceable parts. this fact trans-lates into ease of maintenance. for example, the 30-mm

2-4 SIMPLIFICATION CHECKLISTTable 2-1 should be used in evaluating the design of an

item for simplification. If an answer is “no” for any ques-tion on the checklist, the design should be restudied todetermine whether correction is required.

Figure 2-1. Comparison of the M188 20-mm Gattling Gun to the M230 30-mm Gun

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Figure 2-2. Simple Self-Powered Feed System in M230 Gun

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1.2.3.4.5.

6.7.8.9.

10.11.12.13.

14.15.16.17.

18.

19.

20.21.22.23.24.25.

26.27.

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TABLE 2-1. SIMPLIFICATION CHECKLIST

Has the system been searched for simplified alternatives?Could this function be performed by a standard or existing part?Is this function duplicated on both sides of a work assignment interface?*Could the use of a manifold or multiplexing (electrical) reduce the number of parts required?Can the manual data be understood by an average person with a junior high school (seventh or eighth grademaximum) education?Is the computer software well documented prior to use?Has simplification brainstorming been attempted?Can functions be consolidated in terms of time, place, people, or instruments?Does the design minimize system components while considering requirements for redundancy?Is this function or part really necessary?Is this item simple to maintain but difficult to operate, i.e., has the quest for simplicity been counterproductive?Are requirements for lubrication minimized? When lubrication is required, is there easy access?Have oil sight gages been positioned to be directly visible to the service crew without the use of special stands orequipment’? Is there easy access for adding liquids to maintain their required level? Are see-through containersused for visibility of levels?Are all wrenching or adjustment locations visible in prevailing light?Are all wrenching functions designed for the same size wrench? Same torque values?Has the number of attachments been minimized?Has each requirement for a tool or GSE been analyzed to determine whether the need can be eliminated or thetool made common with those already used?Have quick disconnects been provided for hydraulic, fuel, oil, and pneumatic line couplings for all componentssubject to time replacement or minimum service life and for all modular components?Does the design avoid the use of parts or materials that are known to have caused reliability and maintainability(R&M) problems in earlier designs?Does the design provide components that require little or no preventive or scheduled maintenance actions?Does the design allow for logical and sequential function and task allocations?Have circuits been avoided which require a high degree of voltage regulation?Can adjustable circuits be further reduced?*Are mechanical adjustments held to a minimum?Are the attaching parts for doors and items all the same size in each application? Are any differences of sizenecessary?Are diagnostic techniques simplified?Has human factors engineering been considered in design of equipment?

*A yes answer to these questions indicates a need to reexamine the design for correction.

REFERENCES

1. Ramon Lopez, “UH-60A BLACK HAWK in Pro- International Defense Review, February 1977.duction”, Interavia, November 1978. 3. J. Philip Geddes, “Hughes Helicopters AH-64 in

2. J. Philip Geddes, “Introducing the CHAIN GUN, Advanced Development”, Interavia, August 1978.

BIBLIOGRAPHY

MIL-STD-2165, Testability program for Electronic sys-tems and Equipment, 26 January 1985.

MIL-STD-1472C, Human Engineering Design Criteriafor Military Systems, Equipment and Facilities, 1 0May 1984.

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CHAPTER 3STANDARDIZATION AND INTERCHANGEABILITY

Standardization and nterterchangeability are defined; their close relationship to each other and their influenceon maintainability are discussed. Design goals, principles, and advantages are presented. Specific designapplications and examples of the benefits accruing from standardization and interchangeability are given forseveral commodity areas. Sources of information relative to the Army supply system are detailed. Checklistsfor standardization and interchangeability are included.

3-0 LIST OF SYMBOLS 3-1 INTRODUCTIONContributing maintainability factors are those—other

than the prime factors of diagnostics, accessibility,throwaway modular design. etc. that have a significanteffect on the maintainability of a system. Some of thecontributing factors are prerequisite to consideration ofthe prime factors, some directly affect system maintain-ability without influencing any of the prime factors, a fewaffect system maintainability indirectly, and some havean across-the-board effect. Standardization and inter-changeability are two important contributing factors inthe across-the-board category.

3-2 STANDARDIZATIONThe chapter title, “Standardization and Interchangea-

bility”, is used to emphasize the interrelationship betweenthe two factors. Interchangeability as a maintainabilitydesign factor is closely related to standardization in that itis one of the principal means by which standardization isrealized. Good examples of the close standardization in-terchangeability relationship are the standard size basefor incandescent lamps and the standard size male plug ofelectrical appliances. If these sizes were a function of theirsource of design or manufacture, a chaotic conditionwould exist. A worst case example is the infinity ofshapes, sizes, and materials of construction of washers forhousehold water faucets. Interchangeability is recognizedas a design factor in its own right and is discussed in par.3-3.

The definition of standardization includes more thanparts—i.e., based on the glossary definition, engineeringterms, principles, practices, materials, processes, etc., areincluded. For this discussion, however, standardizationwill be limited to parts, i.e., physical items. This leads tothe definition “Standardization is a design feature forrestricting to a minimum the feasible variety of partswhich will meet the hardware requirement within theArmy and Department of Defense (DoD) inventories.”.Thus standardization encourages the use of commonitems—e.g., the Army uses common items such as Army-Navy (AN) and Military Standard (MS) couplings, truck

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tire sizes, and common materials for uniforms arid canvascoverings.

Standardization is mandated by DoD Directive No.4120.3 (Ref. 1) and implemented by DoD Intruction No.4120.19(Ref. 2) (see par. 3-5); however, the main impetusderives from the willingness of engineers and programmanagers to use existing equipment to satisfy their par-ticular need.

3-2.1 DESIGN GOALS AND PRINCIPLESStandardization was previously defined as a design

feature for restricting to a minimum the variety of partsthat will meet the majority of the hardware requirementsof the system. According), it is important that maintain-ability engineers strive for the design of assemblies andcomponents for a given system that are physically andfunctionally interchangeable with other like assembliesand components of the system. The importance of stan-dardization, a major consideration of maintainabilitydesign, translates into achieving the following primarygoals:

1. Minimizing both the acquisition and supportcosts of a system

2. Increasing the availability of mission-essentialitems

3. Reducing training requirements both in numberof personnel and the level of skill required

4. Reducing inventories of repair parts and theirassociated documentation.

In any attempt to achieve standardization, the follow-ing principles must be carefully considered:

1. Make maximum use of all common parts andassemblies.

2. Reduce to a minimum the variety of assembliesand parts required, and, in doing so, make certain that thebasic types are

a. Used consistently for each applicationb. Compatible with existing uses and practices.

3. Reduce to a minimum, by careful study of thesimplification thus attained. the problems of supply, stor-age, and stocking.

4. Simplify practices, by the same means. in thecoding and numbering of parts.

5. Make maximum use of “off-the-shelf” compo-nents, tools, and test equipment.

3-3.1 GENERAL

3-2.2 ADVANTAGESWhen standardization is carried to the practical maxi-

mum in system design. certain major advantages aregained by the support activities required for the com-pleted system, i.e.,

1. Both the types and quantities of repair parts nor-mally are reduced and required parts are usually at avail-able; this reduces overall support costs.

3-3 INTERCHANGEABILITY

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gram within the North Atlantic Treaty Organization(NATO), which is discussed in par. 3-2.2. The US Alliesusually employ the International System of Units (SI),i.e., metric units. Even though DoD Directive No.4120.18 (Ref. 3) established a policy to consider the use ofthe metric system in all of its activities consistent withoperational, economical, technical, and safety require-ments. materiel fielded prior to 1976 is still in the US andNATO inventories and new equipment being fielded isstill not metric. Ref. 4 presents a thorough discussion ofthe SI system and contains a comprehensive list of con-version factors for transferring from one system to theother. Software programs are available to facilitate theconversion.

3-3.2 RATIONALIZATION, STANDARDIZA-TION, AND INTEROPERABILITY (RSI)PROGRAM

The RSI Program was established by DoD DirectiveNo. 2010.6 (Ref. 5). The overall objective of the programwas to achieve improved NATO operational effectivenessand combat capability by means of

1. Rationalization. Any action that increases theeffectiveness of NATO forces through more efficient oreffective use of defense resources committed to NATO,including both codevelopment and coproduction of NATOStandard Weapon systems

2. Standardization. The process by which nationsachieve the closest practicable cooperation among forces;the most efficient use of research, development, and pro-duction resources: and agreement to adopt on thebroadest possible base the use of common or compati-ble procedures. equipment, and tactical doctrine

3. Interoperability. The ability of systems, units, orforces to provide services to, and accept services from,other systems. units, or forces and to use the services soexchanged to enable them to operate effectively together.

Standardization if and when ever attained wouldensure a major increase in the operational capability ofNATO forces. Total standardization within NATO wouldovercome the present advantage of the Warsaw Pactforces in that their weapons and support systems areessentially standard throughout the Pact. The forces ofthe Pact countries currently possess a high degree ofstandardization because of the unique situation that lim-its their source of’ weapons, ammunition, and follow-onsupplies.

It is interesting to note that the National CodificationBureau Code (NCBC) identifies 24 allied country codesstored in the Defense Logistics Services Center (DLSC)(Ref. 6); this fact indicates that standardization is emerg-ing even though it may be limited to such items as tiresand storage batteries. The national stock number, ofwhich NCBC and DLSC are an integral part, is discussedin par. 3-5.

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3-3.3 DESIGN PRINCIPLESTo attain rnaximum interchangeability, the following

design principles apply:1. Functional interchangeability of’ parts and units

should exist wherever physical interchangeability existsto avoid a dangerous situation.

2. Physical interchangeability should not exist when-ever functional interchangeability is not intended.

3. Whenever complete functional and physical—interchangeability is impractical, the parts and unitsshould be designed for functional interchangeability, andadapters provided to make physical interchange possible.

4. To remove latent doubts, sufficient informationshould be provided undocumented instructions and iden-tification plates to enable the technician to decide posi-tively whether or not two similar parts or units are actu-ally interchangeable.

5. Differences should be avoided in the shape, size,mounting, and other physical characteristics of function-ally interchangeable units.

6. Modifications of parts and units should notchange the manner of mounting, connecting, and other-wise incorporating them into an assembly or system.

7. Complete interchangeability should be providedfor all parts and units that

a. Are intended to be identicalb. Are identified as being identicalc. Have the same manufacturer’s part number or

other identificationd. Have the same function in different applications—

this is especially important for parts and units that have ahigh failure rate.

8. Parts, fasteners and connectors, lines and cables,etc., should be standardized throughout a system, partic-ularly from unit to unit within the system.

9. Mounting holes and brackets should be made toaccommodate parts and units made by different facilities—i.e., make them universally interchangeable.

3-3.4 ADVANTAGESThe advantages to be gained from effective interchange-

ability are essentially the same as those gained by stan-dardization (see par. 3-2.2). In addition, the provision ofinterchangeability is essential to effective standardiza-tion. The greatest advantage is gained when parts are bothstandardized and interchangeable.

3-4 DESIGN APPLICATIONSAreas that are well suited to standardization and inter-

changeability include operating and physical characteris-tics, as well as equipment types. Standard parts, compo-nents, circuits, methods, and practices should be used inthe following applications:

1. Voltage and/or current levels, input or output,

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for circuits2.3.4.5.6.7.8.9.

ing oils,10.

in many

Values of regulators and supply voltagesArrangement and packaging schemesPart identificationLabeling and markingWire identification and codingSelection and mounting of covers and casesSelection and application of fastenersMaterials for servicing—e.g., greases. lubricat-hydraulic fluids, and fuelsSelection of items that have an identical purposeapplications—e.g., starting motors: alternators;

air, oil, and fuel filters; batteries; tires; radiators; drivebelts; and instrument lights

11. Selection of standard sizes and gages of materials12. Design of units that are symmetrical relative to a

centerline to eliminate the necessity for right- and left-handed parts—e.g., a split console cover.

A well-planned standardization and interchangeabilitystrategy will

1. Reduce requirements for special and close toler-ance parts

2. Save manufacturing cost, and maintenance timeand cost

3. Result in more uniformity and predictable reli-ability and maintainability

4. Minimize the danger of misapplication of partsand assemblies

5. Prevent accidents that may arise from improperor confused procedures

6. Reduce errors in wiring, installation, and replace-ment due to the consistency in the physical layout andconfiguration of similar equipment

7. Provide for the testing of many items with a min-imum of standard test equipment.

In addition, the problems associated with procuringand maintaining adequate inventories of parts will bereduced, the required documentation associated withincreased part lists will be lessened. and the number andexpertise of technicians will be reduced.

3-5 EXAMPLES OF BENEFITS OFSTANDARDIZATION/INTERCHANGEABILITY

Three examples are presented, i.e.,1. An increased confidence level -the result of using

a standard item -in performing a maintenance actionwithin a specified time. (See Example 3-1.)

2. Use of an item with an excellent track record forreliability which has demonstrated that the original esti-mated mean time between failures can be increased andthat the original estimated mean time to repair can bedecreased. (See Example 3-2.)

3. Improved logistic support through interchange-ability. (See Examples 3-3 and 3-4.)

3-4

Example 3-1;Maintenance is usually illustrated by an inherent repair

timed distribution (see par. 1-3.1). As indicated in Fig. 1-l.a more skilled and disciplined repair crew can perform arepair action in a shorter mean time and with a reducedvariance. This suggests that repair actions of a repetitivetype on items subject to rapid deterioration. e.g., connec-tors and seals, can be accelerated by the use of standardrepair parts. The reduced variance is a logical expectationbecause familiarity with an operation should lead to con-sistency, i.e., a “tighter” distribution curve. Fig. 3-1(Ref. 7) illustrates quantitatively the effect of a reductionin variance in time distributions resulting from the use ofa standard set of items in a repair operation where themean of every distribution was reduced to unity. Here theinteger shape parameter k for the distribution of serviceitems is defined as

To illustrate the relationship between k and S

2 assutnck = 4; therefore, from Eq. 3-2,

or the standard deviation is

i.e., a mean repair time equal to twice the standard devia-tion. With a k = 20, the mean repair time is 4.5 times thestandard deviation, i.e., a “tighter” distributition curve.

The definitions of other parameters to interpret Fig. 3-1follow:

S(t) = probability of ot completions of repair operationswithin time t, dimensionless

TS = mean duration of repair operations, hµ = 1 /TS, mean service rate, h-1

.

Assume a fixed interval of µ TS, = 0.7 units is available forrepair, then the points at which the dotted line in Fig. 3-1intercepts the A-distributions indicate the probability ofcpmpletions of repair actions e.g., for k = 2, S(t) = 0.5;for k = 20, S(t) = 0.95.

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Figure 3-1. Typical Maintenance Distributions (Ref. 7)

Eample 3-2:A previously used component should be well advanced

in both the reliability and maintenance learning curves.Therefore, an item with demonstrated improved reliabil-ity and reduced maintenance should be considered wherepossible. To illustrate this point, consider the T700 gasturbine that was developed for the BLACK HAWK Util-ity Helicopter program. Because of the excellent in-service performance of the T700 turbine, it was selected topower the Advanced Attack Helicopter (AAH) (Ref. 8).Assume (1) a geometric distribution for the learningcurves for both mean time between maintenance actions(MTBM) and mean time to repair (MTTR) with a con-servative MTBM slope of 0.3 and an MTTR slope of–0.3, and (2) 100,000 h of learning have been accumu-lated with the T700 turbine when the AAH is introducedinto the field. With experience, the MTBM shouldincrease due to improved reliability resulting from im-proved manufacturing techniques and changes in design,and the MTTR should decrease due to a greater familiar-ity with the maintenance process. The improved (MTBM)'and (MTTR)' can be calculated by

(MTBM)’ = A(MTBM), h (3-3)

(MTTR)’ = B(MTTR), h (3-4)

where(MTBM)’ =

MTBM =

projected mean time between mainte-nance actions at 100,000 h, hobserved mean time between main-tenance at 1000 h, h

(MTTR)’ = projected mean time to repair at100,000 h, h

MTTR = observed mean time to repair at 1000h, h

A = ( 100,000/ 1000)0.3 = 3.98B = (100,000, 1000)-0.3 = 0.251.

Given MTBM = 0.8 h and MTTR = 3.6 h, the projectedtimes can be calculated by Eqs. 3-3 and 3-4, respectively,

(MTBM)’ = 3.98(0.8) = 3.2 h(MTTR)’ = 0.251(3.6) = 0.9 h.

As expected, the (MTBM)’ increased and the (MTTR)’decreased, i.e.,

1000 h 100,000 hmean time between maintenance

actions 0.8 h 3.2 hmean time to repair 3.6 h 0.9 h.

Example 3-3:The logistic support of equipment can be improved

through standardization and interchangeability. Forexample, in the wheeled commodity area, a young com-mander in Vietnam took maximum advantage ‘of theinterchangeability of the 2½- and 5-ton truck engines toreturn equipment to the field in the shortest possible time(Ref. 9). The calculations that follow illustrate this case.

Assume that different engines are used for differenttruck sizes and that a time T of 60 days is required toreceive a new stock of engines after placing an order. Also

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assume the following data for this example:

The probability PNORS of not operational ready, supply, isgiven by (Ref. 7)

By use of Eq. 3-5 and the given data. the (PNORS), are

From the data it is obvious that the use of standardengines improves operational readiness.

Example 3-4:In the small arms commodity area, much of the NATO

weaponry has been standardized at 7.62 mm (for person-nel, combat vehicles, and aircraft). This type of standardi-zation increases the availability of small arms ammuni-tion. Since the number of small arms rounds expended isextremely large, a normal distribution can be assumedand used to evaluate the benefits of a standard ammuni-tion size. Assume that the supply of 7.62-mm ammunitionaveraged 100,000 rounds per day with a 30,000-roundstandard error and demand averaged 80,000 rounds perday with a 20,000-round standard error. A standard nor-mal test statistic Zn can be determined by (Ref. 10)

By referring to a Normal Probability Table withZn = 0.55. the probability that demand exceeds supply is29%.

Now. assume that both the supply and demand double,and for the sake of simplicity, also assume that the stan-dard errors double. Then Zn, becomes

and the probability that demand exceeds supply isreduced to 22%. This illustrates how a large inventory ofcommon items will reduce the probability supply willexceed demand.

3-6 SOURCES OF INFORMATIONThe reluctance exhibited by the designer to use existing

parts and standard items is not the result of indifference tothe program; rather, it can be attributed to ignoranceabout the sources of information. Consequently, variousinformation sources that support standardization andinterchangeability are discussed, i.e..

I. DoD Instruction 4120.19, Department of De-fense Parts Control Program

2. A R 700-60. Department of Defense Parts Con-trol Program

3. MIL-STD-965. Parts Control Program4. MIL-STD-143, Standards and specifications,

Order of Precedence for the Selection of5. Department of Defense Index of Specifications

and Standards (DODISS)6. Qualified Products List7. Army Master Data File8. Identification List9. National Stock Number

10. Test, Measurement, and Diagnastic Equipment(TMDE).

Each of these information sources is discussed in theparagraphs that follow.

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3-6.1 DoD INSTRUCTION NO. 4120.19,DEPARTMENT OF DEFENSE PARTSCONTROL PROGRAM

This instruction assigns “responsibility for implement-ing the DOD Parts Control Program as an integral part ofthe acquisition process for the support of systems, subsys-tems, and equipment by the DOD” (Ref. 2).

The objective of this instruction is to “conserve re-sources and to reduce life cycle cost by reducing thevarieties of component parts; promoting the applicationof established or multiuse items of known performanceduring the design, development, production or modifica-tion of equipments and weapon systems; and applyingtechniques to assist system or equipment acquisitionmanagers and their contractors in the identification andselection of established or multi use parts to enhance inter-or intradepartmental system commonality, interchange-ability, reliability, maintainability, standardizatition, andinteroperability” (Ref. 2)

3-6.2 AR 700-60, DEPARTMENT OFDEFENSE PARTS CONTROLPROGRAM

This Army regulation provides for the implementationof DoD Instruction No. 4120.19 and sets forth the Armypolicy and establishes responsibilities for Army participa-tion in the mandatory DoD Parts Control Program(PCP) and for the use of advisory support services ofMilitary Parts Control Advisory Groups (MPCAGs)(Ref. 1l).

The function of the MPCAGs is “to provide ArmyElements and their assigned contractors engineeringadvice and recommendations for assigned Federal SupplyClasses (FSC) on the selection and use of parts during thedesign, development, production, and modification ofsystems, subsystems, and equipment. Decision authority

DOD-HDBK-791(AM)

for the selection and use of parts rests with the Armyelements responsible for the acquisition and support ofthe system, subsystem, or equipment” (Ref. 11).

3-6.3 MIL-STD-965, PARTS CONTROLPROGRAM

Since this is the first time the term “MilitaryStandard” has been introduced, it may be expedient todefine the term to distinguish it from “Military Specifica-tion”. A military standard is a document that establishesengineering and technical requirements for items, equip-ments, processes, procedures, practices, and methods thathave been adopted as standard. Standards may alsoestablish requirements for selection, application anddesign criteria for materiel (Ref. 12). In contrast, a mil-itary specification for the same item describes it in termsof the requirement for procurement.

MIL-STD-965 (Ref. 13) establishes guidelines andrequirements for the implementation of a parts controlprogram, and it is applicable to both new designs ormodifications. The standard emphasizes that the contrac-tor must develop a proposed Program Parts SelectionList (PPSL) in which the number of different part types isminimized and the use of standard parts is maximized.When standard parts cannot be selected, nonstandardparts are to be selected from documents in accordancewith the order of precedence of MIL-STD-143 (Ref. 14).An example of the selection process is shown in Fig. 3-2.

MIL-STD-965 (Ref. 13) describes the function of the1. Parts Control Board (PCB), a formal organiza-

tion established by the contract to assist the prime con-tractor in controlling the selection and documentation ofparts used in equipment, systems, or subsystems.

2. Military Parts Control Advisory Group (MPCAG),a DoD organization that provides advice to the militarydepartments and military contractors on the selection of

Figure 3-2. Example for Selection of Parts for Program Parts Selection List (PPSL) (Ref. 13)

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parts in assigned commodity classes and collects data onnonstandard parts for developing or updating militaryspecifications and standards.

3-6.4 MIL-STD-143, STANDARDS ANDSPECIFICATIONS, ORDER OFPRECEDENCE FOR THE SELECTIONOF

MIL-STD-143 (Ref. 14) establishes the order of prece-dence for the selection of standards and specifications toidentify and describe items, materials. and processes usedby design activities in the design and production of mate-riel. The standard indicates that design and applicationconsiderations- i.e., function, environment. reliability,maintainability, strength, safety, and interchangeabilityas well as economic factors shall govern the selection anduse of materials.

3-6.5 DEPARTMENT OF DEFENSE INDEXOF SPECIFICATIONS ANDSTANDARDS (DODISS)

The DODISS (Ref. 15) is an indispensable publicationin the pursuit of implementing a standardization pro-gram. The DODISS is issued bimonthly—in hard copyand in microfiche by alphabetical and numerical listing.The publication lists the unclassified Federal, Military.and Departmental specifications, standards. and relatedstandardization documents and those Industry docu-ments that have been coordinated for DoD use, i.e.,

1. Military Specifications2. Military Standards3. Federal Specifications and Commercial Item

Descriptions4.5.6.7.8.9.

10.Il.12.13.14.

The1.

105)

2.

Federal StandardsFederal Information Processing StandardsQualified Products ListIndustry DocumentsInternational Standardization DocumentsMilitary HandbooksAir Force-Navy Aeronautical StandardsAir Force-Navy Aeronautical Design StandardsOther Departmental DocumentsMilitary/Air Force-Navy AERO BulletinsAir Force Specification Bulletins.

DODISS can be obtained as follows:For Military Activities:

Commanding OfficerNaval Publications and Forms Center (NPFC

5801 Tabor AvenuePhiladelphia, PA 19120-5099For Government Civil Agencies and Non-Govern-

ment Activities (subscription basis only):Superintendent of DocumentsUS Government Printing OfficeWashington, DC 20402.

3-6.6 QUALIFIED PRODUCTS LIST (QPL)The Qualified Products List (QPL) is a list of products

that have met the qualification requirements stated in theapplicable specification. including appropriate productidentification and test or qualification reference with thename and plant address of the manufacturer and distribu-tor, as applicable )Ref. 16). To establish a QPL, anapproved and dated military or Federal specificatitionmust exist that sets forth the qualificatition examination,tests, and criteria for retention. Ref. 15 contains a list ofQPLs and standards.

QPLs are identified by the symbol “QPL” followed bythe number of the associated specification and an issuenumber to identify the issue of the QPL, e.g., “QPL-8952-1" identifies the initial issue of the QPL associated withmilitary specification MIL-B-8952. Bearing, Roller, RodEnd, Antifriction Self-Aligning. For Federal specifica-tions, both the specification symbol and number are used,e.g., “QPL-ZZ-T-381-2” identifies the second issue of thelist associated with Federal specification ZZ-T-381. Tire,Pneumatic, Vehicular (Highway). Specification revisionindicators tire not used in the QPL number.

3-6.7 ARMY MASTER DATA FILE (AMDF)The AMDF (Ref. 17) is another indispensaible publica-

tion (microfiche) since it is the official source of currentsupply management data for the items maneged or usedby the Department of the Army (DA). Items are listed byNational Stock Number (NSN); the interpretation of thecodes associated with the listing is found in Ref. 18. Thedata have precedence over conflicting data published inany other DA publication except for items within thepurview of SB 700-20, Army Adopted/Other ItemsSelected for Authorized/List of Reportable Items.

Copies of AMDF can be obtained fromCommanderUSAMC Catalog Data ActivityATTN: AMCA-PP

New Cumberland Army DepotNew Cumberland, PA 17070-5010.

3-6.8 IDENTIFICATION LIST (IL)The IL is arranged by Federal Supply Class (FSC)

groupings (see Ref. 19 for groups and classes) and con-tains descriptions of all active items in the Defense Logis-tics Services Center (DLSC). Data elements reflect relatedcharacteristics and or reference number. The principaluses of the IL are to obtain or verify a National StockNumber (NSN) when only the characteristics of an itemare known, to assist in determining interchangeable andsubstitute items, and to obtain data when the NSN isknown.

The ILs contain proprietary information and are forGovernment use only. Army installations may obtain

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microfiche copies of the ILs by mailing a completed DAForm 12-21 to

CommanderUS Army AG PublicationsATTN: AGDL-ODR1655 Woodson RoadSt. Louis, MO 63114.

3-6.9 NATIONAL STOCK NUMBER (NSN)During World War II the military services' supply sys-

tems were fraught with problems. For example, each ofthe services operated several different supply systems.Within the Army alone there were eight Technical Ser-vices, each operating an independent supply system withdifferent stock numbering methods, different specifica-tion guides for identical items. different supply catalogformats. and different descriptive languages. It is obviousthat the system resulted in waste, inefficiency, and lack ofstandardization.

Te Defense Cataloging and Standardization Act,enacted in 1952, resulted in a Government-wide systemjointly administered by he DoD and the General ServicesAdministration (GSA). The Act required “the use ofcommon supply language for naming, identifying, anddescribing each item repetitively used, purchased, orstocked for distribution by the DoD". The Act alsorequired the development of a unique stock numberingsystem to identify each item, part, assembly, or compo-nent so it could be distinguished from another. The result-ing system, called the Federal catalog system, is a com-plex structure that provides services to users world-wide-military services, other DoD components, NATO, friendlyforeign governments. and private sector contractorsdoing business with the US Government. Thus the Fed-eral catalog system supports logistic functions such asmaintainability concepts, repair part management, andprocurement.

The net result of the Act was the evolution of a nationalstock number (NSN) that identifies an item throughoutits entire life cycle in the Federal catalog system. The NSNis the means of access to the various part lists previously

DOD-HDBK-791(AM)

described. The NSN structure consists of 13 digits (seeFig. 3-3). The first four digits comprise the Federal supplyclassification (FSC) code as listed in Ref. 19; the last ninedigits are the national item identification number (NIIN).The NIIN is subdivided into a 2-digit National Codificat-ion Bureau Code (NCBC) and a 7-digit item identifica-tion number sequentially assigned by the Defense Logis-tics Service Center (DLSC). Presently, there are 24country codes stored in the DLSC data bank (Ref. 6).

AR 708-1 (Ref. 20) directs that items in the Armysupply system will not be ordered by commercial partnumbers, i.e., the numbers that commercial firms use toidentify items of their manufacture. The DLSC has beendirected to assign an NSN to every standard item in theArmy supply system. To ensure that items do not enterthe Army supply system without an NSN, the ArmyMateriel Command (AMC) had the DLSC assign NSNSautomatically to all items authorized for central stock ageat the wholesale level. NSNS are also to be listed in partmanuals, and part number-to-NSN cross-reference chartsare to be issued with commercial manuals before releasingnew equipment to the field (Ref. 21).

For improved standardization and parts reduction, it isobvious that the maintainability engineer must be awareof the NSN and the information that it conveys.

3-6.10 TEST, MEASUREMENT, ANDDIAGNOSTIC EQUIPMENT (TMDE)

3-6.10.1 GeneralThe necessary maintenance operations and characteris-

tics of equipment determine TMDE requirements. Toneglect considering TMDE requirements until equipmentdesign and maintenance procedures are finalized mayresult in the demand for many special tools or an unneces-sarily wide variety of standard tools and test equipment.Designers have a tendency to design equipment withproprietary TM DE. Further. they tend to assume that theTMDE in their design facilities will be available at thepoint of use. In fact, such devices, when provided, per-form poorly in the hands of lower level technicians and

Figure 3-3. NSN Configuration

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constitute an extra maintenance burden on the system. Itis not necessary to specify a $2000 oscilloscope to measurea ± 5V on a digital circuit.

3-6.10.2 AR 750-43, Test, Measurement, andDiagnostic Equipment

To bring order out of chaos, AR 750-43 was published.This Army Regulation (AR) (Ref. 22) prescribes policies,establishes objectives, and priorities, and assigns respon-sibilities for the life cycle management of TMDE. It alignswith directives and instructions developed and publishedby the Maintenance Policy Directorate, office of theAssistant Secretary of Defense for Manpower. ReserveAffairs, and Logistics. Primary DoD publications thatapply to the contained policies are DoD Instruction No.4100.40 and DoD Directive No. 4151.16. AR 750-43 alsonames the Commanding General, US Army MaterielCommand (AMC), as the Department of the Army Exec-utive Director for TMDE (EDT) responsible for develop-ing and recommending TMDE policy to Headquarters.Department of the Army—i.e., the central managementof all TMDE support to ensure peacetime readiness whileproviding a smooth transition to wartime support. Theprocedure by which AMC executes this TMDE responsi-bility is described in par. 3-6.10.3.

3-6.10.3 AMC’s Role as Executive DirectorAMC, through the US Army TMDE Support Group

(USATSG), places the TMDE calibration and repairunder a central manager. The TMDE support missionsand resources of all Army major commands, except theSurgeon General, are under the control of the USATSG.A major benefit of this central management has been theability to compile and assess detailed requirements andperformance data and to apply these data on a timelybasis to manage resources. This has led to optimum use of

3-6.10.4 DA TMDE Preferred Items List (PIL_

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Figure 3-4.

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3-7 STANDARDIZATION AND INTER- the design of an item. If an answer is "no" for any question

CHANGEABILITY CHECKLISTS on the checklists, the design should be restudied to deter-

Tables 3-1 and 3-2 checklists for standardization andmine whether correction is required.

interchangeability, respectively—are for use in evaluating

1.

2.

3.4.

5.6.7.

8.

9.

1.

2.

3.

4.

5.

6.

7.

8.9.

10.

11.

TABLE 3-1. STANDARDIZATION CHECKLISTHave all sources of standardization information been searched for common items, materials, and practices?

Has each requirement for a tool or item of ground support equipment (GSE) been analyzed to determine whether theneed can be eliminated or the tool made common with those already used?

Do designs and practices make use of the SI of measures where required?Have special manufacturing techniques been avoided or minimized?Are materials, processes. and components covered by Military Specifications’! Is a QPL available?

Can standard circuits be used that will also be compatible with standardized test equipment?Are circuit types kept to a minimum?Are identical parts used wherever possible in similar equipment or in a series of a given type, such as using the samepiston and cylinder for a series of internal combustion engines?Are parts, fasteners, connectors, lines, cables, etc., standardized throughout the system, particularly from unit to unitwithin the system?

TABLE 3-2. INTERCHANGEABILITY CHECKLISTDoes functional interchangeability exist where physical interchangeability is possible?

Does complete interchangeability exist wherever practical?

Is sufficient information provided on identification plates and within technical manuals to enable the user to decidewhether two similar parts are interchangeable?

Are differences in size, shape, and mounting of components encouraged to eliminate the suggestion that parts may befunctionally interchangeable?

Is complete interchangeability provided for all items intended to be identical, interchangeable, or designed to servethe same function in different applications?Do mounting holes and brackets accommodate units of different makes, such as engines of the same type andhorsepower, built by different manufacturers?Are cable harnesses designed so that they can be fabricated in a factory and installed as a unit?

Is complete electrical and mechanical interchangeability provided on all like removable components?

Are bolts, screws, and other features the same size for all covers and cases on a given piece of equipment?Is interchangeability provided for components having a high mortality?Where complete interchangeability is not practical, are parts of units designed for functional interchangeability, andare adapters provided to allow physical interchangeability, wherever practical?

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for the Preparation of Military standards and Mil-1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

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REFERENCES

DoD Directive No. 4120.3, Defense Standardizationand Specification Program, 10 February 1979.DoD Instruction No. 4120.19, Department of DefenseParts Control Program, 27 June 1984.DoD Directive No. 4120.18, Use of Metrit System ofMeasurement, 10 December 1976.

DARCOM-P 706-470, Engineering Design Hand-book, Metric Conversion Guide, 5 July 1976.DoD Directive No 2010.6, Standardization andInteroperability of Weapons Systems and EquipmentWithin the North Atlantic Treaty Organization(NATO), 11 March 1977.James R. Garver, “The Federal Catalog System”.Army Logistical 16, 6 (November-December 1984).Philip M. Morse, Queues, Inventories and Mainte-nance, John Wiley and Sons, Inc., New York, NY,1958.

J. Philip Geddes, “Hughes Helicopters AH-64 inAdvanced Development”, Interavia (August 1978).

Col. George T. Petersen, “Design for Readiness”.Ordnance (September-October 1967).Robert G. D. Steel and lames H. Tori-e, Principlesand Procedures of Statistics, McGraw-Hill BookCompany, New York, NY, 1980, p. 106.AR 700-60, Department of Defense Parts ControlProgram, 5 October 1977.

MIL-STD-962A, Outline of Forms and Instructions

13.

14.

15.

16.

17.

18.19.

20.

21.

22.

23.

24.

itary Handbooks, 26 October 1984.

MIL-STD-965, Parts Control Program. 26 August1983.MIL-STD-143B, Standards and Specifications,Order of Precedence for the Selection of, 1 2November 1969.

DoD Index of Specifications and Standards(DODISS).

DoD Manual No. 4120.3-M, Defense Standardiza-tion Manual, August 1978.

Army Master Data File (AMDF).

CDA Pamphlet No. 18-1, Code Reference Guide.

Cataloging Handbook H2-1, Federal Support Classi-fication, Part 1, Groups and Classes.

AR 708-1, Cataloging and Supply ManagementData, 1 April 1981.

“National Stock Numbers Increase”, Army Logisti-cian 14, 4 (July-August 1982).

AR 750-43, Test, Measurement, and DiagnosticEquipment, 1 March 1984.

US Army Test Program Sets Procedures Manual,AMCPM-TMDE-T, Fort Monmouth, NJ, 31 Janu-ary 1986.DA Pam 700-12-1, DA TMDE Preferred Items List(PIL).

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CHAPTER 4ACCESSIBILITY

Accessibility, as a significant attribute of maintainability, is discussed in detail. Considerations affecting theaccessibility for three types of maintenance—visual inspection, tools and test equipment, and physical repairand replacement-are presented. Access parameters of size, location, and type are addressed. Specificguidance to access components of major classes of materiel is given. A checklist for maintainability engineers isprovided.

4-1 INTRODUCTIONAccessibility is defined as a design feature that affects

the ease of admission to an area for the performance ofvisual and manipulative maintenance. Thus accessibilityrelates to the hardware configuration, i.e., packaging. Ifan item can be reached quickly requiring the use of a fewstandard tools and only a few simple steps the item isaccessible. However, if an item can be reached quickly butrequires many tools, special tools, many or difficult oper-ations to reach it, the item is really nit accessible. Also themere fact that a technician can “get at something” doesnot mean that he can maintain it. If an awkward bodyposition must be assumed to reach the item. requiring therepairman to be a contortionist, the item is not consideredaccessible and maintainable because the repairman maynot be able to exert the forces required to make the repair.Furthermore, the disassembly or removal of parts thatinterfere with easy access to a component requiring main-tenance is highly undesirable especially if the mainte-nance action is required at the unit level adequate spaceusually is not available for laying out parts as they areremoved. This increases the possibility that the parts willbecome lost, damaged, or contaminated with dirt andmud, and that further malfunctions will be introducedinto the system.

Inaccessibility also embodies psychological effects.Controls: checkpoints; inspection windows; and lubrica-tion, pneumatic, and hydraulic replenishment points aredesigned into the equipment to keep it operating at peakperformance. If it is difficult to access these features, theoperator or repairman will postpone or neglect theseoperations in favor of more convenient tasks.

The maintainability engineer, in his desire to maximizeaccessibility, should be aware of at least two limitingconditions, i.e.,

1. Level of Accessibility. Accessibility should beavailable only to the throwaway and discard level. If theitem is to be discarded at the unit level, the task is simple.However. it the replaced item is to be returned for repairat the intermediate level, accessibility poses additionalconsiderations.

2. Safety. Accessibility must be consistent with theassociated system safety plan required by MIL-STD-882(Ref. 1). The purpose of the safety plan is to minimize oreliminate hazards to which the operator or maintenancepersonnel will be exposed. The safety plan also addresseshazards to the system which may be introduced by main-tenance actions. Maintainability design must interfacewith ground operations, weapon loading. on-loading ofmunitions, refueling, and mission function. For reasonsof safety, maintenance personnel from a psychologicalpoint of view—would rather avoid working in areasexposed to live ordnance and easily ignited gaseousvapors. Also requirements for exposing the technician topotential hazards—moving parts. hot components, build-up of electric charge—should be avoided or minimized.Improved safety associated with maintenance operationswill reduce maintenance time and, consequently, increaseavailability.

The variability in the physical size of personnel isdirectly related to accessibility. What is within easy reachof a 6.5-ft individual may be out of reach for a shorterperson. Conversely, the larger individual may experiencedifficulty placing his larger hands around a component,particularly if he is wearing arctic mittens. Whether oflarge or small stature, maintenance personnel must haveaccess to parts and room enough to operate on the parts-–test, adjust, or replace. Accordingly, the qualitativeaspects of human physiology and psychology as theyaffect access times will be addressed. The material in thischapter is closely related to Chapter 9. “Human Factors”,which presents quantitative aspects of anthropometricmeasures, visibility measures, and frequency of mea-surement errors. A mutual relationship exists between themaintainability engineer and the human factors engineer:

1. The maintainability engineer is responsible forquantifying inherent downtime and inherent availability.

2. The human factors engineer provides the quanti-tative information the maintainability engineer needs toinsure that personnel who fall within the 5th and 95thpercentiles can function within the access design.The final design is a result of cost-effective trade-offs

4-1

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among these and other disciplines.The current “remove-and-replace” philosophy is the

preferred policy; it displaces the "repair-in-place" policyfor restoring materiel to a serviceable condition in thecombat zone (Ref. 2). Thus accessibility is essentiallysynonymous with maintainability where remove-and-replace operations are involved. The sum of many “small”times saved due to a simple feature such as accessibilityadds up to large reductions in maintenance time andeffort, and thus increases availability.

In summary, in planning for accessibility, the items thatwill require the most frequent work must be examined todetermine the type of maintenance each will require.Items that have high failure rates or require frequentservice should be given preferential positioning for access.For example, a control assembly might consist of a powersupply module and a control module. If the power supplyis expected to fail more often than the control module andboth modules are to have a common access, the powersupply should be closer to the access. However, if thecontrol module requires frequent adjustment to keep itsperformance up to par, then it should be allotted thepreferred access position. Next, types of access, covers.methods of mounting, and required dimensions aredetermined. Once the types of access have been selected,the designer—working in conjunction with the maintain-ability engineer—must make certain that each access islarge enough either for physical access or the employmentof service or test equipment. This strategy is presented inthe paragraphs that follow.

4-2 FACTORS AFFECTINGACCESSIBILITY

Considerations of accessibility are influenced by, andin turn exert influence on, virtually all other maintainabil-ity factors; thus its unique importance. Gaining access tocomponents is second in importance only to fault isola-tion as a time-consuming maintenance activity; if auto-matic fault-isolation is embedded in the equipment,access unquestionably will rank first.

Accessibility requirements are determined by the main-tenance action required which may be visual or physi-cal, or both, depending on whether the task is inspecting,servicing, adjusting, repairing. or replacing. Generally,the requirements represent two needs. namely,

1. Placement of items requiring frequent mainte-nance attention where they can be easily seen andreached. Visibility is mandatory where a potential hazardexists.

2. Design of openings to permit access to compo-nents and to provide space in which to perform mainte-nance operations.

Guidelines for meeting these requirements are1. Locate assemblies and parts so that structural

units and other parts do not block access to them.

4-3 MAINTENANCE ACCESSThree general types of maintentance actions require

access, namely.1. Inspection either sight or touch2. Testing3. Part adjustment, repair. or replacement.

To perform these maintenance actions, a means ofaccess is required either merely to observe or to “lay one’shands on the unit”. To determine the type, size, shape,and location of the access opening, it is necessary to havea thorough understanding of (Ref. 3)

1. operational location, setting, and location of theunit within the environment

2. Frequency of using the access3. Maintenance tasks performed through the access4. Time required to perform maintenance functions5. Types of tools and accessories required6. Work clearances required7. Types of clothing operator and technician are

likely to wear8. How far into the access the operator or techni-

cian must reach9. Visual requirements of task

10. Mounting of items. units, and elements behindaccess

11. Hazards in using access12. Size, shape, weight, and clearance requirements

for logical combinations of human appendages, tools.units, etc., that must enter the access.

Inspection primarily is performed by sight or touch.For example, a sight gage can be used to assure properfluid levels. Inspections that require feeling for leaks aretactile and require physical rather than visual access.When inspection or testing locates a failed part, the accessmust be large enough to permit component repair orreplacement.

Table 4-1 provides criteria for decisions relative to thetypes of accesses for the three maintenance actions. In theapplication of this table. inspection that requires touchrather than sight appears in the “For Physical Access”column. The recommendations related to accesses andcovers are illustrated by Fig. 4-1 (Ref. 3). A general dis-cussion of accesses for the three types of maintenance

4-2

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Figure 4-1. Types of Covers and Accesses (Ref. 3)

actions is presented in pars. 4-3.1, 4-3.2, and 4-3.3. The wear or the action of solvents and the environment areparameters of access type, size, shape, and location likely to reduce the transparency of the plastic cover. aapplying the given list of considerations—are discussed in break-resistant glass window should be used. If glass willdetail in pars. 4-4 and 4-5.

4-3.1 ACCESS FOR VISUAL INSPECTIONONLY

Visual access should be unblocked unless exposure islikely to degrade equipment or system performance. A

not meet the stress or other requirements, then a quick-opening, metal cover should be used. Fig. 4-1 illustratesvarious cover types.

Visual inspection usually requires a light intensity of 551m/m2 (lux) or greater. Brightness contrast between thetarget object and its background must be considered as

clear, plastic window can be used if dirt, moisture, or well as glare from both the work and light source. If another foreign materials present a problem. If physical external light source is used, the transparency of the

4-3

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TABLE 4-1. RECOMMENDED EQUIPMENT ACCESSES

FOR VISUALDESIRABILITY

FOR TEST ANDFOR PHYSICAL ACCESS INSPECTION ONLY SERVICE EQUIPMENT

Most desirable Pullout shelves or drawers

Desirable Hinged door (if dirt, moisture.or other foreign materialsmust be kept out)

Less desirable

Least desirable

Removable panel with captive,quick-opening fasteners (ifthere is not enough room forhinged door)

Removable panel withsmallest number of largestscrews that will meetrequirements (if needed forstress, pressure, or safetyreasons)

opening with no cover

Scratch-resistant plasticwindow (if dirt, moisture. orother foreign materials mustbe kept out)

Break-resistant glass (if plasticwill not stand up underphysical wear or contact with

window should allow the required amount of light to betransmitted. When accesses are near components that arehazardous, and if the inspection requires access. theaccess door should be designed to turn on an internal lightautomatically when opened. Also, a highly visible warn-ing label identifying the hazard should be posted on theaccess door.

solvents)

Cover plate with smallestnumber of largest screws thatwill meet requirements (ifneeded for stress, pressure, orsafety reasons)

4-3.2 ACCESS FOR TOOLS AND TESTEQUIPMENT

Openings for tools and test equipment should be un-covered whenever feasible. Where dirt, moisture, or otherforeign matter present a problem, a spring-loaded slidingcap or hinged door can be used (Fig. 4-2). Small slidingcaps are particularly useful for small accesses that do notrequire a tight seal. Spring-loaded caps or lids shouldhave a built-in catch to keep the catch or lid open. A coverplate with quick-opening fasteners is preferred if the capor hinged door does not meet the required standards.

Opening with no cover

Spring-loaded sliding cap (itdirt, moisture, or other foreignmaterials must be kept out)

Removable panel with captive,quick-opening fasteners (ifthere is not enough room forhinged door)

Cover plate with smallestnumber of largest screws thatwill meet requirements (ifneeded for stress, pressure, orsafety reasons)

doors and caps should be designed so that they1. Lock positively2. Do not jam or bind3. Are easy to use and require no tools for operation4. Do not interfere with, cause damage to, or present

potentially harmful contact with electric wires, movingparts, or other items of equipment

5. Are visible enough so that they are not inadver-tently left open.

To improve accessibility), the openings should providefor the use of roll-out, rotatable, and slide-out drawers,sheltes, and racks or other hinged or sliding assemblies(Fig. 4-4) to

Figure 4-2. Examples of Spring Loaded Sliding and Hinged Access Doors

4-4

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Figure 4-3. Sliding Access Door With Handle

1. Optimize work space and tool clearance2. Reduce the need for the operator or technician to

handle fragile or sensitive items3. Facilitate the handling and or positioning of

heavy or awkward items4. Facilitate access to items that must frequently be

moved from the installed position for checking, servicing,or repairing.

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4-3.3 OPENINGS FOR PHYSICAL ACCESSAccesses should be sized and shaped to permit easy

passage of components; this feature is closely related tothe arrangement of components within the volume thatthe access is to accommodate (Fig. 4-5) (Ref. 3). If situa-tions and environmental factors dirt, danger from fal-ling objects, climate– preclude an uncovered opening, ahinged door or sliding cover can be used (Figs. 4-1 and4-3); a hinged door is preferred to a cover plate. The hingeshould be supported at the bottom to remain open with-out manual effort. If this is not feasible, then a catch orbracket– preferably automatic—should be used. Underno circumstances should it be necessary to hold a dooropen manually (Fig. 4-6) (Ref. 3).

Figure 4-5. Example of Access to PermitEasy Removal of Component (Ref. 3)

Figure 4-4. Examples of Rollout, Rotatable, and Slide-Out Drawers (Ref. 3)

4-5

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Figure 4-6. Methods of Securing Hinged Covers

4-4 LOCATION OF ACCESSESIt is important to consider tactical scenarios when

locating components and the access to these components.For example, the loading of ordnance and other mission-essential consumables must be rapid, and the readiness ofmission-essential components must be checked beforegoing into action. Accordingly, access for these essentialoperations must be accorded first priority; however, thesesame accesses can often accommodate maintenance opera-tions. Also, existence of an access door through which toapproach an item that may require maintenance is anecessary, but not sufficient, condition to assure ease ofmaintainability—the arrangement of the componentsbehind the opening also is an important consideration(see par. 4-4.1). Thus a mutual relationship exists betweenthe access and the positioning of the components to beaccessed.

Where possible, accesses should be located1. Only on equipment faces that are accessible as

normally installed2. For direct access and maximum convenience for

job procedures, i.e., arrangement3. On the same face of the equipment as related

displays, controls, test points, cables, etc.4. Away from high voltages or dangerous moving

parts. If this is impossible, provide adequate insulation,shielding, barriers, etc., around such parts so personnelwill not be injured. These factors should be considered inthe system safety plan (Ref. 2).

5. To enable heavy items to be pulled out rather thanlifted out

6. To accommodate items requiring frequent adjust-ment or maintenance

7. So that components behind the access will not beexposed to dripping oil or other fluids, or other contami-nants generated by the equipment

8. To avoid frames, bulkheads, brackets, and struc-tural members which will interfere with maintenance andoperations personnel’s reaching components which they

4-6

must maintain, inspect, or operate (Fig. 4-7) (Ref. 3).These general principles are expanded upon in the discus-sion that follows.

Figure 4-7. Avoidance of StructuralMembers (Ref. 3)

4-4.1 COMPONENT ARRANGEMENTThe relative position of units which can be expected

to require maintenance inside a box-like enclosureaffects the amount of time required to perform the main-tenance task. Consider the four possible mounting posi-tions --i.e., top. bottom, side, or back panel:

1. Top Panel. In this position, quick access requiresthat the panel

a. Be hinged in backb. Have sufficient space above the box to swing

the panel open and to manipulate the item after openingthe box (Fig. 4-8 (A)) (Ref. 3).

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4-8. Component Mounting Positions

4 - 7

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2. Bottom Panel. In this position, it is necessary toremove the box to expose the item. To minimize possibledamage to the item within the box (Fig. 4-8( B)) (Ref. 3).

a. The box should be lifted off the item. andenough clearance should exist around the item to preventdamage and avoid requiring extremely fine or carefulpositioning or handling.

b. Handles should be provided if the box is heavy.3. Side Panel. In this position, there must be space

within the box for manipulative tasks (Fig. 4-8(C)) ( Ref.3).

4. Back Panel. In this position, the panel facing theoperator or technician is hinged on the bottom, and theitem is mounted on the back panel (Fig. 4-8(D)) (Ref. 3).In many cases this is the most compact and accessiblearrangement.

Components should be arranged with the followingconsiderations in mind:

1. Sufficient space should be provided to usescrewdrivers, test probes, soldering irons, wrenches.socket sets, and other required tools (Fig. 4-9). Accessshould be such that straight screwdrivers or adjustmenttools, rather than offset tools, can be used.

2. High-failure rate components and all assembliesor parts that will require servicing or replacement shouldbe accessible without removal of other items.

3. Plug-in items should be oriented in one directionto facilitate replacement. Fig. 4-10 illustrates this featurefor vacuum tubes; however, it applies to other compo-nents as well.

4. Resistors, capacitors, and wiring should notinterfere with plug-in part replacements. This considera-tion applies to nonelectrical systems as well.

5. Delicate components should be located wherethey will not be damaged while the unit is being repaired.

6. Fuses should be located so that they can be seenand replaced without removing other parts or sub-assemblies.

7. Tools should not be required for replacing fuses,filters, etc. Access to the fuses, filters, etc., should be witha minimum of time and tools.

Figure 4-9. Clearance for Nuts and Bolts

4-8

Figure 4-10. Common Orientation of TubeSockets to Facilitate Tube Replacement

8. Internal controls such as switches and adjust-ment screws should not be located close to dangerousvoltages or moving parts. Also minimize the likelihood ofinadvertently’ jarring a switch or adjustment screw.

9. Components that retain heat or electrical chargeafter the equipment is turned off should be equipped withbleeder networks or should not be located where opera-tors or technicians are likely to touch them while perform-ing maintenance tasks.

10. Orient components such that, for example, aslipped screwdriver will not puncture a high-pressurehydraulic line.

11. Do not stack units. If stacking is required toconserve space, place the unit requiring the least frequentaccess in the back or on the bottom.

12. Group together components that will be main-tained by the same technician. They should be arranged tominimize movement from one position to another duringsystem checking.

4-4.2 COMPONENT DISPERSALDispersal of components that are expected to require

frequent maintenance can permit various elements of theequipment to be serviced simultaneously and thus improveequipment availability. However, this multiaccessiblefeature requires that the maintainability engineer investi-gate the manning studies performed because it would beridiculous, for example. to provide five access parts forthis purpose if the Table of Organization and Equipmentauthorized only one repairman.

Fig. 4-11 illustrates simultaneous maintenance opera-tions on a modern helicopter. Seven repairmen are servic-ing the aircraft at different work stations. Multiple main-tenance actions are the rule rather than the exception onsome Army weapon systems. Under these conditions,downtime is not the sum of the individal maintenancetimes; rather, it is the longest of the parallel action times,assuming that operations at one station are not dependenton a signal or function from an adjoining work station.

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4-9

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4-5 SIZE OF ACCESSGenerally, one large access is better than two or more

small ones; however, access openings must be sized for theequipment that will be maintained and for the actions thatwill be performed. This requires considerations of sight,type of tool required, light intensity, and required main-tenance operations within the access—i.e., the maintain-ability engineer determines what the operator or techni-cian will have to see and do before deciding on the accesssize. In some instances a small hole that admits a screw-driver or grease gun is sufficient access. In other instancesthe technician may have to get a hand or, perhaps, hiswhole body into a piece of equipment. Also, the frequencywith which the maintenance must be performed is a con-tributing factor. Fig. 4-12 indicates that 70% of the main-tenance actions on electronic equipment are accom-plished by the use of a screwdriver, pliers, wrench, andsoldering iron. Obviously, the access opening for thisequipment should be large enough to permit the entranceand manipulation of these tools.

Access size is also a function of the movementsturning, pulling, pushing, twisting—of the human bodypart or parts once access is gained and a function of thedimensions of the body part or parts that must be ad-mitted through the opening. These two factors are deter-mined by dynamic and static body measurements. Humanfactors analyses also consider anthropometric sizing insystem functions. (See Chapter 9, “Human Factors”.) Thefact that the operator or technician maybe wearing arcticclothing while performing a maintenance action will alsoinfluence access size. Table 4-2 and Figs. 4-13, 4-14, and4-15 provide anthropometric data relative to gainingaccess through ports of various sizes.

The size of access openings is also determined by thesize and shape of parts, components, or assemblies towhich access is desired. This is particularly true if the itemmust be removed and replaced through the openings.Access panels should be sized to avoid opening more thanone panel to gain access to any single item behind thepanel. The panel should be sized and componentslocated—to permit units to be removed along a straight orslightly curved path rather than a less direct path (Fig.4-5). This will facilitate removal and decrease the possibil-ity of damage to fragile components during withdrawal.

4-6 EXAMPLESDesign for accessibility will be discussed for the follow-

ing seven areas:1.2.3.4.5.6.7.

4-10

ElectronicsFire controlMissilesMobile equipment trailersTank-automotive materielArmy marine equipmentArmy aircraft.

4-6.1 ELECTRONICSElectronic and electrical components should be located

for easy access and removal and should be designed ifnot of the throwaway type for case of access of testequipment and/or tools required for repair at the inter-mediate or depot level. Other accessibility decisions areinfluenced by considerations of equipment protection,operator or technician protection, layouts, openings,interference, vision. and handle design.

Fig. 4-16 illustrates a foldout construction layout for-anelectronic chassis. This construction should be used forsubassemblies whenever feasible. l-he parts and wiringshould be positioned to prevent damage when opening orclosing the assembly.

Braces or similar items should be provided to holdhinged assemblies in the “out” position while they arebeing maintained (Fig. 4-17); this arrangement leavesboth hands free to perform required rnaintenance opera-tions. Rests or stands should be provided to presentdamage to delicate parts. If feasible, these rests or standsshould be part of the basic chassis as shown in Fig. 4-l8.Side aligning devices, similar to the ones shown in Fig.4-19, are desirable for heavy components because thecomponent can be slid into place.

Some commonly used handles shown in Fig. 4-20are finger recess. hand recess. T-bar, and J-bar. Minimumacceptable dimensions of these handles together withminimum curvature or edges when carrying various loadsare also given. Handles should be provided on the coversof units and positioned for balance to facilitate re-moving the cover and carrying the item; the use of fingers(Fig. 4-21) or levers to pry off covers is not good practice.Covers of cases should be designed to be lifted off the unit;the unit should not have to be lifted out of its cover(Figs.4-8(B) and 4-22). Handles can also serie various supple-mentary applications as shown in Fig. 4-23.

Whenever practical, wire connections should be madewith U-type lugs rather than O-type lugs as shown in Fig.4-24 to facilitate removal. Termimals for soldered wireshould be far enough apart so that work on one terminaldoes not compromise the integrity of an adjacent terminalas shown in Fig. 4-25. The terminals should be longenough to prevent damage to insulation from the hotsoldering iron as shown in Fig. 4-26.

Cables should be routed so they1. Are not pinched by doors, lids, and slides2. Are not walked on they should not be on the

same side of the panel reserved for access or operationsas shown in Fig. 4-27

3. Are accessible to the technician i.e., not underfloorboards, behind panels or components that are diffi-cult to remove, or routed through congested areas

4. Are not bent or unbent sharply, as shown in Fig.4-28, when connected or disconnected.

Cable connections that are maintained between sta-

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Screwdriver

Pliers

Wrench

Soldering Gun/Iron

Hammer

Flashlight

Alignment Tool

Spin Tight

Socket set

Screwholding Screwdriver

Tube Puller

Soldering Aid

Tuning Wand

Screw Starter

Knife

Heat Dispenser

Rachet

Punch

Wire Stripper

Tweezer S

File

Power Drill

Burnishing Tool

Mirror Stick

Bearing/Gear Puller

Wire Straightener

Mechanical Fingers

Scissors

Chisel

Drill press

Cleaning Brush

Wire Brush

Figure 4-12. Frequency With Which Hand Tools Are Used at Least Once in 427 MaintenanceTasks on Electronic Equipment

4-11

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TABLE 4-2. APERTURE DIMENSIONSFOR REGULARLY CLOTHED

TECHNICIAN

Body Member or Position Dimensions

Passing head breadth 178 mm (7 in.)

Passing shoulder width 508 mm (20 in.)

Passing body thickness 330 mm (13 in.)

Passing through access in 788 mm (31 in.) high, 508a crawling position mm (20 in.) wide

Passing through access in 1638 mm (65 in.) high,kneeling position (with 508 mm (20 in.) wideback erect)

Two men passing through 914 mm (36 in.) wideaccess abreast (standing)

A. Arm to Elbow

Light clothing: 102 mm X 114 mm or 114 mm dia (4.0 in. X 4.5 in. or 4.5in. dia)

Arctic clothing: 178 mm (7.0 in.) square or diaWith object: clearances as above

B. Arm to Shoulder

Light clothing: 102 mm (4 in.) square or diaArctic clothing: 216 mm (8.5 in.) square or diaWith object: clearances as above

Minimal Finger-Access (First Joint)

C. Operating Push Button Bare Hand: 32 mm (1.25 in.) diaGloved Hand: 38 mm (1.5 in.) dia

D. Twisting With Two Fingers Bare Hand: 51 mm (2.0 in.) diaGloved Hand: 64 mm (2.5 in.) dia

Figure 4-13. One-Hand Access Openings (Ref. 3)

4-12

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Figure 4-14. Access-Opening Dimensions (Ref. 3)

4-13

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Figure 4-14 (cont’d)

4-14

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Figure 4-15.

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4-15

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Figure 4-16. Foldout Construction for Electronic Chassis (Ref. 3)

Figure 4-17. Hinged Assemblies Braced in "Out" Position Leave both Hands Free (Ref. 3)

Figure 4-18. Standards for Maintenance asPart of Chassis Will Prevent Damage toParts (Ref. 3)

Figure 4-19. Side-Alignment BracketsFacilitate Correct Mounting (Ref. 3)

4-16

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4-17

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Figure 4-21. Handles Facilitate Removal of Covers and Carrying of Units

Figure 4-22. Cases Should Lift Off Units

Figure 4-23. Additional Use of Handles

Figure 4-24. U-Type Lugs Facilitate Repairs

Figure 4-25. Spacing Wire Leads FacilitatesRepairs

Figure 4-26. Terminals Should Be LongEnough to Prevent Damage to InsulationDuring Repairs

4-18

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Figure 4-27. Route Cables So They Are Not Likely to Be Walked On (Ref. 3)

Figure 4-28. Route Cabling to Avoid SharpBends (Ref. 3)

tionary equipment and sliding chassis or hinged doorsshould have service loops to permit movement, such aspulling out a drawer for maintenance, without breakingthe electrical connection. The service loops should con-tain a return feature to prevent interference when theremovable chassis are returned to the cabinet. Fig. 4-29shows two methods of recoiling the cable.

Cable connectors should be far enough apart to insurefirm gripping for connecting and disconnecting (Fig. 4-30). The space required will depend on the size of theplugs plus a minimum separation of 64 mm (2.5 in.).

Figure 4-29. Methods for Recoiling ServiceLoops in Sliding Chassis (Ref. 3)

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DOD-HDBK-791(AM)

Figure 4-30. Connectors Should beArranged So That They Can Be GraspedFirmly for Disconnection

4-6.2 FIRE CONTROL EQUIPMENTFire control equipment – optical sights, laser range

finders, laser designators, radars, infrared sources, andelectronic computers-–are usually readily accessible be-cause they must be accessed to locate and track the targetand to aim the weapon. Access to computer and radarcomponents, being essentially electronic and electrical, isaddressed in par. 4-6.1. Maintenance for optical sightsand laser equipment, except for replacement of rubbereye pieces, is confined to the depot level; therefore, ready,ease of access is not a critical factor.

The fixtures that attach the optical sights to the weaponor laser range finders should be readily accessible andrequire no special tools to operate—quick disconnecttype fasteners should be used wherever possible.

Laser and optical sight equipment require a clean roomatmosphere and precise adjustments to achieve alignmentof components and collimation—tasks too delicate to beperformed at the unit or intermediate maintenance levels.Maintenance to be performed at the depot level does notrelieve the designer of providing access to components orfor operations that will facilitate maintenance, e.g.,

1. A means for purging and charging instrumentsshould be provided and be easily accessible. The accessshould be located so as not to interfere with seals orexpose lenses to abrasion.

2. Aligning, collimating, and triggering devicesshould be so located as to insure that the repairmancannot accidentally expose himself to the emerging laser

4-6.3 MISSILESMissiles should be provided with suitable access doors

or removable covers for servicing operations such asinspection, test, lubrication, drainage, adjustment, fuzesetting, and replacement of parts. In particular, the adap-tion kit—providing the safing, arming, and fuzingfunctions—must be readily accessible.

The access openings should be of sufficient size andproper shape to furnish an adequate view of the parts tobe serviced. The access opening should be large enough to

4-20

permit entrance of a gloved hand whenever possible. (SeeFig. 4-15 for gloved-hand dimensions. ) The access doorsshould be externally flush e.g., retractable handles topreserve the aerodynamic shape of the missile—easy toopen, and held securely closed by the appropriate fasten-ers. Fig. 4-31 shows examples of various types of fastenersand their actions. The doors should also be designed sothat the action of the slipstream will tend to keep thedoors closed in flight.

4-6.4 MOBILE EQUIPMENT TRAILERSThe word “accessibility” has a different connotation

from that previously used in this chapter. In this para-graph, accessibility does not relate to the maintenance ofthe trailers or mobile equipment; instead, accessibilityrelates to the design of the mobile equipment to make themateriel it services more accessible. The trailer chassisusually are constructed of commercial components andhave been engineered for maintenance, and the requiredmaintenance is minimal.

It could be argued that some of the presented guidelinesare human factors features; however, this is too narrow aninterpretation—where a mutual relationship appears toexist, a guideline is stated.

4-6.4.1 Component TrailersThis discussion relates only to the manner in which the

component trailer facilitates access to the materiel forwhich it was designed to service. Design guidelines orconsiderations follow:

1. Design component trailers with precise position-ing controls in the x-, y, and z- directions if precise posi-tioning is necessary. Personnel should not be requiredmanually to push or lift heavy components in order tomate them. If a primitive, common type of lifting devcewill do the job, do not provide a special materiel-specificitem.

2. Design component trailers of sufficient heightand configuration to enable the transported item to bedirectly in line with its component part. This feature isillustrated in Fig. 4-32 in which a warhead is beingmounted on a missile.

3. Equip the trailer with brakes so that it can beimmobilized for precise positioning with cradle controlsafter coarse positioning movements have been made bymaneuvering the trailer. Locate the brake controls so thatpersonnel can reach them while manually restraining thetrailer.

4. Design trailers to allow for individually swivelingall four wheels to reduce positioning time.

5. Design independent controls such as roll, pitch.or yaw—for component trailers. These controls will allowthe technician to position components properly forrequired maintenance if the trailer is to be used as amaintenance stand.

beam.

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Figure 4-31. Fastener Examples (Ref. 3)

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DOD-HDBK-791(AM)

Figure 4-32. Mating Surface of Component Trailers

4-6.4.2 Van TrailersVan trailers generally are used to provide protected

space for specific operations—e.g., command post, firedirection center, and computer center. Accordingly, forthe access design features to enhance their maintainabil-ity, the appropriate paragraphs of this handbook shouldbe consulted—e.g., if the van houses electronic and elec-trical equipment, refer to par. 4-6.1.

The chassis of the vehicles are made of commercialcomponents which have been engineered for mainte-nance. Special consideration should be given to the rout-ing of cables into and out of the van (see Figs. 4-27,4-28,and 4-30). Ready access to filters—particulate, aerosol,and gas —should be available to permit changing from theinside so that air quality inside the van can be maintained.

4-6.4.3 StandsThe proper work stand is important physically and

psychologically in accomplishing a maintenance task effi-ciently and expeditiously—a stand is essentially one of thenecessary tools to “do a job correctly”. Accordingly, if astand is to be especially designed, it should embody fea-tures that maximize its use. Considerations in the designof stands follow:

1. Design maintenance stands so that they can beused on inclined surfaces of 15 deg without tipping whenthe weight of personnel and/or the component is concen-trated on one side. Fig. 4-33 illustrates the advantage of astand supported by two members instead of a singlepedestal.

2. Use anchors or outriggers for stands that havehigh centers of gravity and that may be overturned bywinds.

3. If stands are an integral part of the equipment,insure that personnel can reach all items they mustmanipulate without falling off.

4. Provide brakes for auxiliary stands equipped withwheels.

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Figure 4-33. Physical Stability forMaintenance Stands

5. Design the walking surfaces on stands and plat-forms to afford good traction under all weather conditions.

6. Specify the use of engine stands which are slightlylarger than the engine diameters to prevent damage fromforklifts and other handling devices.

7. Use properly balanced and supported fuselagestands with low centers of gravity so that the stands willnot tip under unevenly distributed weight.

8. Incorporate rails in the power plant shell struc-ture where consistent with aerodynamic and weightrequirements of the aircraft in order to roll the engine inand out of the shell. The rails should be matched to theheight and size of the engine transporter dolly.

4-6.5 TANK/AUTOMOTIVE MATERIELAccessibility of components in tank-automotive equip-

ment can have a major impact on the maintenance indicesof the vehicle. Design for accessibility should consider thefollowing factors:

1. Type of maintenance2. Maintenance environment3. Task frequency4. Performance and,’ or design considerations.

Each of these factors is discussed in the paragraphs thatfollow.

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4-6.5.1 Type of MaintenanceAs with other types of equipment, tank-automotive

maintenance will be either corrective or preventive. Theimpact of accessibility on each type follows:

1. Preventive Maintenance:Preventive Maintenance Checks and Services

(PMCS) other than daily operator and/or crew checksnormally will account for approximately 20% of totalmaintenance time. The overwhelming problem regardingPMCS—including daily checks is to reduce the timerequired for these services to the absolute minimumnecessary. A daily PMCS checklist that cannot be exe-cuted by the crew in 20 min for a truck and 45 min for atank will most likely be rushed over or ignored; therefore,optimum placement of components to be checked is criti-cal. Assure good accessibility of fluid reservoirs, filters,and other checkpoints. Eliminate the need to removearmor plates or other obstacles in order to reach thecomponents involved in the PMCS checklist. The quar-terly PMCS checklist—which will include monthly ser-vices—-should take no more than eight clock hours toperform. Semiannual and annual services naturally willtake longer, but since they are performed less frequently,their contribution to total maintenance time, and hencethe maintenance ratio. is not as significant.

Accessibility of a specific component may deter-mine whether the part will be serviced during PMCS orallowed to fail. For example, a U-joint on a specific lightcombat vehicle was designed to be lubricated at the quar-terly service. With this scheduled service. the reliabilitywas effectively the life of the vehicle. However. the loca-tion of the U-joint necessitated removal of the powerpack. Thus this quarterly lubrication required over 8clock hours. The U-joint, therefore, was changed at aslight increase in cost to a permanently lubricated designwith a projected mean time between failures (MTBF) of1000 h and a replacement time—mean time to repair(MTTR)—of 9 clock hours. With an annual usage of 350h, failure could be expected approximately once everythree years. By accepting this small risk of failure-–-withapproval from the user community—design engineersreduced the maintenance ratio and increased the avail-ability of the system. A small increase in unit cost wastraded off for a lower life cycle cost. If no other failuresare assumed, the inherent availability calculations are

Scheduled Service = M T B M

MTBM + MTTR

— 350/4— = 0.92(350/4) + 8

Run to Failure = M T B FMTBF + MTTR

— 1000— = 0 . 9 9 .1000 + 9

DOD-HDBK-791(AM)

2. Corrective Maintenance. Accessibility considera-tions and ground rules discussed in the beginning of thischapter can be applied to tank-automotive equipment.Time spent getting to the faulty part is wasted time. If it isassumed that reliability cannot be further improved, thenthe lower the expected reliability of a part, the moreconsideration should be given to its accessibility. Refer tothe discussion on frequency of maintenance in par.4-6.5.3.

4-6.5.2 Maintenance EnvironmentTank-automotive equipment is unique in that mainte-

nance will be required in most every imaginable environ-ment. In peacetime, maintenance generally will be per-formed on a hardstand in a sheltered bay, often withpower lift capability, and a pit. No such amenities willexist under wartime conditions. Do not rely on having apit to gain access to maintenance or service points underthe vehicle. Jack stands that support a vehicle on a hard-stand may sink in soft ground or mud if not properlydesigned. Lift capability used to remove components forservice or better access will not be available unless amobile wrecker is called in. Also a component that isaccessible under “classroom conditions” may not beaccessible when caked over with hardened mud anddebris.

4-6.5.3 Task FrequencyIf a power pack is removed only once a year, it really is

not important if the task requires 30 man-hours to per-form. If the pack must be removed monthly, however, thecontribution of this task to the total maintenance burdenis multiplied by 12. The importance of task frequency tothe total maintenance burden is shown in Tables 4-3 and4-4. Table 4-3 lists the top ten replacement parts, byfrequency of occurrence, for two types of vehicles. Table4-4 lists the top ten replacement parts, by man-hours toreplace, for the same vehicles.

TABLE 4-3. LIST OF TOP TENREPLACEMENT PARTS BY FREQUENCY

OF OCCURRENCE

M113 Family M915 Series(Ligh Combat Vehicle) (Heavy Tactical Truck)

Engine Oil Gear ShiftTransmission Oil GasketTrack Pads CableAntifreeze ShaftTrack Shoes BreatherFuel Filter Decreasing TireRubber Cushion Frequency Fuel FilterOil Filter Oil FilterRoad Wheel Air FilterBattery Battery

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TABLE 4-4. LIST OF TOP TENREPLACEMENT PARTS BY MAN-HOURS

TO REPLACEM113 Family M915 Series

(Ligh Combat Vehicle) (Heavy Tactical Truck)

Track Shoe Engine ValveEngine InsertEngine Oil SealTransmission Decreasing TireRubber Cushion Man-Hours SpringRoad Wheel TransmissionSprocket GasketFinal Drive Fuel FilterTransmission Oil Oil FilterBattery Air Filter

For combat vehicles, track is usually the number onemaintenance burden. An accessible location for breakingand joining track is essential if the time to perform trackmaintenance is to be minimized. For all tank-automotiveequipment, engine oil and filter change also is a signifi-cant contributor to total maintenance time. In general,the maintainability engineer should place emphasis onthose items in Table 4-4 since they are the top contribu-tors to the maintenance burden. Tasks such as replacingthe gearshaft are performed more often than replacing theengine valve, but they do not add as much to the totalmaintenance burden.

4-6.5.4 Performance and/or DesignConsiderations

Because tank-automotive equipment is mobile, it issubject to dynamic stress and strain. To maintain vehicleintegrity regarding stress and strain calculations, accessi-bility must be considered during design and not as anafterthought. For example, rectangular hull access panelshave maximum stress at the corners (Fig. 4-34). Theoptimum solution from a stress consideration is a circularaccess hole. However, this shape access may be totallyimpractical from the viewpoint of production, i.e., cuttingor machining difficulties. A reinforced edge around theperimeter of the rectangular access may serve to streng-then the hull and prevent cracking. The use of the access

Figure 4-34. Corner Stress Cracks

plate in the first place, however, may degrade ballisticprotection. The goal is to conduct the appropriate acces-sibility performance trade-offs to arrive at the optimumsystem design.

4-6.6 ARMY MARINE EQUIPMENTThe accessibility of marine components that may require

maintenance is affected by fouling, i.e., the growth ofanimals and plants on the surfaces of submerged objects.A principal source of trouble is the fouling in pipes andconduits used to conduct water inside ships. For example,fouling may prevent closure of an isolation valve thatmust be closed to access a failed component. Fouling canmake underwater structural repairs difficult or impossi-ble. Consequently, more ships will require dry-dock facil-ities, i.e., making dry docks less available.

Fouling is controlled primarily by the selection ofappropriate materials of construction and the use of pro-tective coatings such as paint. The common antifoulingpaints contain copper, mercury, or arsenic compounds invarious combinations; concentrations of about 1 milli-gram per liter are effective in reducing fouling. Di-isobutyl phenol and chlorophenarsamine are consideredvery effective in combatting fouling. As a preventive mea-sure, pipes and conduits should be made of bronze orother copper alloys because these materials are the leastlikely to foul.

4-6.7 AIRCRAFT (Refs. 4 and 5)Aircraft components that require maintenance should

be easily accessible. However, the designer should con-sider the expected frequency of maintenance to determinethe degree of accessibility and to insure that the effortexpended to provide accessibility is warranted.

4-6.7.1 General Inspection and AccessRequirements

The aircraft designer should provide every possibleconvenience for performing periodic inspections andreplacements of functional components in a minimumperiod of time and for decreasing possible in-flighthazards. Some design recommendations that merit con-sideration follow:

1. Do not use doors that are welded or riveted tothe airframe, panels, or other accesses requiring the re-moval of permanently attached structures.

2. Provide doors or access panels in the fuselage.airfoils, nacelles, control surfaces, and any location nototherwise accessible from the interior for the inspectionand servicing of actuators, controls, jack screws, pulleys,cables, guides, electric junction boxes, the Pitot-staticsystem, fuel tanks and system, boost pumps, and similaritems.

3. Uniquely mark all removable inspection andaccess doors or otherwise identify their locations to expe-

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dite reinstallation. When hinged doors are used, locatethe hinges so that the airstream tends to keep them closed.

4. Provide a door opening of at least 150 deg, pref-erably 180 deg. Piano hinges may be used and are desir-able as locking devices for inspection and access doors.

5. Do not locate inspection or access doors on theengine air intake duct or near its opening because theymight be sucked into the engine if they become unfastened.

6. Use flush, quick-opening fasteners on inspectiondoors that conform to MIL-F-5591 (see Fig. 4-31 forexamples). Do not use screw-retaining doors when fre-quent inspection, servicing, or maintenance is required.

7. Design access doors and cowling so that whenclosed the fasteners do not appear fastened if they are not.Unlatched cowlings have been lost in flight, with resultantaircraft damage, due to this design inadequacy.

8. Size and locate all inspection and access panelsso that a mechanic dressed in arctic clothing, includinggloves, may accomplish the necessary work. (See Fig.4-15 and Chapter 9.)

9. Provide each fuel cell with its own access doorfrom the exterior of the aircraft. (See Fig. 4-35.) Access tofuel cells in large aircraft presents special problemsbecause ofthe number of separate fuel cells and thecomplexity of the interconnecting and regulating hard-ware in and around the cells. In a large aircraft there maybe hundreds of valves, float switches. circuit breakers,fuel manifolds, or clamps. Accordingly. inspection, trou-bleshooting, replacing, and repairing become formidabletasks. Where possible, the complexity a subsystemshould be reduced. Reducing the number of parts in asubsystem will improve access for maintenance.

10. Where possible, make equipment accessible forin-flight maintenance and operation. Place equipmentrequiring access. operation, or adjustment during flightwithin easy reach of the operator. During flight whencrew members are properly restrained, i.e., shoulder har-nesses and seat belt fastened, components that requireadjustment should be within easy reach. Crew membersshould not be required to release themselves from ejectionseats.

Figure 4-35. Provide Access Door for EachFuel Cell

DOD-HDBK-791(AM)

11. Make control components accessible for inspec-tion and maintenance; make actuators accessible forstroke adjustment and replacement of motor brushes.Temperature-setting adjustments on thermostatic con-trols should be readily accessible, and test points requiredfor checking waveforms, voltages, hydraulic pressure,and gas pressure should be readily available and identifi-able (see Chapter 9).

4-6.7.2 Propulsion SystemsEase of accessibility to all parts of the power plant will

minimize the time and manpower required for mainte-nance. The designer should consider the accessibility ofall parts for inspection, cleaning, and adjustment. Re-moval of components for servicing should be possiblewith a minimum need to remove other parts of the powerplant or aircraft. Without neglecting vulnerability thelayout of these components should allow a maximumnumber of mechanics to work on the installation withminimum interference from one another.

Tables 4-5 and 4-6 list engine accessories that should beaccessible for inspection, cleaning, and adjustment whileinstalled on the aircraft without removal of the engine,fuel tanks, or other important parts of the aircraft struc-ture. Table 4-5 is for reciprocating engines; Table 4-6, forturbine engines. The tables also list the accessories thatshould be accessible for removal or replacement. Theseaccessories should be designed or arranged so that onlytools normally found in the mechanic’s tool kit are neededfor the required work.

Additional accessibility design recommendations forpropulsion systems follow:

1. Provide access to engine parts and accessorieswithout necessitating removal of the ring cowling.

2. Provide large, quick-opening access doors andsufficient space in the engine accessory area for servicingand replacing of components.

3. Hinge aircraft skin, where possible, for ease ofaccess to engine maintenance tasks.

4. Use split-line design whenever possible for max-imum accessibility to engine components. For example,split the compressor and combustion chamber housingsfor easy inspection, service, or removal of blades andcannular chambers.

5. Design engine rail brackets as a part of the mainchassis to facilitate removal of the engine.

6. Design engine mounting in normal installations—e.g., the engine is installed in the nose or in the nacelle—sothat the mount, complete with cowling, is readily detach-able.

7. To facilitate quick power plant changes, use self-aligning mounting bolts employing ball-and-socket ortapered ends. This type of fastener should be used onlywhere stress requirements permit.

8. For maximum access mount the accessory gear

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Magneto breaker pointsOil pressure relief valveOil tankOil cleaner or strainerFuel tankFuel pressure relief valveFuel strainerDrain valves

DOD-HDBK-791(AM)

TABLE 4-5. RECIPROCATING ENGINE ACCESSORIES THAT SHOULD BEACCESSIBLE FOR INSPECTION, CLEANING, ADJUSTMENT,

REMOVAL, AND REPLACEMENT

Inspection, Cleaning, and Adjustment

Carburetor or fuel injection systemCarburetor air filtersSuction relief valveFeathering pumpPropeller governorTurbosuperchargerTurbosupercharger regulatorAutomatic controls

Removal and Replacement

Spark plugsMagnetoStarterGeneratorTachometer generatorHigh tension wiringRadio shieldsTemperature control actuatorTemperature control actuator motor brushesOil tanks, not integralOil cleaner or strainerOil pumpOil pressure relief valveOil booster pumpFuel pumpFuel booster pumpFuel strainerCarburetor fuel strainerCarburetor air filter

drives and their related accessories in the bottom, or sixo’clock, engine position along the compressor sectionbecause present aircraft design makes this the mostaccessible position.

9. Mount engine accessories so they are accessiblefor inspection, cleaning, adjustment, or removal withoutremoval of the engine or other important power plantstructures.

10. Design the power plant installation so that alldaily and preflight inspections can be made in coldweather when the operator is wearing heavy gloves andbody clothing. In particular, provide proper accessibilityto fuel and oil drains.

11. Provide an opening in the engine cowling forground heaters. This opening should have either an acces-sory door with a minimum diameter of 305 mm (12 in.) oran easily removable section of cowling of equivalent size.Locate the door or opening so that it may be used forconvenient servicing of oil and fuel drains. Considergrouping the drains, especially the main oil and oil tanksump drains, near the accessory door opening. Stencil the

4-26

Water injection pumpExhaust stacks and collectorFlame damperExhaust gas heat exchangerTurbosuperchargerTurbosupercharger regulatorAutomatic engine controlsSuction relief valveVacuum pumpHydraulic pumpDeicing pumpAccessory gear driveCabin supercharger (mechanical)Fuel injection pumpsFluid shutoff valvesOil coolerOil cooler control valveFuel injection controlCarburetor or fuel injection system

accessory door or cowling section with “OPEN FORGROUND HEATER DUCT”.

12. In turbine-powered aircraft adequate provisionsshould be incorporated into the cooling system and struc-ture for rapid inspection, repair, and replacement of thetailpipe, flexible coupling, and all components of thesystem, such as ejector shroud, insulation blanket, cool-ing-air shutters, diverters, and controls. If the systemdesign incorporates an ejector shroud, it should be fabri-cated or assembled on the tailpipe so that it is removablewith this unit.

13. Design the turbine with a removable housing sothat the rotor blades are visible. This feature makesinspection of every blade possible by rotation of the tur-bine wheel.

14. Mount the turbine stator and rotor blades sothey can be removed, inspected, and installed individuallyby hand (see Fig. 4-36). If it is necessary to change a blade,the retaining plate is unscrewed and blades are slipped outby hand and inspected. When a damaged blade is reached,a new blade is slid in, and the retaining plate is replaced.

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TABLE 4-6. TURBINE ENGINE ACCESSORIES THAT SHOULD BE ACCESSIBLE FORINSPECTION, CLEANING, ADJUSTMENT, REMOVAL, AND REPLACEMENT

Inspection, Cleaning, and Adjustment

Main fuel controlStarting fuel controlEmergency fuel controlGovernor (speed)Suction relief valveActuator motor brushesAutomatic controlsSump plugs and drain valvesSpark plugs

Oil cleaner or strainerOil pumpOil pressure relief valveOil coolerOil temperature relief valveBooster pumpFuel pumpFuel strainerFuel regulator unit or control unitFuel nozzleDrip valveFlow divider (fuel system)

Variable nozzle area unit and controlOil pressure relief valvesOil tankOil cleaner or strainerFuel filterFuel pressure relief valveFuel nozzleBarometric unit (fuel control)Control valve (idling speed)

Removal and Replacement

Pressure bypass valve (fuel system)Main fuel controlStarting fuel controlEmergency fuel controlThermal unit (fuel system)Barometric unit (fuel control)Water tank or thrust augmentation fluid tankAutomatic engine controlsDrain valves

Figure 4-36. Mount Stator and Rotor Bladesto Facilitate Removal of Individual Blades

Spark plugsIgnition coilStarterGeneratorTachometer generatorHigh tension wiringRadio shieldingTemperature control actuatorTemperature control actuator motor brushesOil tanks, not integralSuction relief valveVacuum pumpHydraulic pumpAccessory gear driveAir filter (bearing cooling)Air filter (vents)Tailpipe (extension)Reducer (variable nozzle area)Governor (RPM)Oil shutoff valve

4-6.7.3 Landing GearsConsider the following accessibility recommendations

in the design of landing gear:1. Design all units of the alighting gear to be accessi-

ble for lubrication, service, inspection, and replacement.2. Design all hydraulic mechanisms so that their

filler plugs, bleeder plugs, and air valves may be readilyserviced with air and fluid.

3. Provide sufficient clearance between the shockabsorber packing gland nut and adjacent parts of theaircraft when the shock absorber is fully deflated so thatthe nut may be readily adjusted with a wrench.

4. Design all shock absorber struts so that the extentof inflation may be determined without removing thecowling or using any measuring device other than a scale.

4-6.7.4 Mechanical ItemsConsider the following accessibility recommendations

in the design of mechanical items:

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1. Provide the greatest accessibility for either theremoval of bearings for bench relubrication or their purg-ing and relubrication in the aircraft. This requirementstems from the need to replace bearings and change lubri-cant to suit the range of temperatures and conditions atwhich certain aircraft may operate.

2. In using split bearings optimize accessibility bymaking the plane of the split correspond with accessports. For example, split the crankshaft bearing on anengine connecting rod to permit removal of the bearingthrough an external access without removing the crank-case cover.

3. To permit changing of the bearings without disas-sembly of the entire component, mount two bearings intandem rather than using a single bearing of larger size.This will permit changing one bearing while the otherbearing supports the load.

4. Where accesses are located over dangerousmechanical components, design the access door so that itturns an internal light on automatically when opened.Also provide a highly visible warning label on the accessdoor.

4-6.7.4.1 Drawer-Type HousingAssemblies, such as instrument panels and electrical

and hydraulic units, can be made more accessible by theuse of drawer-type housings. Basically, a unit should bemade up of a housing with many cavities. Each cavityshould contain a component that would serve, as far aspractical, a separate portion of the overall system. Theoverall unit should be mounted on a rugged frame withthe front cover also serving as the control panel for theenclosed compartments. The frame should be supportedby hinges that come apart readily to permit withdrawal ofthe whole component. Rests, limit stops, guards, and/ orretaining devices should be provided as part of the basicchassis to prevent the unit from falling from the aircraft.No special tools should be required to withdraw the com-ponent. The front panel of the component should have a

dust seal to prevent contaminants from entering the cav-ity in the housing. When ventilation is required to cool thecavity, suitable filters should be installed in the ventilatingintake and exhaust ducts. Component test points, ifrequired, should terminate at an easily accessible terminalboard or strip.

The design of drawer-type housings offers the follow-ing desirable maintenance features:

1. The component may be withdrawn and rapidlycalibrated, serviced, inspected, or repaired.

2. The component maybe removed completely fromthe housing, placed on a bench in front of the housing,and, with the use of an adapter cable(s), serviced andmaintained with full access to the component.

3. The component may be taken to a test area andrepaired, calibrated, tested, or inspected under idealconditions.

4. The component maybe replaced with an identicalunit from stock, which rapidly returns the system toavailability.

4-6.7.4.2 Major Unit HousingsMajor unit housings, such as engine nacelles, should

have hinged or removable housings that can be rapidlyopened and closed for inspection and repair. The fasten-ers required to secure the housings should be kept to aminimum and should be removable with speed tools, suchas speed wrenches and screwdrivers.

4-7 ACCESSIBILITY CHECKLISTTable 4-7 summarizes the important design recom-

mendations to be considered when designing for main-tainability. The checklist contains several items that arenot discussed separately in the text. These items areincluded here because their necessity in the design is soobvious that they might otherwise be overlooked. If theanswer to any item on the checklist is “no”, the designshould be reexamined to determine the need forcorrection.

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TABLE 4-7ACCESSIBILITY CHECKLIST

A. GENERAL:1.

2.3.4.

5.

6.7.

8.9.

10.

11.

12.

13.14.

15.

16.

17.18.19.

20.21.22.23.

24.25.

26.27.

28.

29.

30.31.32.33.

Is optimum accessibility provided in all equipment and components requiring maintenance, inspection, removal,or replacement?Is a transparent window or quick-opening metal cover used for visual inspection accesses?Are access openings without covers used where they are not likely to degrade performance?Is a hinged door used where physical access is required (instead of a cover plate held in place by screws or otherfasteners)?If lack of available space for opening the access prevents use of a hinged opening, is a cover plate with captive,quick-opening fasteners used?Are parts located so that other large, difficult-to-remove parts do not prevent access to them?Are components placed so that there is sufficient space to use test probes, soldering irons, and other requiredtools without difficulty?Are units placed so that structural members do not prevent access to them?Are parts mounted on a single plane, i.e., not stacked one on another?Are components placed so that all throwaway assemblies or parts are accessible without removal of othercomponents?Is equipment designed so that it is not necessary to remove any assembly from a major component totroubleshoot that assembly?Are units laid out so that maintenance technicians are not required to retrace their movements during equipmentchecking?Can screwdriver-operated controls be adjusted with the handle clear of any obstruction?When adjustments with a screwdriver must be made by touch, are screws vertically mounted so that thescrewdriver will not fall out of the slot? Are Phillips head or Allen head screws used rather than slotted ones?Is enough access room provided for tasks that necessitate the insertion of two hands and two arms through theaccess?If the maintenance technician must be able to see what he is doing inside the equipment, does the access provideenough room for the technician’s hands or arms and still provide an adequate view of what he is to do?Are irregular extensions, such as bolts, tables, waveguides, and hoses, easy to remove before the unit is handled?Are units removable from the installation along a straight or moderately curved line?Are heavy units (more than about 110 N (25 lb)) installed within normal reach of a technician for purposes ofreplacement?Are provisions made for support of the units while they are being removed or installed?Are rests or stands provided on which units can be set to prevent damage to delicate parts?Is split-line design used wherever possible and necessary?Are access points individually labeled so they can be easily identified with nomenclature in the job instructionsand maintenance manuals?Are accesses labeled to indicate what can be reached through this point (label on cover or close thereto)?Are accesses labeled to indicate what auxiliary equipment is needed for service, checking, etc., at this particularpoint?Are accesses labeled to specify the frequency for maintenance either by calendar or operating time?Are parts that require access from two or more openings marked to so indicate to avoid delay and/ or damage bytrying to repair or remove through only one access? Are double openings of this type avoided wherever possible?Are human strength limits considered in designing all devices that must be carried, lifted, pulled, pushed, andturned?Are environmental factors (cold weather, darkness, etc.) considered in design and location of all manipulatableitems of equipment?When necessary, are internal parts illuminated?Are fuses located so that they can be seen and replaced without removal of any other item?If fuses are clustered, is each one identified?Are fuse assemblies designed and placed so that tools are not required for replacement?

(cont’d on next page)

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TABLE 4-7 (cont’d)

B. ACCESS DOORS AND COVERS:1. Are clearance holes for mounting screws in cover plates oversize to obviate the need for perfect alignment?2. Are the cases designed to be lifted off units rather than for the units to be lifted out of the cases?3. Are the cases made larger than the units they cover to preclude damage to wires and components?4. Are guides or tracks provided to prevent the cocking of the case to one side?5. If the method of opening a cover is not obvious, are instructions provided on the outside of the cover?6. When covers are not in place and secure, are means provided to make this error obvious?7. Are no more than four fasteners used to secure the case?8. Is the same type of fastener used for all covers and cases on given equipment?9. Are ventilation holes with screening of small enough mesh provided to prevent entry of probes or conductors that

could inadvertently contact high voltages?10. Are access doors made in whatever shape is necessary to permit passage of the components and implements that

must pass through?11. On hinged access doors, is the hinge placed on the bottom or is a prop provided so that the door will remain open

without being hand-held?12. If maintenance instructions are placed on the door, are letters oriented to be read when door is open?13. Because military equipment must be maintained on ground covered deep in mud or snow, in extreme tempera-

ture, and tactical blackout at night, do access doors permit equipment to be maintained from above rather thanbelow and from inside rather than outside?

C. HANDLES:1. Are handles used on units weighing over 45 N (10 lb)?2. Are handles provided on smaller units that are difficult to grasp, remove, or hold without using components or

controls as handholds?3. Are handles provided on transit cases to facilitate the handling and carrying of the unit?4. Are handles placed above the center of gravity and positioned for balanced loads?5. For handles requiring a firm grip, are bale openings at least 115 mm (4.5 in.) wide and 50 mm (2.0 in.) deep?6. Do handles have a comfortable grip while the unit is being removed or replaced?7. Are handles placed where they will not catch on other units, wiring, or structural members?8. Are recessed handles located near the back of heavy equipment to facilitate handling?9. Are handles located to prevent accidental activation of controls?

10. Are handles placed to serve as maintenance stands for equipment?11. For heavy equipment that requires two people for lifting, are four standard size grips or two large size grips

provided?12. Are handles or other suitable means for grasping, handling, or carrying provided on all units designated to be

removed or replaced?

D. WIRE AND CABLE:1. Are electrical cables of sufficient length so that each functioning unit can be checked in a convenient place?2. Is it possible to move units that are difficult to connect, when installed, to convenient positions for connecting

and disconnecting?3. Are cables and lines directly accessible to the technician wherever possible, i.e., not under panels or floorboards,

which are difficult to remove?4. Are cables routed so they need not be sharply bent or unbent when being connected or disconnected?5. Are cables and wire bundles routed so they cannot be pinched by doors or lids, or so they will not be stepped on or

used as handholds by maintenance personnel?6. Are means provided for pulling out drawers and slide-out racks without breaking electrical connections when

internal, in-service adjustments are required; for reeling cabling when drawers and racks are returned to theirpositions?

7. Are parts mounted on one side of a surface with associated wires on the other side?

(cont’d on next page)

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8.

9.10.

11.12.13.

14.

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TABLE 4-7 (cont’d)Is a 75-mm (3.0-in.) minimum clearance provided wherever possible between control cables and wiring, or is aphysical means provided to prevent chafing? (The designer must anticipate a potential chafing hazard.)Is electrical wiring routed away from all lines that carry flammable fluids or oxygen?Is care taken in the design of cable conduits to prevent collection of water or debris, which could interfere withoperation of a control system (freezing or short circuiting)?Is the necessity for removing connectors or splicing lines avoided?Is direct routing through congested areas avoided wherever possible?Are cable entrances on the fronts of cabinets avoided where it is apparent they could be “bumped” by passingequipment or personnel?Are adjacent solder connections far enough apart so work on one connection does not compromise the integrityof adjacent connections?

E. SAFETY:1.2.

3.

4.

Are access openings free of sharp edges or projections that could injure the technician or snag clothing?Are parts that retain heat or electric charge after equipment is turned off located so that the technician is notlikely to touch them while servicing the equipment? If this potential hazard cannot be avoided, does the accessdoor contain a label alerting the technician to the hidden hazard?Are access doors located away from moving parts or do they conceal moving parts that present a potentialhazard? If the concealed hazard cannot be avoided, does the access door contain a label alerting the technician tothis hazard?Are internal controls—switches and adjustment screws—located away from dangerous voltages or movingparts?

REFERENCES1. MIL-STD-882B, System Safety Program Require- 4. J. W. Altman, et al., Guide to Design of Mechanical

ments, 30 March 1984. Equipment for Maintainability, ASD-TR-6l-381,

2. DA PAM 750-40A, Guide to Reliability Centered Wright-Patterson Air Force Base, OH, August 1961.

Maintenance (RCM) for Fielded Equiment, 1 5 5. P. H. Newman and G. L. Murphy, Human FactorsFebruary 1980. Handbook, Vol. III, For Design of Protective and

3. MIL-HDBK-759, Human Factors Engineering Storage Ground Support Equipment, AFSWC-TR-

Design for Army Materiel, 31 December 1985. 59-13, Air Force Special Weapons Center, KirtlandAir Force Base, Albuquerque, NM, 1959.

BIBLIOGRAPHYF. L. Ankenbrandt, et al., Maintainability Design, Arsenault and Roberts, Reliability and Maintain-Engineering Publishers, 1963. ability of Electronic Systems, Computer ScienceBlanchard and Lowery, Maintainability Principles Press, Potomac, MD, 1980.and Practices, McGraw-Hill, New York, NY, 1969. D. J. Smith and A. H. Babb, Maintainability Engi-Cunningham and Cox, Applied Maintainability neering, Pitman Publishing, New York, NY, 1973.Engineering, John Wiley and Sons, New York, NY,1972.

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CHAPTER 5MODULARIZATION

The role of modularization in reducing maintenance time and improving operational availabililty isdiscussed. The advantages and disadvantages of throwaway modules are presented. Guidelines are lisled toassist the engineer in deciding on the physical attributes of a module and techniques of component grouping.Examples of the application of the modular concept to Army materiel are given. A design checklist formodularization is presented.

5-1 INTRODUCTIONThe support concept for a new system ultimately con-

sists of a series of support requirements—one set for eachlevel of support together with the procedures, tech-niques, and services needed for satisfactorily meetingeach requirement. Every system must be provided with itsunique support plan designed to meet its peculiar needs.As has been mentioned in previous chapters, the determi-nation o! the support requirements depends to a consid-erable extent on the operational requirements of the sys-tem in question. For example, the operational availabilitydesired for a system may dictate a mean downtimerequirement of such short duration that it can be met onlyby providing for a maximum repair capability at the unitlevel. Because this requirement maintenance at the userlevel usually employs personnel with limited skills forsupport work. a need for designing the system for easyidentification and isolation of faults is established. Theadditional requirement that rapid repair be facilitatedonce a fault has been identified calls in turn for modulardesign. Once these steps have been taken, general guid-ance principles to govern the design evolve, e.g.,

1. Modules will be removable by user personnelwithout the use of special tools.

2. Modules will be interchangeable without therequirement for maintenance adjustments.

3. Modules will cost no more than a given amount orwill not cost more than a given amount to repair. (This isto make throwaway at the user level economicallyfeasible.)

4. Modules will be replaced within a specified periodof time, i.e., a maximum limit on time to replace module.(This is necessary to meet the downtime requirement forthe system as a whole.)

This discussion leads to the definition of a modulei.e., “A module is a part, subassembly, assembly, or com-ponent designed to be handled as a single unit to facilitatesupply and installation, operations, and or main-tenance.” It can be either repairable (at the intermediateor depot level) or nonrepairable (discard-at-failure).Modularization is achieved by dividing equipment intophysically and functionally distinct parts or modules. The

module must be functionally complete to permit testingand verification apart from interfacing items. Thus modu-larization enables subsystems, assemblies, and subassem-blies to be designed as removable entities that can meetthe criteria for line-replaceable units (LRUs). (An LRU isan item whose removal and replacement with a like ser-viceable item is considered the optimum method of repairor restoration of a higher order system.)

The modularization concept is not confined exclusivelyto hardware items; software modules also lend themselvesto this concept. Software modules are discussed in par.5-4.

5-2 ADVANTAGESThe concept of modularization creates a divisible con-

figuration that is more easily maintained. Troubleshoot-ing and repair of modularized assemblies, therefore, canbe performed more rapidly. Use of this technique to themaximum improves accessibility, makes possible a highdegree of standardization, provides a workable base forsimplification, and provides an optimum approach tomaintainability at all maintenance levels.

To realize these advantages, the module should be1. Easily identified or determined without the use

of sophisticated test equipment and procedures to havefailed or malfunctioned: failure should be recognizable atthe module level rather than at the system level of whichthe module is a part.

2. Readily and easily accessible (see Chapter 4); theneed for accessibility to be in direct ratio to the specificfailure rates of the modules

3. Replaceable in less time and with fewer techni-cians with a lesser skill level than would be possible byrepair-in-place

4. Replaceable by technicians without the need forspecial tools and instructions. Mating connections betweenmodule and end-item should be easily recognized so thatthe module may be quickly detached and replaced with-out recourse to maintenance manuals for instructions.

5. Designed to be emplaced without adjustment orcalibration in order to be compatible with the system

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6. Designed if not a "throwaway” (see par. 5-3)to insure that components of the module are accessible sothat a higher level of maintenance can service or repair thecomponents

7. Designed for standardization and interchangeabil-ity (see Chapter 3) if the same function is performed inother subassemblies of the overall system.

Additional advantages of modular design are1. New equipment design may be simplified and

design time may be shortened by using previously devel-oped standard “building blocks”, i.e., modules. Of course,the building blocks must be consistent with the reliability,compatibility, and integrated system testing requirementsof the adopted system.

2. Current equipment can be modified in servicewhen newer and better functional units become availableprovided the new unit does not affect input-outputcharacteristics.

5-3 THROWAWAY MAINTENANCE

5-3.1 GENERALModularization lends itself to throwaway maintenance.

Strictly defined, throwaway maintenance is a mainte-nance policy whereby components or items of equipmentto a given level are discarded at failure rather thanrepaired. It embraces the terms “discard-at-failure” main-tenance and “nonmaintenance design”. As a policy, it isbased on the principle that every system design has a levelof repair at which it is more practical and economicallyfeasible to throw away a failed item or component than itis to repair that item or component.

The level of throwaway to be selected for a given designis dependent on many factors and may be established atany point between the complete system and any of thepiece parts of its subsystems. the higher levels of throw-away obviously provide for increased system availability,but they may dictate costs of such magnitude that a lowerlevel must be chosen. Higher levels of throwaway areuniversally acceptable whenever costs are not a determin-ing factor.

As a means of increasing the availability of equipment,throwaway maintenance is a logical extension of the“maintenance float” concept. This maintenance concept.i.e., the need to have major items of equipment on hand toreplace or substitute for equipment removed from servicefor either scheduled or unscheduled maintenance, hasbeen used by industry and the Army. Since most of theitems in this category are expensive, costs preclude theirdiscard—hence a pool of repaired items as well as newreplacement items. On the other hand, no one wouldconsider attempting to repair an electrical fuse, a lightbulb, or a fan belt; such items would be thrown away, andnew ones substituted. Somewhere between these two

5-3.2 ADVANTAGESAmong the typical advantages obtained by selecting a

high throwaway level for a given system that is beingdesigned are the following:

1. Costs of record keeping are drastically reducedbecause the number of expendable items is high.

2. Because the number of line items needed to sup-port a system is drastically reduced, the possibility isincreased that a part needed to repair a malfunctioningsystem will be available.

3. Greater possibility in packaging is affordedbecause access to the interior of throwaway modules isnot required; this makes possible both a very high packag-ing factor and greatly reduced overall volume.

4. System reliability is increased by the extent towhich unitized packaging makes possible encapsulationwith its protection against corrosion and humidity, andthe use of rigid assemblies with their high resistance toshock and vibration with fewer make and break con-nections.

5. Periodic replacement of assemblies (modules)because of normal failure makes possible a system that is.for all purposes. rebuilt. This feature is not exclusive tothrowaway maintenance.)

6. In-line changes that do not affect input-outputcharacteristics can be made in assemblies without addi-tional repair parts and without changes in field documen-tation being required.

7. Manufacturing costs per unit are minimizednot only because the volume of production is increased.but also none of the assemblies are designed to berepairable.

8. Modifications and all problems associated withthem are minimized because design imperfections can becorrected by changes being made in the complete module.

9. Documentation of equipment is reduced becausethe user or maintenance personnel do not need to knowthe internal makeup of individual assemblies.

10. Requirements for maintenance personnel andtheir respective skill levels are reduced because assembliesare replaced, not repaired. Similarly, test equipmentrequirements are reduced. This results in considerablesavings in many areas e.g., manpower, training, facili-

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ties. documentation, repair parts, and the test equipmentitself.

5-3.3 HAZARDSApplication of the throwaway concept creates several

important hazards, each of which requires provisions toprevent the development of major disadvantages. Thesehazards are

1. The frequent failure of a single, relatively inex-pensive component necessitates discard of an expensiveassembly, which results in increased maintenance dollarcosts. The obvious counteraction to be taken is the use ofcomponents of approximately equal reliability in eachassembly. This should be required even if more expensivecomponents are required to obtain an approximation ofequal reliability.

2. Frequent failure of a module, resulting in discard,does not reveal the cause of failure. A postmortem analy-sis of the failed module may discover that (a) the samecomponent causes failure and, if replaced by a morerelitible one, could extend the life of the module consider-ably or (b) the module is a victim of a false alarm thaterroneously signals failure of the module, i.e., the moni-toring or diagnostic equipment is at fault.

3. Procurement times for assemblies tend to begreater than those for components, which possibly leadsto excessive system downtime. To prevent this, increasedemphasis must be placed on stocking the critical items insufficient quantities at the unit level.

4. The technical capability of the personnel of theunit in the area of self-support generally deterioratesbecause of the decreasing need for piece-part repair activi-ties. Obviously, one of the objectives of throwaway main-tenance is the elimination of a requirement for high skilllevels for technicians.

Accordingly. a decision to adopt a high-level throw-away concept for the design of a system is predicated on anumber of conditions, i.e.,

1. Replacement assemblies should be available inadequate quantities at the user level.

2. The reliability of components within each assem-bly should be approximately equal.

3. The reliability of the assemblies comprising a sys-tem should be sufficiently high to compensate for theirrelatively high cost.

Despite these hazards and conditions, wherever con-siderations of operational availability are paramount, adefinite attempt should be made to design a system foremployment of throwaway assemblies.

5-4 SOFTWARE MODULESSoftware modules are similar to hardware modules in

that they may be designed to perform specific functions.Software modular design, however, cannot be applied to

all types of equipment with equal advantage. Its greatestapplication is in electronic equipment to monitor ordetermine the status of a circuit, subassembly, or system;diagnose or troubleshoot a failure or malfunction; orperform a self-check on an item of test equipment. Sepa-rate programs (modules) can be written, using the samemicroprocessor, to perform each of these functions foreach of the different hardware modules that requires it.

The advantages of software modularization are (Ref. 1)1. By using tailored software programs, i.e., focus-

ing on a specific item or function, the software is lesscomplex. Therefore, it requires less frequent maintenanceand fewer steps to locate a fault. Programs can be readilychanged to accommodate a retrofit replacement item.

2. Lesser routines maybe more easily understood bythose responsible for subsequent program maintenance.

3. Less likelihood that a modification to a specificprogram will affect other programs. This reduces thepossibility of maintenance-induced failures.

4. The number of discrete problems possible in aspecific subroutine is often many orders of magnitude lessthan the number possible in the complete overall pro-gram. Thus testing is more manageable. Software modulescan be tailored to the demands of the system components.

5. Software programs can be easily modified basedon operational experience.

6. Imperfections in module software that remainedunidentified up to the time of failure–-although they wereintroduced when the software was originally written orsubsequently modified-can be corrected without impact-ing the complete system.

5-5 DESIGN CRITERIA

5-5.1 MODULARIZATION VERSUS PIECE-PART DESIGN

The general criteria for modularization versus piece-part design are

1. Feasibility. Feasibility criteria often lead to aquick decision; if it is not within the state of the art todevelop a modularized design as an alternative, piece-partdesign wins by default.

2. Life Cycle Cost. When modularization is a feasi-ble alternative, its cost-effectiveness must be consideredas it applies to the life cycle of the item. The cost aspect isparticularly important if a throwaway module is beingconsidered.

3. Compatibility With Logistical Support Plan.Most “module versus piece-part” decisions are made tosatisfy the demands imposed by the logistical supportplan to accommodate the operational demands of thesystem. The requirement for immediate repair at the unitlevel by unskilled personnel demands modular replace-ment (see par. 5-l).

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Guidelines to assist the designer in the “module versuspiece-part’’ consideration are presented in the paragraphsthat follow.

5-5.1.1 General "ModuIe Versus Piece-Part”Criteria

The following are general considerations in the trade-off between modular and piece-part design: (A “yes”answer favors a modular design effort.)

1. Can the design effort be simplified by using pre-viously developed standard “building blocks"? (Assumethat the reliability and input-output characteristics areconsistent with the new system.)

2. Can deployed equipment be upgraded with newerand better functional units that replace older assembliesof component parts?

3. Does modular design take advantage of auto-mated manufacturing methods?

4. Could the module be procured commercially?5. Does modularity provide a more effective distri-

bution of effort among the maintenance echelons?6. Are recognition, isolation, and replacement of

faulty units facilitated by modularization—which resultsin an increase in operational readiness?

7. Will modular design ease the problem of trainingmaintenance personnel? Are fewer and less skilled main-tenance personnel required?

8. Is the use of automatic diagnostics facilitated bymodularization?

5-5.1.2 General Module CriteriaIf a decision has been made to “go modular”, then the

basic philosophy of the modular concept must be pursuedproviding a design that can be maintained at the leastpossible cost, with minimal downtime, and that willrequire lesser skills and less manpower at the unit level.The following guidelines apply:

1. Design equipment so that a maximum number ofmodules can be fault isolated by using the instrumenta-tion provided as part of the equipment.

2. Provide the modules with as much self-fault test-ing and isolation capability as possible.

3. Install modules to be accessed, disconnected. andreplaced and connected without special tools or handlingequipment under ambient conditions.

4. Provide modules that require minimum post-maintenance servicing or calibration.

5. Provide module designs, i.e., encapsulation, thatprotect critical parts from environmental damage duringstorage and handling in forward areas.

6. Design modules to maximize the potential fordiscard-at-failure, rather than to repair, for all modulesplanned for replacement at the unit level.

5-5.1.3 Throwaway (Discard-at-Failure) CriteriaThrowawy (disposable) modules are easy solutions

for difficult and potentially time-consuming maintenanceproblems. The advantages and hazards associated withthrowaway modules are addressed in pars. 5-3.2 and 5-3.3. respectively.

Disposable modules should be designed. manufac-tured. and installed to meet the following criteria:

1. Expensive parts art not discarded because offailure of inexpensive parts.

2. Long-lived parts are not scrapped because offailure of short-lived parts.

3. Low-cost. noncritical, and generally availableitems are typically made disposable.

4. Disposable modules are encapsulated for pro-tection and ease of handling but remain compatible withperformance and reliability requirements.

5. Test procedures, to be applied before disposal,provide clear and unequivocal results.

6. Items are clearly marked for disposal at failureand so indicated in maintenance manuals and supplycatalogs.

7. Discard instructions are documented in opera-tor and maintenance manuals and in supply catalogs.

8. The precious metal parts of disposable modulesare designed for ease of salvage.

9. Parts that can become contaminated are desig-nated for proper protective measures.

10. Devices bearing a security classification aremarked to provide the proper channels for disposal.

5-5.2 FUNCTIONAL GROUPING FORMODLUARIZATION

Functional relationships are used to facilitate fault iso-lation and to provide repair parts that arc compatiblewith the logistical support plan. Thc conscious effort tolocate and package components in self-contained func-tional units facilitates both the operation and mainte-nance of a system (Ref. 2). Techniques emplyed toachieve a functional relationship are

1. Logical flow grouping2. Circuit grouping3. Component grouping4. Standard construction5. Frequency grouping.

Each of these techniques is discussed in the paragraphsthat follow.

5-5.2.1 Logical Flow GroupingLogical flow, as its name implies, is the technique of

grouping components of the overall system to parallel theinputs of individual assemblies or subassemblies as theyfunctionally relate to the overall system. For example, thelogical grouping for an automotive vehicle would be

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power plant, transmission, and power train---the direc-tion of the impetus (power) is from the engine to thepower train, not the reverse.

Guidelines to be followed are1. Package and locate components parallel to their

functional relationships as established by a flow chart.Fig. 5-1 illustrates this idea.

2. Select methods and subassemblies so that only asingle input check and single output check are required toisolate a fault within an item.

5-5.2.2 Circuit GroupingCircuit grouping is the technique of grouping circuits—

electric, hydraulic, or power for a specific function, e.g.,the audio and video circuits of television receivers andrecorders. Guidelines to be followed are

1. Locate all parts of a given circuit or logicallyrelated group of parts—e.g., an automotive transmission- -together in a common volume.

2. Place each circuit within the group in a separatemodule, e.g., a printed circuit board of the plug-in typethat can easily be replaced.

5-5.2.3 Component GroupingComponent grouping is the technique of grouping

together components with similar functions or that pos-sess common characteristics. Guidelines to be followedare

1. Locate items together that perform similarfunctions—e.g., amplification circuits, fuses, and relays.

2. Segregate resistors, capacitors, and transistorsinto a minimum number of different locations on subas-semblies and terminal boards.

3. Group inexpensive components on a single chas-sis to Facilitate throwaway at failure.

4. Group gages and instrument readouts—necessaryfor the control or monitoring of a function—into aninstrument panel to facilitate operator surveillance.

5. Segregate components on the basis of significant

variations in the required maintenance tasks. For exam-ple, items that are to be cleaned by different methods—steam or solvent--should be packaged so that cleaning ispossible with minimum masking.

5-5.2.4 Standard ConstructionStandard construction is a technique that follows no

preconceived set of rules. This construction creates a finalproduct by balancing a number of factors- heat loss,component size, final unit size and weight, and eyeappeal—to arrive at a compromise, which varies for eachapplication of the standard item. Examples of such acompromise are the simple radio receivers available onthe civilian market—they are configured in headsets,hand-held portable radios, clock radios, etc., for consum-er appeal.

5-5.2.5 Frequency GroupingFrequency grouping is the technique of grouping mul-

tiple, similar parts that are likely to require replacement atthe same time. This technique is most beneficial if thedominant failure mode is wear, fatigue, or a similar age-dependent mode. Periodic replacement of assembliesbecause of anticipated failure makes possible a systemthat is, for all purposes, rebuilt.

5-5.2.6 Evaluation of Grouping MethodsAn empirical evaluation of the previously described

equipment grouping techniques was performed by tech-nicians of different skill levels on a simple system and acomplex system (Ref. 3). Based on the performance dataused to make the evaluation—troubleshooting time,amount of information gained per unit of time, techni-cian’s subjective performance, and engineering criteria-it was determined that the logical flow method is superiorto the standard construction method because the logicalflow method clearly enhanced the ease of equipmentmaintenance. The other grouping techniques also werepreferred to the standard construction method; however,

Figure 5-1. Design for Functional Utilization That Corresponds to Modularization

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their superiority was not as clearly demonstrated as thatof the logical flow method.

The selection process is not mutually exclusive how-ever; choosing one particular method does not excludethe subsequent use of another method as the systemevolves e.g., the logical flow method having been decidedupon as the principal technique, circuit grouping could beused to augment the grouping technique.

5-5.3 TYPICAL DESIGN GUIDELINESThe following are typical design guidelines relative to

modularization that will facilitate maintenance:1. Make modules and component parts approxi-

mately uniform in basic size and shape for best packaging.2. Minimize interconnections between neighboring

modules.3. Emphasize modularization for unit level replace-

ment to enhance the operational capability. Modulariza-tion versus part replacement for intermediate and depotmaintenance can be largely determined by cost factors.

4. Standardize modules and receptacles; however,particular care must be taken to prevent inadvertentplugging into the wrong receptacle.

5. Use guide pins for plug-in modules to permiterror-free insertion.

6. Use quick-disconnect hold-down devices to per-mit easy removal.

7. Design all repairable modules so that the rapid,easy removal and replacement of malfunctioning partswithin the module can be accomplished by intermediateand depot level maintenance technicians.

8. Divide equipment into as many units as are elec-trically and mechanically practicable while consideringthe efficient use of space.

9. Use an integrated approach i.e., simultane-ously consider materials, design, and application toachieve cost-effectiveness.

10. Optimize the components of a module for agiven single function rather than for multiple, divergentfunctions—many compromises may be necessary to accom-modate multifunction units, which will render the moduleless than optimum and will defeat the purpose ofmodularization.

11. Design electronic modular units—if indicated tobe in an unserviceable condition by built-in test equip-ment (BITE) or monitor to permit operational testingwhen removed. The immaturity or unreliability of theBITE may have resulted in a false alarm. This is particu-larly important if the module is expensive.

12. Match the physical separation of equipment intoreplaceable units with the functional design of the equip-ment. This will maximize the functional independence ofunits and minimize interaction between units.

13. If a major assembly can be made of two or moresubassemblies, design the major assembly so that a subas-

sembly can be removed independently, i.e.. without re-moval of the other subassemblies. This unitization isespecially vaauable when the subassemblies have varyinglife expectancies.

14. Unless it is structurally infeasible, design allequipment so that rapid, easy removal and replacement ofmalfunctioning components can be accomplished by onetechnician.

15. Where possible, make modules small and lightenough for one person to handle and carry. Removableunits should have a mass less than 18 kg(401b). Units witha mass greater than 4.5 kg (10 lb) should have handles.

16. Consider part weight and size of a module inrelation to its installed location to assure that handlingloads are compatible with both male and female mainte-nance personnel, i.e., from the 5th percentile female to the90th percentile male.

17. Where possible, make each module capable ofbeing checked independently. If adjustment is required,design so that each module may be adjusted independent-ly of other units.

18. Design control levers and linkages so they can beeasily disconnected from components to facilitate moduleremoval and replacement.

5-6 EXAMPLESTypical examples of good modularization techniques

will be1.2.3.4.5.

5-6.1

presented for five areas, namely,Electronic and electrical equipmentSectionalization of missiles and rocketsTank-automotive equipmentModularization in armamentHelicopter engine.

ELECTRONIC AND ELECTRICALEQUIPMENT

Logical flow and circuit packaging are both logicoriented: however, circuit packaging usually is on a levelbelow that of logic flow because, as indicated in par.5-5.2.6, the selection of one technique does not excludethe use of other grouping techniques at subordinate lev-els. Consider, for example, the Army's Targct Acquisitionand Designation System (TADS) (Ref. 4), which featuresthe power supply as a black box (module); one of thepower supplies for the four TADSS is shown in Fig. 5-2.In contrast, the stabilizing control system on the Army'sAdvanced Attack Helicopter (AAH) features the powersupply as a component circuit board within a black box(module). In both cases the black boxex are modulesbased on the logic that supplying power is the first logicalfunction and, at the unit level, they represent identicalmaintenance actions—i.e., replace when unserviceable.However, the AAH black box which is designed formaintenance at the intermediate or depot level—uses the

5-6

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Figure 5-2. Modules in Target Acquisition and Designation System (TADS)

circuit grouping technique for the power component con-tained within the black box. Some other modules com-prising the TADS are illustrated in Fig. 5-3.

Component grouping might subsequently be employedto segregate similar parts into modules. For example, allrelays could be located on one card within a black box.Army missiles often have their gyroscopes segregated by apackaging technique, i.e., a packaged component as amodule, because gyroscopes frequently fail if left dor-mant in storage (Ref. 5).

The Navy has established an extensive program referredto as the “Navy Standard Electronic Module Program”(SEM) (Ref. 6). The SEM (Ref. 6) is based on the princi-ple of limiting redundant design through the use of stan-dard functions, which achieves cost benefits through largeproduction volumes and wide competition. The basicobjectives of SEM are

1. To partition electronic functions so that they arecommon to a majority of equipment applications

2. To document modules that have functional speci-fications (to preclude dependence upon a specific vendor,

design, or technology). This results in long-term availabil-ity and cost savings through vendor innovation andcompetition.

3. To achieve high reliability through stringent qual-ity assurance requirements for module design andproduction.

4. To discard modules upon failure; this is madepossible by high reliability and low cost.

5. To provide flexible, modular, mechanical packag-ing requirements that employ various circuit and packag-ing technologies, and adapt to the various mechanicalconfigurations of the equipment.

6. To ease the logistic support burden on the con-gested supply system by extensive intersystem commonal-ity of a limited number of modules.

The Air Force has investigated the benefits that couldaccrue to their programs through the SEM approach(Ref. 7). Results indicated that the application of SEMconcepts to simulation equipment could prove beneficial.For small and medium production quantities of singletypes of simulators, the SEM version had the lowest life

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Figure 5-3. TADS Turret Assembly—Day Side Modules

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cycle cost.Although the SEM has been applied to the Navy’s

electronic equipment, the circuit modules could be ap-plied advantageously to new Army equipment design.This would provide initial cost savings and possibleimprovement in the availability of repair parts for theNorth Atlantic Treaty Organization.

SEM documentation can be found in1. MIL-STD-l378, Requirements for Employing

Standard Electronic Modules2. MIL-STD-1389, Design Requirements for Stan-

dard Electronic Modules3. Navy Pub 0101-073, Module Design Handbook4. Navy Pub 0101-051, Program Manager's Hand-

book.

5-6.2 MISSILES AND ROCKETSTo facilitate the handling, transportation, and storage,

large missiles and rockets should be divided into func-tionally packaged modules or sections i.e., warhead,adaption kit, guidance, and motor—that are adaptable torapid field assembly. This division allows for the separa-tion of the explosive or other hazardous materials frominert components.

The Army’s HELLFIRE missile system contains amodular guidance package that permits the missile to beused in three different modes. Although this is not modu-larization primarily for ease of maintenance, the HELL-FIRE demonstrates the application of modularization ofa component to permit the use of the basic system indifferent operational modes.

5-6.3 TANK-AUTOMOTIVE EQUIPMENTModularization as it applies to the tank-automotive

area is already well-developed and present in vehicles.Consider the ancilliary equipment related to the engine—plug, distributor, wiring harness, alternator, starter, oilfilter, etc. which are good examples of modularization.Room for improvement exists. Examples of fuel injec-tion, cylinder packages, and total vehicle modularization—discussed in the paragraphs that follow are examples ofhow the design can be improved to facilitate maintenance.

5-6.3.1 Fuel InjectionIn many diesel engines the fuel injection pump can be

factory timed and calibrated; thus it requires no adjust-ments when replacing an unserviceable system. The fuelinjection system can be simply removed and replaced as aself-contained module and, when installed, is ready foroperation. other assemblies that lend themselves to thiskind of no-calibration or -adjustment are prime candi-dates for exploitation.

5-6.3.2 Engine ModulesFig. 5-4 shows the three major modules–-gearbox,

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forward engine, and rear engine—of the M1 turbine tankengine. Each module is a packaged assembly ready forinstallation. These modules significantly reduce the over-haul time previously required to disassemble, clean,inspect, recondition, and reinstall individual components.Special fittings and rigging are provided to remove themodules from their shipping containers for assembly intothe tank as illustrated for the rear engine module in Fig.5-5.

5-6.3.3 Total VehicleA study was undertaken to determine whether a con-

cept of modular design for an automotive vehicle wouldeffect a significant reduction in the maintenance load andimprove the strategic posture of the field Army (Ref. 3).The significant results of this study indicated

1. Maintenance man-hours reduced 27%2. Active maintenance downtime reduced 25%3. Ratio of inactive to active downtime reduced 44%4. Unavailability of trucks reduced 54%.

The modular units designed for quick disconnect andease of replacement were

1. Engine Assembly. (See 1, Fig. 5-6.) Quick-dis-connect fuel line, ignition wiring, starter cable, generatorcable, and throttle controls

2. Transmission. (See 2, Fig. 5-6.) Quick-disconnectcontrols, features to permit ease of separation fromengine by rail guides or lifting device

3. Transfer Assembly. (See 3, Fig. 5-6.)4. Differential and Axle Assembly. (See 4, Fig. 5-6.)5. Propeller Shaft Assemblies. (See 5 and 6, Fig.

5-6.)6. Rear Wheels (Hub and Brake) With Tires. (See 7,

Fig. 5-6.) Quick disconnect on brakes7. Front Wheels (Hub and Brake) With Tires. (See

7, Fig. 5-6.) Quick disconnect on brakes8. Steering Gear Assembly. Quick disconnect.

Real data on fielded trucks were used in a Monte Carlosimulation to compare parameters of a standard vehicleto a modular vehicle. Fig. 5-7 summarizes the results,which indicate

1. Reduction in occupational specialists2. Considerable savings in training costs3. Substantial savings in special tools and test

equipment4. Improvement in the availability status.

5-6.4 MODULARIZATION IN ARMAMENT

This subparagraph describes examples of modulariza-tion in armament equipment, e.g., small arms and can-nons. The first example is the modularization of a func-tional group in a gun; the second example is the modulariza-tion of a complete gun.

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Figure 5-5. Fittings and Rigging for Rear Engine

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Figure 5-6. Modular Vehicle Design

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ConsiderationPERSONNEL

Man-Hours (active maintenance)Man-YearsManpower Utilization (average)

TRAININGNumber of Military Occupational Specialty (MOS)

Types InvolvedTraining Cost per MOS

Total Training Cost

TOOLS AND TEST EQUIPMENT

Special Tools, Kits, Test Equipment

REPAIR PARTSNumber of Federal Stock Numbers (FSNS) Involved

UnitNo. of Type Items StockedQuantity of Items Used

IntermediateNo. of Type Items StockedQuantity of Items Used

AVAILABILITYActive Maintenance DowntimeInactive Maintenance Downtime

StandardVehicle

86804.34

(Undetermined)

3$ 5626.00(2 types)

$22,504.00

$2143.42

1260

(Unknown)1130

(Unknown)2350

833547,510 (est)

(per fleet per year)

Figure 5-7. Data Output Balance Sheet

ModularVehicle

62273.113

12.5%

10

0

0

277 (est)

28 (est)1088

249 (est)2265

625720,223

(per fleet per year)

5-6.4.1 Feed and Eject ModuleCHAIN GUNS are externally powered guns, in

which the ammunition feed is powered by an electricmotor rather than by gas from a fired projectile (Ref. 8).Fig. 5-8 shows the bolt assembly and drive train moduleswithin the CHAIN GUN. Failure in any of the parts of thechain drive module requires only removal and replace-ment of the module.

5-6.4.2 Complete Gun ModularizationFig. 5-9 shows the assembled 5.56-mm M231 Sub-

machine Gun. Its modularization has resulted in simplifica-tion, reduced weight, improved maintainability, andpotentially decreased production costs. Fig. 5-10 illus-trates the modular assemblies. The barrel assembly isdesigned for quick change. These five modular units—except for removal of the barrel from the upper assembly-cannot be reduced further to the usual array of unman-ageable piece parts associated with earlier submachinegun models. The weapon can be disassembled in approxi-

mately 10s without special tools. Compared to previouslyfielded standard submachine guns, the five modular corn-ponents represent a major reduction in parts.

5-6.5 HELICOPTER ENGINEThe complete Lycoming T-53 gas turbine engine is built

of many subassemblies that can be exchanged as completemodules (Ref. 9). In case of extensive damage within onesubassembly, the engine can be made operable again in ashort time by exchanging an entire module. Also it ispossible to replace the combustor assembly, the mainreduction gear, the fuel control, and other accessorymodules. The axial compressor housing is split lengthwiseso that compressor vanes or blades can be replaced.

The design of the T-53 permits removal of the entirecombustor assembly, which contains the power turbinewithin the exhaust diffuser, by simply unbolting at thecombustion chamber flange—an operation accomplishedwithout removing the engine from its mounting in theairframe. In this way all hot end parts—turbine blades,

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Figure 5-8. CHAIN GUN Bolt Assembly and Drive Train Modules

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Figure 5-9. 5.56-mm M231 Submachine Gun

Figure 5-10. Modules for 5.56-mm M231 Submachine Gun

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fuel vaporizing tubes, nozzles, rotors, combustion cham-ber liner, etc.—can be replaced or inspected. In one casethe “hot end” inspection was carried out during fieldmaneuvers. An inspection revealed a defective oil seal.The inspection was completed, the seal replaced, and theaircraft made ready for operation in less than 4 h.

5-7 MODULARIZATION CHECKLISTTable 5-1 summarizes the important design recom-

mendations to be considered when designing for modu-larization. The checklist contains several items that werenot discussed in the text. They are included because theirnecessity in the design is so obvious they might otherwisebe overlooked. In using the checklist, if the answer to anyquestion is “no”, the design should be reexamined todetermine the need for correction.

TABLE 5-1. MODULARIZATION CHECKLISTA. GENERAL

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

5-16

Is the equipment divided into as many modular units—mechanical, electrical, electronic—as practical in keepingwith the effective use of space and overall equipment availability requirements?If all components of a module except one or two are extremely reliable, has consideration been given to unitizingthe module with the unreliable components removable from the exterior of the package? Has consideration beengiven to improving the reliability of the unreliable items?Has an integrated approach—considering simultaneously the problem of materials, component design, andapplication of the modular concept—been used?Are modules and component parts approximately uniform in basic size and shape for best packaging whenfeasible?Do the modular units contain components that are optimized for a given function rather than providingmultiple, divergent functions?Do the modular units permit reliable operational testing when removed from the equipment and require little orno adjustment after replacement?Does the physical separation of equipment into replaceable modules match the functional design of theequipment?Where an assembly can be made up of two or more module subassemblies, does the major assembly consist ofmodules that can be removed or serviced independently without removal of the other modules? NOTE: This isparticularly important when the modules have widely varying life expectancies.Have modules been designed so that the rapid and easy removal and replacement of malfunctioning units can beaccomplished by one technician unless it is structurally or functionally not feasible?Where possible are units small and light enough for one person to handle and carry? Do units have a mass lessthan 18 kg (40 lb)? Do units that have a mass greater than 4.5 kg (10 lb) have handles?Is each module capable of being checked independently? If adjustment is required, is the module designed so thatit can be adjusted independently of other units?Has the modular concept for major subsystems and components of vehicles—to permit replacement as a unit orbeing repaired or tested outside the vehicle or parent item—been considered?Have control levers and linkages, and other hookups been designed so that they can easily be disconnected, andeasily and correctly reconnected, from the modules and thus enhance the modular concept of ease ofmaintenance?Has modularization been emphasized to permit maintenance at the operational (unit) level to enhance opera-tional availability?Are standardized modules and receptacles used to facilitate replacement? Is particular care exercised to preventinadvertent plugging into the wrong receptacle? Has receptacle coding by shape, size, or color been employed?Are quick-disconnect holddown devices used to permit easy removal and replacement of module?

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B.

1.

2.

3.

4.

5.

6.

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TABLE 5-1 (cont’d)

THROWAWAY DESIGN1.2.

3.4.

5.6.

7.

8.

9.10.

Does the failure of inexpensive parts result in the disposal of an expensive module?*Does the failure of short-lived parts result in the disposal of long-lived parts?*Are low cost and noncritical items made disposable?Are all encapsulated modules designed for disposal at failure? If not, has the module been designed to permitaccess for repair at intermediate or depot level?Is the maintenance level and or criteria, i.e., time to repair and, or cost of module- clearly specified?If test procedures are to be applied before disposal, are they clearly specified, and do they provide clear andunequivocal results?Does the module identification plate or marking contain the statement “Dispose at Failure”? Do the repairmanuals and supply catalogs so indicate?If the module contains precious metals that should be recovered, is the module designed for ease of salvage ofthese components? Is module labeled to indicate that it has salvageable components? Do manuals so indicate?If module contains contaminants, is it so labeled and instructions provided in pertinent manuals for its disposal?Are modules bearing a security classification properly labeled, and are instructions provided in pertinentmanuals for their disposal?

*A yes answer to these questions indicates a need to reexamine the design for corection.

REFERENCESRobert H. Dunn and Richard S. Ullman, ModularilyIs Not a Matter of Size, 1979 Proceedings of AnnualReliability and Maintainability Symposium, 23-25January 1979, p. 342.AMCP 706-133, Engineering Design Handbook,Maintainability Engineering Theory and Practice,January 1976.R. I. Moeller and R. E. Redfern, Materiel SupportSystem Model-Automotive, Annals of Reliabilityand Maintainability, Fourth Annual Reliability andMaintainability Conference, 28-30 July 1965, p. 481,B. J. Baskett, C. M. Tsoubanos, V. M. Weiner, “TheTADS/PNVS 'Eyes’ for the AH-64 Attack Helicop-ter”, Vertiflite, American Helicopter Society, Novem-ber December 1980.

Storage Reliability of Missile Material Program, USArmy Missile Command, Huntsville, AL, June 1974.

NAVSEA Journal, September 1975, p. 28.

7.

8.

9.

Robert Laskin and William L. Smithhisler, The Eco-nomics of Standard Electronic Packaging, 1979 Pro-ceedings of Annual Reliability and MaintainabilitySymposium, 23-25 January 1979, p. 69.Brochure on 25-mm M242 Automatic Cannon, Ord-nance Marketing, Hughes Helicopter. Subsidiary ofMcDonnell-Douglas, Culver City, CA.

Stanley Duhan, Maintainability at Work—The T-53Gas Turbine Engine, Annals of Reliability and Main-tainability, Fourth Annual Reliability and Main-tainability Conference. 28-30 July 1965, p. 501.

BIBLIOGRAPHYLyle R. Greenman, Is Maintainability Keeping Up With

Electronics?, Proceedings of the 1976 Annual Reliabil-ity and Maintainability Symposium, 20-22 January1976, p. 157.

NAVORD OD 39223, Maintainability EngineeringHandbook, 1 February 1970.

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CHAPTER 6IDENTIFICATION AND LABELING

This chapter emphasizes the importance of the proper identification of parts and the labeling of function toimprove equipment maintainability. basic principles associated with labeling are discussed, and the subse-quent presentations are devoted to the application of these principles. Methods of labeling, positioning, andlabel design to include letter fonts and sizes, colors, and color contrasts are presented. an identificationchecklist is provided.

6-1 INTRODUCTIONThere are four distinct aspects of identification and

labeling, namely,1. What marking is required or necessary?2. What information should the mar-king contain?3. How should the marking be applied?4. Where should the marking be applied?

These four aspects are covered in detail in subsequentparagraphs.

Labels, legends. placards, signs, or markings should beprovided wherever it is necessary for an operator or tech-nician to identify, interpret, follow procedures, or avoidhazards in the use or maintenance of systems or equip-ment except where it is obvious to the observer what anitem is and what to do about it. The proper identificationand labeling of equipment components, parts, controls,instruments, and test points simplify the technician’s taskand reduce both the task time and risk of error. However,identification, when considered in isolation, does notconstitute maintainability; the fact that an item is ade-quately labeled does not mean that it can be maintained.

Melvin S. Majesty (Ref. 1) states, “... analysis of 600recent rocket engine Failure and Consumption Reportsby Chase and Tobias ( Ref. 2) showed that 35% of thefailure reports indicated equipment failure or malfunc-tion directly attributable to human interaction with theequipment during maintenance.” The study indicatedthat the error rate would have improved (1) by unambig-uous identification of parts and equipment functioning,which would have minimized the need to reference repairmanuals and other data sources. and (2) by properlylabeled positions where equipment is installed. operatingprocedures. and operational limits.

Identification can be defined as the adequate markingor labeling of parts, components. controls. and test pointsto facilitate repair or replacement during maintenanceoperations (Ref. 1). Proper identification is present if thecomponent is readily identified for repair, replacement,or service with minimum effort by the technician

Included in identification is the process of determiningwhat markings are required to identlify a part correctly or

to designate a function; it also determines the best methodfor accomplishing the process. Identification markingsusually are identified with instruction plates, function oroperation information, and caution or warning signs ap-plied directly to the item. whereas labels are usually iden-tified with the precise nomenclature or function of theitem, or they are diagrammatic instructions for the opera-tion or maintenance of the equipment. Labels frequentlyare used on the exterior of access locations to describe theequipment or components to be accessed, to help reduceaccess time, and to eliminate possible confusion duringmaintenance operations particularly if the opening pro-vides access to several similar components. For example,a circuit diagram on the inside of an access cover caneliminate the need to obtain a manual, which simplifiesthe acquisition of necessary data during maintenance.Regardless of whether the identification is marking orlabeling, the same guidelines apply.

Since materiel may be exported or integrated into theNATO forces, markings should conform to appropriateinternational specifications and labeling—i. e., size, color,symbols, and units of measure.

The information in this chapter relative to purpose,use, format, and size of labels and identification imple-ments the general guidance expressed in pars. 9-3.1 and9-4, Chapter 9, “Human Factors”. Labeling and identifi-cation are not exact sciences; the guidance presented inthis chapter and the referenced documents are the resultof experience, observation, and lessons learned.

The referenced Military Standards and Military Speci-fications do not reveal the complete story with regard tolabeling and identification, e.g.. MIL-STD-130 (Ref. 3)refers to many other specifications. To include additionalspecifications with each reference would be cumbersomeand would clutter the text; accordingly, a bibliographyhas been included to augment the references. To gain afull appreciation of the art of labeling and identification,the reader is urged to refer to the references and bibliog-raphy in implementing the guidance contained in thischapter.

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6-2 BASIC CHARACTERISTICSThe characteristics of the label, legend. or marking

should be determined by such factors as (Ref. 4)1. Accuracy of identification required2. Time available for recognition and other responses3. Distance at which device must be read4. Illumination level and color characteristics of the

illumination5. Criticality of the label, legend, or marking.

Labels and identification should conform to these prin-ciples (Ref. 4):

1. Labels should give the user the informationrequired to assist in performance of the task.

2. Labels should be located consistently through-out the equipment.

3. Labels should use familiar words; avoid overlytechnical or difficult words.

4. Labels should be brief and unambiguous; omitpunctuation.

5. Labels should be printed to read horizontally leftto right, not vertically unless the device demands it.

6. Labels should be supplemented where necessarywith other coding procedures such as color and shape.

7. Labels should be placed where they can be seeneasily, not where other units in the assembly will obscurethem.

8. Labels should be large enough so that the opera-tors and technicians can read them easily at the normalworking distance.

9. Generally, labels should be printed in capitalletters; however, if a label has several long lines. use bothupper- and lowercase letters.

10. Labels should be printed in boldfaced lettersonly for short words or phrases that require emphasis.

11. Labels should be placed on or very near the itemsthey identify; eliminate confusion with any other itemsand labels.

12. Labels should be etched, molded, or embossed.where practical, into the surface for durability rather thanstamped, printed, or stenciled. Decals are acceptable butless desirable.

Subsequent paragraphs will expound upon these char-acteristics and principles.

6-3 TYPES AND USES OFIDENTIFICATION

6-3.1 GENERALVarious types of markings are used on equipment.

parts, and diagrams to assist operators, technicians, andsupply personnel. The purposes of these markings are to

1. Assign a unique nomenclature describing theitem—name, model number, serial number for logisti-cal and accountability controls. Definitions should be inaccordance with MIL-STD-280 (Ref. 5).

2. Identify function.3. Indicale operational and hookup or connection

instructions to minimize error during operation andrepair and to reduce the need to refer to manuals forcritical information.

4. Indicate hazardous conditions.The manner in which these purposes are implemented ispresented in the paragraphs that follow.

6-3.2 EQUIPMENT IDENTIFICATIONEquipment in thiis context, to distinguish it from minor

subassemblies and or parts, refers to items issued for aspecific tactical or administrative role e.g. , a tank,truck, artillery piece, or radio. Major subsystems, e.g.,laser range finder for a tank, would also qualify as equip-ment. (Part identification and marking are discussed inpar. 6-3.5.) Equipment should be marked in accordancewith MIL-STD-130 (Ref. 3) with a permanent identifica-tion e.g., stamping, engruving, molded in, attachedplate, or decal. The identification label should be securelyattached to the equipment and be resistant to water, oil.fuel, cleaning solvents, corrosion, and wear. The perman-ent plate should allow for a revision in model numberwhen equipment has been retroofitted, e.g., 8-in. Gun.M101, to 8-in. Gun, 101A1.

Markings should contain the following information(Ref. .3):

1. Item nomenclature and type designation2. Design activity Federal supply code for manu-

facturers (FSCM) or NATO supply code for manutactur-ers (NSCM)

3. Manufacturer’s identification the manufactur-er’s name, FSCM, or NSCM, which identities the place ofmanufacture

4. Procurement instrument contract or purchaseorder identification number

5. Serial number a unique notation that idenfies itsa single unit of a family of like units; normally assignedsequentially (set Ref. 5)

*6. US to denote Government ownership*7. Special characteristics pertinent rating, oper-

ating characteristics. and other information neccssary foridentification of item

*8. NATO and national stock number ( NSN)*9. Configuration item identifier (CII) the number

assigned to identify a configuration item. When assigned,it is the unchanging base number to which serial numbersare assigned.The items marked with an asterisk (*) are shown onlywhen specified in the contract or purchase order. Fig. 6-1is an example of an identification plate.

If the item is warranted by a contract Statement ofWork or other contract clauses, the nature of the war-ranty should be conspicuously displayed by a label. Fig.6-2 is an example of a warranty marking.

6-2

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DOD-HDBK-791(AM)

Figure 6-1. Example of Identification Plate (Ref. 3)

Figure 6-2. Example of Warranty Marking (Ref. 3)

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6-3..3 EQUIPMENT FUNCTIONAL AND/ORINSTRUCTIONAL MARKINGS

No specific guidelines or criteria exist for determiningwhen to install functional and or instructional markingson equipment. Instruction plates are not required whenthe steps to be taken are obvious to the average mechanic.Conversely, instructions should be provided when it isdetermined that a technician trained on past equipmentwould have a high risk of error if he applied the samemaintenance procedures to the new equipment. Accord-ingly, each equipment item should be evaluated carefullyto determine what instructions are required and theproper content of these instructions. It is safer to overin-struct than to underinstruct. Basically, the decisioninvolves the evaluation of the anticipated maintenanceoperation to determine what information the technicianrequires. For example, a simple operation such as chang-ing a drive belt should be illustrated if the belt is crossedover. If reference to data can be eliminated by placingsimple instructions on the equipment, an instruction plateor diagram is warranted. Areas that merit considerationare basic operating instructions; calibration data; simplewiring or fluid flow diagrams; adjustment instructions;location of test points: location of piece parts on elec-tronic circuit cards; valve and ignition settings; types offuel, oil, and lubricant to be used; and other data requiredto perform routine servicing and maintenance. Guidelinesfor the placement of instruction plates are provided inpar. 6-5.2.

When considered important, the identification of per-tinent data with regard to function, capacity, capabilities.limits, ranges, frequencies, voltage, current. power, revo-lutions per minute, weight, etc., should be indicated. Thedata may be displayed on a separate plate or integratedwith the nomenclature plate as shown in Fig. 6-1. To becurrent on instruction plate identification, consult thelatest revision of MIL-P-514 ( Ref. 6).

6-3.4 HAZARDOUS CONDITIONMARKINGS

Hazards and associated risks should be eliminated bydesign wherever possible. Components should be locatedso that required access during operation, servicing, main-tenance, or adjustment minimizes personnel exposure tohazards. When alternate design approaches cannot elimi-nate the hazard, only then should warning and cautionlabels be displayed (Ref. 7). The use of warning andcaution markings should not be used as a substitute forproper engineering design.

Hardware, per se, is not the only source of hazards. Ananalysis of work areas and maintenance operationsshould be made by human factors personnel and indus-trial safety personnel to identify hazards that may resultfrom unsafe acts by technicians and maybe introduced by

signs is1. Install appropriate labels to remind the techni-

cian that he must consult a technical manul be before work-ing on equipment.

2. Erect high visibility warnings it peronnel maybe subjected to harmful gases. noise levels, or suddenincreases or decrcases in pressure, laser beams, electro-magnetic radiation, or nuclear radiation.

3. Identity areas of operation or maintenance inwhich special protective clothing, tools, or equipmente.g., insulated shoes, gloves, suits, hard hats, and orbreathing masks are necessary.

4. Mark all electrical receptacles with their voltage,phase, and frequency characteristics, as appropriate; spe-cific details are contained in MIL-STD-454 (Ref. 8).

5. For aircraft, missile, and space sytems clearlyand unambiguously label pipe, hose, and tubing for fluids(gas, steam, hydraulic fluids), and label or code them as tocontents. pressure, heat, cold, or other hazardous proper-ties in accordance with MIL-STD-1247 (Ref. 9). Markand color code pipelines for other systems in accordancewith MIL-STD-101 (Ref. 10).

6. Provide “NO STEP” markings where necessaryto prevent injury to personnel or damage to equipment.

7. Labeljacking and hoisting points conspicuouslyand unambiguous). and describe any special handlingrequirements for these operations.

8. Distinctly mark the center of gravity and theweight of equipment where applicable.

9. Indicate weight capacity on stands, hoists,cranes, lilts, jacks, and similar weight-bearing equipmentto prevent overloading.

10. Prominently display labels instructing the tech-nician in hazardous situations, for example, instructionsfor sequential operations as shown in Fig. 6-3 (Ref. 11).

11. Make caution and warning signs as informativeas possible, yet they should be consistent with limits oninformation required and space available (see Fig. 6-4).The content of the label will vary, but it should informpersonnel

a. Why a hazardous condition existsb. Places to avoidc. Behavior to avoidd. Sequence to follow to obviate a dangere. Where to refer for additional specific informa-

tion.The language level of the signs should be consistent withthe reading level of the target audience.

6-.3.5 PART IDENTIFICATIONIdentification is essential throughout the life of any

part. The nomenclature, with its associated number

6-4

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Figure 6-3. Placement of Labels forHazardous Tasks (Ref. 11)

Figure 6-4. Examples of Informative Labels(Ref. 11)

DOD-HDBK-791(AM)

manufacturer and or National Stock Number (see par.3-6.9) uniquely defines the item for ordering from themanufacturer. supply, and inventory control.

When parts cannot be physically marked because oflack of space, e.g., too small, or because marking wouldhave a deleteriouss effect. the information specified in par.6-3.2 should be marked on the container (Ref. 3) in addi-tion to the identification marking information specified inMIL-STD-129 (Ref. 12). Where polarity or impedancerating are critical, the identification should so indicate.Some parts, because of their small size, e.g., resistors, arecolor coded to reveal their ratings and quality. For exam-ple, a gold marking on a resistor indicates ±5% of its ratedvalue, a silver marking, ±10%.

A part number and the drawing number detailing thepart are the same. As a drafting practice, each part on thedrawing or schematic should be keyed to the descriptionof the part shown elsewhere on the drawing or schematic.A wiring diagram prepared in accordance with the sche-matic should carry identification for wire, sockets, plugs,receptacles, resistors, transistors, capacitors, etc. Termi-nals on all assemblies and parts should be suitablymarked, and the wiring should have all terminal mark-ings. Circuit cards should show the part number or a codeto assure the correct positioning of parts to be affixed tothe card. Each mechanical part that will require repair orreplacement must be identified by a unique name andnumber.

6-4 METHODS OF LABELINGThere are numerous marking processes available, and

each one has advantages and limitations. A marking pro-cess should be selected only after a review of the partdesign (material, type of construction, marking spaceavailable, and environment in storage and use); the typeof surface to which it will be applied; the best location forvisibility or durability; and any remarking requirementsthat may result from engineering changes. Markings anddesignations are applied either directly to the item—i.e.,part, framework, panel, chassis, or end item—or by at-tachment of separate plates bearing the desired designa-tions. The commonly used methods of applying the char-acters are ink stamping, steel stamping, engraving, mold-ing-in, decalcomania transfer, stenciling, photoetching,metal plates, tags, photocontact, screen printing, andadhesive-backed labels of metal or plastic. Special fea-tures that pertain to each of these processes, their limita-tions, and preferred types of applications are described inthe subparagraphs that follow.

Despite the fact that this is a maintainability hand-book, the production advantages and disadvantagesassociated with the various application methods areincluded because—since labeling is such a seeminglyunimportant consideration—they may not be well-known. This information will enable the maintainability

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engineer to engage in a meaningful dialogue with theproduction engineer relative to trade-offs. If any of sev-eral labeling methods will satisfy the labeling require-ment, the method chosen should be the one that simplifiesthe production task; conversely, if only a particular-method will satisfy the requirement, the maintainability’engineer must insist on the method regardless of theincreased production effort.

them ineffective where the item is engraved. this methodalso may produce sites susceptible to stress to which thematerial may otherwise be immune. It can be applied tononwearing surfaces that are too thin for steel stamping.The "electric pencil" marking device is one type of engrav-ing tool that is often used for marking metal parts. Thismethod is not generally accepted for formal identificationpurposes because the resulting label may be illegible.

6-4.1 INK STAMPINGThis method is widely used, primarily for piece parts. In

this technique ink is applied with a rubber stamp. Water-er petroleum-soluble inks should not be used. Ink stamp-ing by hand can be awkward, especially when the lengthof the stamp is much greater than the height.

The chief production advantages of this method arethat it is easy to apply and can be changed during manu-facture or in the field. The maintainability advantages arethat the process does not alter the surface finish, and it canbe used in small, restricted areas. The disadvantages arechiefly lack of durability, the characters may not besharply defined, the stamp does not conform to unevensurfaces, ink is subject to smudging, and ink can be ap-plied unevenly.

6-4.2 STEEL STAMPINGThis method of marking provides identification of

mechanical parts at a low cost; it is permanent and notsubject to deterioration in any environment. However,care must be taken to assure that the stamping proceduredoes not damage the part. Steel stamping will penetrateanticorrosion coatings and painted surfaces and willrender them ineffective where the item is stamped. It mayalso produce sites susceptible to stress corrosion to whichthe material may otherwise be immune. This methodcannot be used on surfaces that are subject to wear or onitems that are installed through a gland or packingbecause the roughened surface will cause damage.

One of the more common uses of steel stamping is toadd specific identification data to preprinted metal identi-fication plates, which are attached subsequently to pro-duction run hardware. The plates may be brass or stain-less steel (avoid bare, dissimilar metals in contact witheach other because of galvanic action), printed or reverse-etched, with spaces allocated for stamping the part identi-fication information on as one of the final steps in themanufacturing process.

Declacomania transfer (decals) are a printed design orcharactes on thin plastic material that is mounted on apaper backing for handling and storage. It is applied bysoaking in water until the transfer is loosened from thepaper and then carefully sliding the transfer off the paperand into place on the item. The transfer is held on thesurface by a lacquer-type adhesive. some decals are of theopen-letter type, i.e., individual letters and numerals,which apermit on-the-spot composition of lebels. An over-coating of lacquer or clear varnish—compatible with thefinish coating on the part—usually is applied to add sta-bility to the composed label. Decals are not wear resistantand are not acceptable for areas subject to repeated han-dling or to any type of abrasion. They can be applied tometal, glass, plastic, or organically finished surfaces.Decals with water-soluble coatings should not be used.Additional advntages of decals are that they can be ofany size and can be multicolord.

6-4.3 ENGRAVINGEngraving is the act of cutting characters into the sur-

face with a tool; it has the same wearing qualities as steelstamping. However, the cost of engraving is high andproduction is slow. Engraving will penetrate anticorro-sion-coated surfaces and painted surfaces and will render

6-4.4 MOLDING-INFor flat surfaces requiring no additional marking or

work after casting, molding-in is an effective and inex-pensive method of permanently identifing a part. Theidentification is permanent. When used with sand cast-ings, care must be exercised to ensure that the identifica-tion is not washed out in the molding process. A washoutmay make numerals such as 0, 6, and 8 indistinguishablefrom each other or completely eliminate the partidentifier.

6-4.5 DECALCOMANIA TRANSFER

6-4.6 STENCILINGStenciling is applying ink to an item through an outline

of the characters the stencil. It is it common method formarking the outside of shipping containers and is oftenused for vehicle identification. Stencils are made of heavy,treated paper or thin plastic, and the characters arepunched out by a stencil-cutting machine. Stenciling is arelatively slow process, but stencils can be applied touneven surfaces and to any material if the stenciling ink iscompatible with the surface finish.

If stenciling is to be applied to plastic, a cover coatingallowing the stencil to take usually may be omitted pro-vided the stenciling ink meets the requirements of FederalSpecification TT-1-1795 (Ref. 13). For some plastics.however, that exhibit a slick surface , e.g., polyethylene. acover coating is necessary.

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6-4.7 PHOTOETCHINGPhotoetching—a photomechanical process by photo-

graphing an image on a metal plate and then etching—canbe applied to metal surfaces. Reverse photoctching isnormally specified for metal marking. In this process thecharacters are printed or drawn to an enlarged scale and aphotographic reproduction, reduced to proper size, ismade on the surface to be marked. The characters arethen treated to make them resistant to the etching agent,acid. When acid is applied, the unprotected area (back-ground) is etched out, which leaves raised characters. Thephotoetching process is frequently used for makingstandard identification plates that are later stamped toinsert information. for the specific item to which it isattached.

6-4.8 METAL PLATESMetal plates are generally used to label vehicles, tanks,

aircraft, and ground support equipment. The plates aremarked with the categories of required identificationdata, and spaces are left for inserting the data for thespecific item of equipment. The metal plate can be at-tached with screws, rivets, or adhesive, as desired. Thechosen method of attachment should be compatible withthe operational environment anticipated for the item. Theequipment, plate, and fasteners should be of the samemetal unless they have a protective coating to preventcorrosion resulting from galvanic action.

6-4.9 TAGSTags are pieces of paper, plastic, or metal that are

attached to an item when it is not possible or convenientto apply information directly to the item. Tags are oftenused for attaching shipping information to a loose item,and they can be used to carry imprinted identificationinformation when the item is too small to accept thenecessary characters on its body (see par. 6-3.5). Usually,the tag is used only during stocking and shipping and isremoved at the time of installation. It is common practiceto mark failed items by attaching a rejection tag thatremains attached until the item is repaired. The disadvan-tages of tags are that they can get in the way of work, canbecome tangled with other items, and can be easily sepa-rated from the item.

6-4.10 PHOTOCONTACTThe photocontact process should be used where preci-

sion markings are required, e.g., on dials. This processexposes a sensitized surface by means of a photonegativecontaining the desired legend. The exposed surface isprocessed to develop the image. This process can be ap-plied to metallic and nonmetallic materials. An advantageis that a lot of information can be displayed in a smallspace.

DOD-HDBK-791(AM)

6-4.11 SCREEN PRINTINGScreen printing (silk screening) is a relatively inexpen-

sive method for marking parts. It can be applied to a widevariety of materials and items, and almost any size of itemcan be marked. It is particularly suitable and economicalfor small and medium production runs. Screen printing isa paint process and, therefore, should not be used on thehandling or wear surfaces of controls. This process isuseful for applying function data to valves, and controland dial labels to control panels.

6-4.12 ADHESIVE-BACKED LABELSAdhesive-backed plastic or metal labels are a common

method for marking items for function and for markingpart locations on equipment. Photoetched metal platesused for part identification are often applied with anadhesive, rather than with screws, whenever the environ-ment allows this method. Adhesive-backed labels may beapplied on painted surfaces and directly on metal orplastic.

6-5 LABELINGLabels are lettered or diagrammatic indications of the

name, identifying number, and function of equipment;they are affixed on or near the relevant equipment orfunction. They may include lettered warning signals andabbreviated instructions (both lettered and diagrammatic)relating to the operation or maintenance of the equip-ment. Operation and maintenance instruction manualsare not often readily available at the equipment, andmeaningful labeling can be a satisfactory substitute inmany instances. When evaluating the need for and con-tent of labels, it is usually better to overlabel than tounderlabel. The characters, markings, and symbols onlabels and signs should remain sharp, have high contrast,and be resistant to wear.

The subparagraphs that follow present recommenda-tions for organization and wording, displacement andpositioning, size, and color of labels. The discussion onplacement and positioning also presents some recom-mendations for the application of specific types oflabels--nomenclature, warning, instructions, and identi-fication of specific subsystems.

6-5.1 LABEL CONTENTThe label content must be consistent with the criticality

of the information that it attempts to convey—i.e., partidentification, function of the equipment, or hazards asindicated in par. 6-2. The label content also will be deter-mined by equipment function, i.e..

1. Describe in terms that the typical observer, opera-tor, or technician understands. Engineering characteris-tics, nomenclature, or other terminology should only beused when a commonly understood term does not exist.

6-7

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2. Label each control and display.3. Indicate the functional result of the control

movement—e.g., increase, decrease, slower. Where appli-cable, the label may include calibration data. Labelsshould be visible during operation (see par. 6-5.2). Thefunctional result term should be at the end of an arrow.

4. Label instruments in terms of what is being mea-sured or controlled; the use and purpose should also beconsidered.

5. Indicate functional relationships when controlsand displays must be used together, e.g., adjustmenttasks.

6. Avoid highly similar names for different controlsand displays.

Once the purpose or intent of the label has been deter-mined, the following guidelines are applicable for display-ing the information:

1. Make label content brief but explanatory– use asfew words as possible to convey the intended meaning.Special markings or symbols, e.g., arrows and pictorials,should be considered only when they will unambiguouslyconvey meaning in a more direct manner than severalwords. Labels should only provide reminders, as shown inFig. 6-5, for the trained technician; they need not providecomplete instructions.

2. Use abbreviations only when they will be mean-ingful in the overall statement. When used, standardabbreviations should be selected in accordance with MIL-STD-12 (Ref. 14), MIL-STD-411 (Ref. 15), MIL-STD-783 (Ref. 16). If these references specify the same abbrevi-ation for more than one function, such abbreviationshould not be used for more than one function. If a newabbreviation is required, its meaning should be obvious tothe intended observer (Ref. 4).

3. Itemize—when a label lists a number of steps to beperformed in an established sequence—each step, ratherthan compose the steps, in narrative form as illustrated byFig. 6-6.

Figure 6-5. Example of Label Brevity (Ref.11)

6-8

Figure 6-6. Example of Step-by-StepInstructions

4. Make part identification and equipment labelsconsistent with both the technical manuals and supplycatalogs.

5. Compose labels so that they read from left to rightas indicated in Fig. 6-7. Vertical arrangements should beused only when they are not critical for personnel safetyand task performance and where space is limited. Whenused, vertical labels should read from top to bottom (Ref.4).

6. Specify directional arrows that are as clearly rec-ognizable and identifiable as possible when read at adistance. The direction of arrows with sharp angles andclean lines, as illustrated in Fig. 6-8 (Ref. 9), is less easilymisinterpreted at a distance than that of arrows withwider angles and broader overall width-to-angle ratios.

The following Military Standards and Military Speci-fications should be referred to to insure that the format ofthe label content is consistent with the military require-ments:

1. MIL-STD-130 (Ref. 3)2. MIL-STD-195 (Ref. 17)3. MIL-STD-411 (Ref. 15)

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Figure 6-7. Example of Horizontal Labels toFacilitate Reading

Figure 6-8. Examples of Preferred ArrowShapes (Ref. 11)

6-5.2 LABEL PLACEMENT ANDPOSITIONING

The positioning of labels and information is important.Labels, signs, and markings should be positioned so thatthey are visible from the normal or expected observer’s

viewpoint reference when he needs to see them, i.e., theobserver should not be required to assume an unusualposition to see a label. The following guidelines should beapplied in the determination of the proper location forlabels and markings:

1. Locate identifying labels for a major assemblya. On main chassis of the equipmentb. Externally in a position so that the label is not

masked by adjacent assemblies or componentsc. On flattest, most uncluttered surface available.

Avoid positioning of labels on curved surfaces, particu-larly surfaces with sharp radii because—when viewedfrom the side–-the entire label may not be in view.

2. Place labels consistently above or below a controlor display on a given panel. The “above” position ispreferred except when the panel is located considerablyabove the observer’s eye (Ref. 4). Fig. 6-9 illustrates the“above” position arrangement.

Figure 6-9. Example of ConsistentPositioning of Labels

6-9

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DOD-HDBK-791(AM)3. Locate label so that a control handle will not

mask the label as illustrated in figure. 6-10 (Ref. 4).4. do not locate label on a control that could rotate

the label to an upside down position as illustrated in Fig.6-11 (Ref. 4).

5. Do not place labels near the floor, as illustrated byFig. 6-12 (Ref. 4), or other positions that preclude theobserver from getting his eye in an adequate position forreading the label.

6. When a frame is used to inclose a functionalgroup to define its boundaries, center the label at the topof the group either in the frame or just below the frameboundary as illustrated in Fig. 6-13. 7. Place labels where they will not become obliter-

ated by grease, filings, dirt, or moisture. If a label ispartularly susceptible to being obliterated by materialsdropping from above or masked by manuals placed onthe surface, locate the label on a vertical surface as illus-trated by Fig. 6-14. 8. When the direction of motion to perform a func-

tion or the position of a control is critical to an operation.locate labels to indicate the direction of motion or condi-tion as illustrated in Fig. 6-15.

Figure 6-10. Examples of Method ofPositioning Labels to Avoid Masking byControl Handles (Ref. 4)

Figure 6-11. Example of Label PlacementWhich Can Result in Upside-DownPosition (Ref. 4)

6-10

Figure 6-12. Example of a Poor Placementof Label (Ref. 4).

Figure 6-13. Example of Label Location fora Panel

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6-5.3 LABEL DESIGN

Figure 6-14. Example of Placement of Labelon Vertical Surface

Labels and placards should be designed for easy andaccurate reading at the expected reading distances, vibra-tion or motion environments, and illumination levels. Toachieve ease of reading, the following factors must beconsidered:

1. Color contrast between lettering and immediatebackground

2. Height, letter width, stroke width, spacing, andstyle of letters and numerals

3. Method of application—e.g., adhesives, etching,decal, silk screen.Each of these factors is discussed in the paragraphs thatfollow.

6-5.3.1 Color Contrast and Background (Ref. 4)Label background colors should contrast visually with

equipment background specified in MIL-STD-1473 (Ref.20). No special additional background for the labelshould be used on the equipment without approval of theprocuring authority. Placards or signs that include theirown independent background should provide maximumcontrast between lettering and immediate background.Shiny metallic backgrounds should not be used for opera-tional labels, placards, signs, or markings.

Instruction plate markings should be printed in whiteletters on a black background. Caution plates (see Fig.6-16) or decals should be printed in yellow letters on ablack background in conformance with AR 385-30 (Ref.21). Danger signs (see Fig. 6-17) should be printed inwhite letters on an oval red background upon a blackbackground (Ref. 22).

Figure 6-15. Labels for Valve Controls(Ref. 11)

Figure 6-16. Example of a Caution Sign

(Ref. 21)

Figure 6-17. Example of a Danger Sign(Ref. 22)

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6-5.3.2 Letter and Numeral Size and Font(Ref. 4)

Type style (font), letter or numeral width, characterstroke width, stroke continuity, character spacing, wordspacing, line spacing. and character height all contributeto the ease with which a label may be read. Each of thesefactors is discussed in the paragraphs that follow.

6-5.3.2.1 Type StyleTo identify the type to be used for label composition.

specify the style, the name, and the height in units oflength. The printer’s measure of height, the point = 1/72in., is an ambiguous measure for designating height thesame point size of a given style may be interpreted incor-rectly, e.g., a lowercase “x” has a different point size thana lowercase "f".

Letters and numerals should be of simple style withoutserifs except as may be necessary to distinguish betweencharacters that would otherwise be confused, e.g., I and 1.Acceptable styles are listed in Table 6-1 (Ref. 4).

6-5.3.2.2 Letter or Numeral Width (Ref. 4)The width-to-height ratio should be between 3:5 and

1:1 for all characters and styles except the “I” and "1". The1:1 ratio is appropriate for use on curved surfaces such ascounter drums, small pipes, and cables.

6-5.3.2.3 Character Stroke Width (Ref. 4)When characters are used on a light background, the

stroke width should be approximately 1/6 the height ofthe character. When light characters are used on a darkbackground, the stroke width should be 1/7 to 1/8 theheight of the character. These ratios apply regardless ofhow high characters are made for distant viewing. How-ever, for certain applications, characters with differentstroke widths may be used for emphasis. In this case thethinnest character stroke should be no less than 1/8 northe thickest character stroke greater than 1/5 respectivecharacter heights.

TABLE 6-1CLOSE EQUIVALENT TYPEFACESSUITABLE FOR LABELING (Ref. 4)

Type Font

Airport Bold CondensedAirport Demi-BoldAirport Medium CondensedAirport Semi-BoldAlternate Gothic #2Alternate Gothic #3Alternate Gothic #51Alternate Gothic #77Franklin Gothic Condensed

6-12

Futura Demi-BoldFutura MediumFutura BoldGroton Condensedlining Gothic #66Spartan HeavySpartan MediumTempo BoldVogue Medium

6-5.3.2.4 Stroke Continuity (Ref. 4)Continuous sroke characters should be used where

appllicable and practical for all equipment labels, legends,placards, and signs. Stencil characters may be used forshipping containers; however, stencil characters shouldnot have stroke breaks greater than 1/2 the characterstroke width. Stencil stroke widths should conform to therequirements of par. 6-5.3.2.3.

6-5.3.2.5 Character Spacing (Ref. 4)The minimum space between characters in a word

should be one stroke width. However, character spacingshould be adjusted to provide an appearance of "openarea balance” within single words i.e., when adjacentvertical strokes between adjoining characters are com-pared to adjacent characters in which vertical compo-nents are far apart, the word will appear to be properlyspaced. In such cases, the space between adjacent verticalstrokes should be slightly wider than between verticalhorizontal or horizontal horizontal strokes.

6-5.3.2.6 Word and Line Spacing (Ref. 4)The preferred space between words is the width of one

character, except for "I" or "1". The minimum spacingbetween words should not be less than 1/2 the width ofone character.

The minimum space between lines should be 1/2 thecharacter height.

6-5.3.2.7 Character Height (Ref. 4)Character height for labels, legends. and signs should

be determined on the basis of the critcral in Figs. 6-18 and6-19.

The linear equation represented by Fig. 6-19 is

wherey = numerical or character height, mmy = viewing distance, m.

Eq. 6-1 provides a convenient method for calculating thenumeral or letter height for any viewing distance.

If a panel within a given piece of equipment or consolemust be labeled to identify it from other-s, i.e., when apanel integrates a specific operating function as distinctfrom another panel, functions on the panel and individualcomponents should be differentiated in terms of the lettersize (height). The size encoding should progress as follows

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DOD-HDBK-791(AM)Letter Height versus Viewing Distance

and Illumination Level

(Minimum Space Between Characters, 1 Stroke Width;Between Words, 6 Stroke Widths)

Figure 6-18. Character Height Criteria for Instruments, Panels, and Equipment Viewed in CloseProximity Under Various Illumination Levels (Ref. 4)

6-13

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Figure 6-19. Character Height for Viewing Signs at Extended Distances (Ref. 4)

4. Smallest label for individual components. dis-plays, and controls.Label should be compatible with the expected viewingdistances. However, to provide discriminable differencesamong label sizes, each label character height should be atleast 25% larger or smaller than the next function label.Fig. 6-20 illustrates this hierarchy.

6-5.4 METHOD OF APPLICATIONMethods of application together with their advantages,

disadvantages, legibility, and permanence are discussedin par. 6-4.

6-6 COLORS FOR LABELS AND SIGNS

Color combinations of printing and background shouldbe selected to maximize legibility. The best color combi-nations in descending order are given in Table 6-2.

If color codes, labels, and signs are necessary, selectcolors on the basis of recognizable differences. the colorsindicated in Table 6-3 are ideal for surface coding becausethey are easily recognizable by both normal and colordeficient observers. when related displays and controlsare color coded, they should be coded the same color. allemergency controls should be coded in red.

Color coding for hose, pipe, and tube lines for aircraftand missiles should be in accordance with par. 5.1.1,MIL-STD-1247 (Ref. 9). Color codes for pipelines shouldbe in accordance with par.4, MIL-STD-101 (Ref. 10).

Caution and danger signs should be color coded asdescribed in par. 6-5.3.1 and illustrated in Figs. 6-16 and6-17, repectively. Signs indicating a radio frequencyradiation hazard should be coded as indicated in fig. 6-21(Ref. 24). signs indicating a nuclear radiation hazardshould be coded as shown in fig. 6-22.

6-14

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Figure 6-20. Label Size-Hierarchy Example for Equipment, Panel, Subpanel, and ComponentIdentification (Ref. 4)

TABLE 6-2. BEST COLORCOMBINATIONS IN

DESCENDING ORDERBlue on WhiteBlack on YellowGreen on WhiteBlack on WhiteGreen on RedRed on Yellow

TABLE 6-3. RECOGNIZABLE COLORS

*Red No. 11136 may be used instead of 11105.**Yellow No. 13538 may be used instead of 13655.

6-15

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Figure 6-21. Radio Frequency Radioation Hazard Warning Symbol

6-16

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1.2.

3.4.

5.6.

7.8.9.

10.

11.

12.13.

14.

15.

16.17.18.19.

6 - 7 I D E N T I F I C A T I O NCHECKLISTS

Table 6-4 lists items to consider for proper identifica-tion. Several items are included that were not discussed inthe text, but they are included here because they areimportant to good design and might otherwise be over-looked. The designer, maintainability engineer, humanfactors engineer, product engineer, and ConfigurationManagement Office must work together during design toinsure that all facets of proper identification are covered.When evaluating the design in accordance with the check-list, if the answer to any question is “no”, the designshould be studied carefully to determine the need for, andproper application of, improvements in identification.

Figure 6-22. Nuclear Radiation HazardWarning Symbol

TABLE 6-4. IDENTIFICATION CHECKLIST

Are all units labeled and, if possible, with full identifying data?Are parts stamped or labeled with relevant characteristics information?

Are structural members stamped with physical composition data—e.g., can be welded; is flammable?Is each terminal labeled? Does the label have the same code symbol as the wire attached to it?Are labels on components or chassis (not parts) etched or embossed in lieu of stamping or printing?

Are labels placed for full, unobstructed view?

On equipment using color coding, is the meaning of the colors given in manuals and on an equipment panel?IS color coding consistent throughout the system; are displays and controls the same color?

Are numeral and letter designs used that have simple configurations equivalent to typed letters?Are capital letters used for labels and standard capitalization, and lowercase type used for extended text material?Are the lowercase "ell” (1), I, and 1 distinguishable from each other for the selected font; capital “oh” (O) from zero(0)?Have standard abbreviations been used?Are instructions brief, i.e., unnecessary words and punctuation omitted?

Are display labels imprinted, embossed, or attached in a manner that they will not be lost, mutilated, or becomeunreadable?

Do display and control labels clearly indicate their functional relationship? Are displays labeled by functionalquantity—i.e., gal, psi, ohms—rather than by operational characteristics?

Does displayed printed matter always appear upright to the technician from his normal viewing position?Do adequate labels appear on every item the technician must recognize, read, or manipulate?Does display of the sequence of use of controls appear as a number on each control (for fixed procedure operation)?Are display labels attached to each test point, and do they show intolerance or limits that should be measured at thatpoint?

(cont’d on next page)

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TABLE 6-4 (cont’d)20. Are schematics and instructions attached directly to, or adjacent to, the chassis for all units that may require

troubleshooting?

21. Do display labels on component covers provided relevant information concerning the electrical, pneumatic, orhydraulic characteristics of the part?

22. When selector switches may have to be used with a cover panel off, is a duplicate switch-position label provided onthe internal unit so the technician does not have to refer to a label on the case or cover panel?

23. Are displays labeled so they correlate with notations found in system diagrams, in technical manuals, or in relatedliterature?

24. Do display schematics on separate assemblies show clearly any relationships to other or interconnecting schematics?25. Are color codes for identifying test points or tracing wire or lines easily identifiable under all conditions of

illumination, and are they resistant to damage or wear?

26. Is functional organization of displays and controls emphasized by use of such techniques as color coding. markedoutline, symmetry of grouping, and/or differential plane of mounting?

27. Are all potted parts labeled with current, voltage. impedance, or terminal information?28. Are lubrication points properly labeled?

29. Are labels used to indicate the direction of movement of controls. especially where lack of such knowledge may resultin damage to equipment?

30. Are labels used to indicate type of fluids at fill or service points and on lines?

31. Are labels on panels, components, and subassemblies differentiated in terms of letter size to indicate the hierarchy?

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1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

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REFERENCESMelvin S. Majesty, “Personnel Subsystem Reliabilityfor Aerospace Systems”, Proceedings. ISA Aero-space Systems Reliability Symposium, 16-18 April1962, p. 199.W. P. Chase and P. R. Tobias, “Rocket EngineDamage Causes and Cures”, Western Aviation,Missiles and Space, August 1961.MIL-STD-130F. ldentification Markings of US Prop-erty, 2 July 1984.

MIL-HDBK-759A, Human Factors EngineeringDesign for Army Materiel, 31 December 1985.

MIL-STD-280A, Definitions of Item Levels, ItemExchangeability, Models and Related Terms, 7 July1969.MIL-P-514D, Plate, Identification, Instruction andMarking, Blank, 7 July 1971.

MIL-STD-882B, System Safety Program Require-ments, 30 March 1984.

MIL-STD-454J, Standard General Requirementsfor Electronic Equipment, 30 June 1985.

MIL-ST D-1247B, Marking Functions and Hazarddesignations of Hose, Pipe, and Tube Lines for Air-craft, Missile, and Space Systems, 20 December1968.MIL-STD-101B, Color Code for Pipelines and forCompresed Gas Cylinders, 3 December 1970.J. W. Altman, et al., Guide to Design of MechanicalEquipment for Maintainability, ASD-TR-6-381,Air Force Systems Command. Wright-Patterson AirForce Base, OH, 1961.MIL-STD-129J, Marking for Shipment and Stor-

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.24.

Fed Spec TT-I-1795A, Ink, Marking, Stencil, Opaque(Porous and Nonporous Surfaces), 17 March 1975.MIL-STD-12D, Abbreviations for Use on Drawings,Specifications, Standards, and in Technical Docu-ments, 29 May 1981.

MIL-STD-411D, Aircrew Station Signals, 30 August1974.MIL-STD-783D, Legends for Use in Aircrew Sta-tions and on Airborne Equipment, 18 December1984.

MIL-STD-195, Marking of Connections for Electri-cal Assemblies, 7 February 1958.MIL-E-11991E, E1ectronic, Electrical, and Electro-mechanical Equipment, Guided Missiles and Asso-ciated Weapon Systems, General Speecification for,26 July 1983.NAT-STD-2027, Marking of Miliary Vehicles, 15December 1982.

MIL-STD-1473A, Standard General Requirementsfor Color and Marking of Army Materiel, 29 July1983.

AR 385-30, Safety Color Code Markings and Signs,15 September 1983.

ANSI Z35.1-1972, Specifications for Accident Pre-vention Signs, American National Standards Insti-tute, 1430 Broadway, New York, NY 10018, 1972.

FED-STD-595A, Color, 30 August 1984.

ANSI C95.2-1982, Radio Frequency RadiationHazard Warning Symbol, American National Stan-dards Institute, 1430 Broadway, New York, NY10018, 1972.

age, 25 September 1984.

BIBLIOGRAPHYANSI Z53.1-1971, Safety Color Code for Marking Physi-

cal Hazards, American National Standards Institute,1430 Broadway, New York, NY10018, 1971.

MIL-STD-1472C, Human Engineering Design Criteriafor Military Systems, Equipment, and Facilities, 1 0May 1984.

MIL-STD-190C, ldentification Marking of Rubber Prod-ucts, 17 October 1977.

MIL-STD-1480C, Color Codes for Webbing, Textile;Manufacturer's Identification, 22 July 1974.

FED-SPEC-TT-L-50G, Lacquer, Nitrocellulose, Acrylicand Acrylic Butyrate, Aerosol, 12 August 1977.

MIL-M-18012B, Markings for Aircrew Station Displays,Design and Configuration of, 17 February 1972.

MIL-I-43553A, Ink, Marking, Epoxy Base, 22 November1985.

MIL-M-11745C, Marking and Labeling of US ArmyMarine Craft, 25 July 1980.

MIL-T-23991E, Training Devices, Military; GeneralSpecifications for, 20 February 1974.

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CHAPTER 7TESTABILITY AND DIAGNOSTIC TECHNIQUES

Testability} and diagnostics are defined, and their differences are discussed. The importance of introducingtestability in early design is presented. The contribution of good diagnostic techniques to facilitate mainte-nance is emphasized. Built-in test equipment and automatic test equipment are defined, and examples of theiruse are given. Functional tests—such as detection, isolation, and prediction—are anallyzed. Critical designconsiderations—such as personnel, software, test sequence, and stimuli—are detailed. The application ofartificial intelligence and expert systems to improve programming techniques is presented. Examples of thedesign considerations are provided in several commodity areas. Checklists for testability and diagnosticscomplete the chapter.

7-1 INTRODUCTION

The preponderance of the repair time required for anyitem. subsystem, or system normally is a direct function offault isolation. Rapid advances in system complexityhave aggravated the problem by reducing the effective-ness of conventional testing and diagnostic techniques.During the era when discrete components were used inquantity, it was possible to probe troubleshoot-elcctronic assemblies to isolate failures. Now there arelayers of hermetic seals and programs stored in memorythat obscure the technician’s view into the physical pro-cesses of system operation (Ref. 1). Accordingly, it isimperative that provisions be made for the most effectivediagnostic routines possible. The application of newtechniques is not restricted to electronic systems; newtechniques can also be applied to mechanical systems (seepar. 7-3.4).

It is necessary that system testability as a discipline beincorporated into the design process for cost-effectivenessand to insure that diagnostic techniques and supportingtest equipment are mature enough to support materieldelivered to the field. Decisions regarding testing alsoaffect the logistical support plan. Factors involved in thedecision include mission and operational characteristicsfor the equipment, anticipated reliability, maintenancestructure, automatic fault isolation capability of built-intest, built-in test equipment (BIT)/(BITE), equipmentand skill level of personnel available for maintenance,operational environment, development time, and cost.

Historically, testability has received a lower priority—effort and funding—than classic performance character-istics (Ref. 2). A change in philosophy is emerging, how-ever, for complex and sophisticated weapon systems asevidenced by the award by the US Air Force of a $3.2million contract to the Boeing Military Aircraft Com-pany to develop a prototype ground-based diagnosticsystem for the B-lB avionic system (Ref. 3).

The paragraphs that follow discuss testability, testabil-ity analysis, the relationship of testability to availability,diagnostic techniques and aids, functional testing, anddesign considerations.

7-2 TESTABILITY

7-2.1 GENERALTestability is defined as a design characteristic that

allows the status of a unit or system to be determined in atimely and cost-effective manner (Ref. 4). This definitiondistinguishes testability from diagnosis which describesthe functions performed and the techniques used indetecting and isolating the cause of a malfunction orfailure, e.g., the application of a physical or electricalstimulus to a device to produce a measurable response.Closely associated with testability and diagnostics are theterms BIT, BITE, and automatic test equipment (ATE).These terms are defined in pars. 7-3.5.2.3 and 7-3.5.2.2,respectively.

To illustrate the difference between testability anddiagnostics, consider the function indicator lights on thedashboard of an automobile. The temperature indicator,which lights up when the engine overheats, representscontinuous BITE. It will not isolate which component ofthe cooling system is at fault; it is not a diagnostic tool inthe strictest sense. Now consider a pneumatic tire. The tirevalve represents a designed-in test feature. The determi-nation of the air pressure inside the tire by a pressure gagerepresents a diagnostic technique.

Good testability does not “just happen”; on the con-trary, it is achieved by establishing a well-planned test-ability program that accomplishes the following generalrequirements (Ref. 4):

1. Establishment of sufficient, achievable, and afford-able testability, built-in and off-line test requirements

2. Integration of testability into equipment and sys-tems during the design process in coordination with themaintainability design process

3. Evaluation of the extent to which the design meetstestability requirements

4. Inclusion of testability in the program reviewprocess.

7-2.2 TESTABILITY ANALYSIS (Ref. 5)Testability analysis is defined as the element ‘in the

equipment design analysis effort related to developing the

7-1

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diagnostic approach and then implementing that ap-preach. The analysis includes BIT analysis, nodal analy-sis for partitioning, test point design, fault simulation.and diagnostic preparation for all levels of maintenance.

To be effective, testability analysis must be used todefine diagnostic requirements before the detailed designof the major equipment is begun. During the detaileddesign, test point analysis and nodal analysis for parti-tioning should be major inputs to the layout or packagingdesign that contributes to modularization. Analysis ofprojected BIT performance and fault simulation studiesshould be used to evaluate the process to determinewhether the BIT objective is being met and as inputs to aBIT maturation program. Design improvement datashould also be extracted from test and operational data.For example, system components coming off the produc-tion line should be tested with equipment designed foreventual use by repair personnel. An analysis of the testresults will reveal whether the test equipment can confi-dently and satisfactorily perform its diagnostic purpose.

Chapter 13, “Test and Evaluation (T&E)”, AMC-TRADOC Materiel Acquisition Handbook (Ref. 6), setsforth procedures used to plan, evaluate, and report on thetest and evaluation of materiel systems and or items. thetypes of test and evaluation performed. and the responsi-bilities of the US Army Materiel Command (AMC), USArmy Training and Doctrine Command (TRADOC),operational tester, and other organizations in the test andevaluation process. The purposes and time frames as theyrelate to the various test programs from operational test-ing to production testing are defined.

Guidelines for conducting the testability analysis are1. Question the process by which the developer

defined his testability approach—how, why, and logicemployed.

2. Insure testability design is concurrent with majorequipment design.

3. Insure test point selection and design, and test-ability partitioning play a major role in system layout andpackaging.

4. Insure that a failure modes and effects analysiswas available and used as part of the testability analysis.

5. Insure that BIT analysis and fault simulation wereused to evaluate the coverage and effectiveness of the BITdesign if BIT is used. BIT development, integration, andcheckout must be concurrent with system performancedevelopment.

6. Insure that the testability design approach evolvesas information is obtained from analysis and test expe-rience. Compare the results with the requirements.

7-2.3 INTERFACE RELATIONSHIPSEquipment availability. measured as a probability, has

two elements, i.e.,1. The probability that an item is operable because it

has not failed, i.e., is reliable2. The probability that, if the item has failed or is

down for maintenance, it can be restored to serviceablecondition within the time permitted by mission constraints.

Any failed condition that is present and undetectable isnot corrected and is, therefore, a part of the unreliabilityof a mission. This highlights the importance of testabilitybecause the incidence of undetectable failures can bereduced by designing equipment for a high degree ofreliable testability. This attribute of the system is referredto as test effectiveness, or BIT efficiency, i.e., the percentof all faults or faults that the BIT system detects. one ofthe more complex tasks in the acquisition of modernweapon systems is the specifying of performance mea-sures or figures of merit for BIT. It is becoming commonpractice that contracts for electronic subsystems andcomponents specify the false alarm rate and the percent offailures that can be detected and isolated. Ref. 7 discussesthe analysis performed on a complex digital data systemwherein the BIT specifications required that 98% of allpossible failures be detectable and that 90% of all failuresbe isolatable to one plug-in assembly. Par. 7.2-4 discussesand illustrates by example characteristics external to BIT.

It is obvious that testability interfaces with reliability,modularization, end-item configuration, space alloca-tion, BIT, automatic test equipment (ATE), and logistics.Trade-off studies involving these factors will result in themost cost-effective method for achieving availabilitygoals consistent with the mission of the system. Diagnos-tic techniques are important inputs to this analysis (seepar. 7-2.2), and in some cases diagnostic techniquesevolve from the analysis.

In the trade-off between reliability and testability,availability may be enhanced by designing a piece part toa higher level of reliability or by providing standbyredundancy. Testability, with its BIT, introduces anothcrcomponent into the system that may fail, i.e.. the BITequipment may not reveal an undetected fault or maysignal a false alarm. However, added redundancy mayraise the reliability of a component to a level where testingis considered unnecessary. Consider the following re-dundant system

where the reliability Rc of a component is 0.8. The equa-tion for calculating the improved reliability of this paral-lel redundant system, where k = 2, is

Since the reliability of the redundant system has beenincreased from 80% to 96%, which enhances overall sys-tem availability, testing of the circuit may be consideredunnecessary and the BITE eliminated. Unfortunately,redundancy almost always increases the maintenance

7-2

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workload, complicates fault detection, and increases cost.Depending upon the size of the added component, theincreased weight and volume also may be a factor. Offset-ting these negative aspects are the cost of providing testequipment, the time to conduct the test, and the timerequired to repair a mission-critical end-item.

7-2.4 CHARACTERISTICS EXTERNAL TOBIT (Ref. 8)

There are two important considerations external toBITE that must be addressed in any discussion of BITEand diagnostics, namely,

1. Reliable performance of the weapon system deter-mines, to a large extent, the criticality of BIT perfor-mance. Therefore, if the basic system is very reliable,more than expected, a shortfall in the BIT performancemay have very limited impact on the operational utility ofthe system.

2. All system faults that are correctable by mainte-nance action must eventually be detected and isolated.The Failure Modes, Effects, and Criticality Analysis(FMECA) is an effective tool for evaluating BIT effec-tiveness. The FMECA can be used in defining test andcheckout procedures to insure that all essential parame-ters, functions, and modes are verified. Therefore, thetechniques, tools, manuals, test equipment, and person-nel required to isolate non-BIT detectable faults can be amajor maintenance consideration.

Example 7-1, which follows, illustrates the impact ofBITE on the overall maintenance planning effort. It alsoillustrates the effect of external factors on BIT equipmentperformance.

Example 7-1:Assume the radar of an attack aircraft is composed of

five line-replaceable units (LRUS) with the followingBITE and system characteristics:

System:Five LRUS

Mean Time to Repair (MTTR) B— with BITE:2-h—includes failures that have been bothdetected and isolated

Mean Time to Repair (MTTR) NB —non-BITE:5-h—includes failures that may not have beenisolated but may have been detected

Mean Flying Hours Between Failures MFHBF50 flying hours

Time Period of Interest TPI:2500 flying hours

BIT Specifications:Percent detect ion R delect = 90%Percent isolation Risol = 90% (to LRU level)False alarm rate RFA = 5% (of all BITE indica-tions).

For this example, operating time is assumed to be flighttime.

DOD-HDBK-791(AM)

Before beginning the analysis, note that a relativelyhigh BIT system capability has been specified. A casualexamination would likely conclude that, with this exten-sive BIT coverage, there is minimal maintenance actionrequired.

The notationfollows:

AFIC =( FB) detect =

( FB) LRU =

F FA =

(F B) total =

F T =

R E A =MFHBF =

( M T T R )B =

(MTTR) NB =

R detec t =R isol =

(T FA ) total =

(T LRU)total =

(T NB)total =

(T NB+FA ) total =

T total =

TPI =

used in the conduct of the analysis

automatic fault isolation capability, %number of failures of the total numberof failures FT that BITE will detect astrue, failuresnumber of BITE detected failures thatcan be isolated to LRU level, failuresnumber of false alarms expected duringTPI, dimensionlesstotal BITE indications of failure, i.e.,true failure plus false alarms, failurestotal failures expected during T P I ,failuresfalse alarm rate indicated by BITE, %mean flying hours between failures, fly-ing hoursmean time to repair BITE detected andisolated failures hmean time to repair non-BITE detectedand isolated failures, hBITE detection rate, %BITE isolation rate to LRU level, %total maintenance time to resolve falsealarms, htotal corrective maintenance time torepair BITE detected and isolated fail-ures, htotal corrective maintenance time torepair non-BITE detected and isolatedfailures, htotal non-BITE corrective maintenancetime to repair non-BITE detected andisolated failures plus false alarm main-tenance time, htotal corrective maintenance time dur-ing TPI, i.e., sum of BITE and non-BITE corrective maintenance time, htime period of interest, 2500 flying hours.

The analysis follows:1. Total number of failures F total to be experienced,

on the average, are

whereTPI = time period of interest, flying hours

MFHBF = mean flight hours between failures,flying hours.

7-3

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7-4

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From Example 7-1 it is apparent that even with a highAFIC of 81%, the non-BITE-oriented corrective mainte-nance represents 60/140 = 43%—i.e., (TBN)total/ T total—ofthe total anticipated corrective maintenance hours. Theexample did not consider the impact of any scheduledmaintenance since it is not associated with BITE. AlsoExample 7-1 has been greatly simplified in that it ignoredBITE-addressable errors such as cable connectors. In thisplanning type of example, it was assumed that the BITAFIC will be 81%. If, in fact, the AFIC is 81%, then80/140 = 57%—i.e., (TLRU)total/Ttotal —of the maintenanceeffort will be oriented toward BITE detected and isolatedfailures. However, if the true AFIC is determined to belower than 81%, it may be necessary to reevaluate theoverall effectiveness of the entire maintenance and logis-tic programs as well as total mission effectiveness.

7-3 DIAGNOSTICS

7-3.1 GENERAL (Ref. 7)Diagnostics refers to the functions performed and the

techniques used in determining and isolating the cause ofmalfunctions in an operating system or in determining itsoperational status. The primary objective of the main-tainability engineer in the field of diagnostics is an overallreduction of system downtime by providing a strategy forthe rapid location of faults.

Observations from various case studies of materiel nowin the inventory reveal that diagnostic system develop-ment is an immature discipline when compared to reliabil-ity or maintainability. One of the chief reasons is thatthere are no accepted definitions of requirements that aredirectly understandable and that can be related to fieldperformance. Diagnostic tests also are less mature—although fault insertion tests can be diagnosed in thelaboratory, they are poor predictors of field performance.A comparison of results from laboratory fault insertiontests and field operational tests for the F-16, APG-66 FireControl Radar, is shown in Table 7-1 (Ref. 9). Table 7-1indicates that a successful demonstration in a laboratorysetting is no guarantee of success in the field. Demonstra-tion by fault insertions is necessary, but not sufficient, tovalidate a diagnostic design.

DOD-HDBK-791(AM)

Lack of knowledge in the diagnostic area presents asignificant challenge to the developer to improve thediagnostics of current weapon systems and acquisitionmethods for improved diagnostics in future weapon sys-tems. Where a mature diagnostic capability has beenachieved in systems that required sophisticated tech-niques, it is obvious that success was no accident; successevolved as the direct result of a carefully defined processconsisting of the following elements:

1. Planning2. Management strategy3. Motivation4. Technical activity and innovation5. Adequate funding that spanned system acquisi-

tion from the definition of requirements through deploy-ment.As indicated in par. 7-1, the user’s requirements shouldaddress diagnostic capability in the larger context of theoperational mission and environment as well as the sup-port constraints of manpower, skill-level maintenanceconcept, deployment, and logistic burden. The require-ments, constraints, environment, and economics shouldthen drive the architecture of the system with diagnosticsbeing one of the fundamental characteristics.

7-3.2 DESIGN NEEDS (Ref. 9)In the area of design of diagnostic systems, case studies

have identified the following design needs:1. Strategies to minimize “cannot duplicate”, “bench-

checkd serviceable", "retest OK”, and false alarm condi-tions

2. Techniques to maximize vertical testability. i.e.,from system, to subsystem, to subassembly, to part level

3. Flexible diagnostic systems that will permitchanges to be incorporated in diagnostic algorithms, dis-plays, and tolerances with minimal hardware impact

4. Fault-free software development techniques5. Techniques to enable more concurrent hardware

and software development, and earlier integration of thetwo

6. Computer-aided engineering techniques forenhancing design for testability. Some techniques such asLOGMOD and STAMP may already be available tomeet this need.

7. Experienced engineers who understand how toachieve good diagnostic techniques

8. Tools for predicting, measuring, and managingdiagnostic designs

9. Better design practices such as the control of tim-ing margins in high-speed circuits and systems.

7-3.3 DEVELOPMENT ANDDEMONSTRATION TESTING (Ref. 9)

Case studies have shown that improvements in devel-opment and demonstration testing will aid in diagnosticdevelopment. The following guidelines have been sug-gested by the experts:

1. Use reliability and other test events as opportuni-ties to discover problems with BITE performance. Envi-

7-5

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TABLE 7-1. TYPICAL FAULT INSERTION TESTRESULTS VERSUS FIELD RESULTS (Ref. 9)

ronmental testing may be particularly useful for discover-ing false alarm indications such as induced intermittentand transients.

2. Increase the number of repair parts and the timebudgeted in the laboratory to investigate diagnosticanomalies.

3. Expand the set of faults inserted.4. Increase the allowable cost of demonstrations to

include repair costs. This action will permit the insertionof a better cross section of faults.

5. Develop a library of computer simulation modelsto test BIT (hardware and software).

6. Adopt comparability analysis as a useful tool foridentifying a realistic set of faults for insertion.

7. Develop improved demonstration techniques topredict diagnostic performance in the field.

Field maturation is essential to achieve inherent diag-nostic potential. When a system is first fielded, it is com-mon to find that not all the hardware and software provi-sions of the diagnostics have been fully implemented. Inaddition, the operational use patterns and the environ-ment produce new failure modes and diagnostic indica-tions. Unfortunately, these new indications which theBIT equipment may not deal with properly- are resolvedby the judgment of operators and maintainers (who maynot have been trained to deal with them) with the aid oftechnical data that may not have been developed toaddress them. Thus a structured diagnostic maturationeffort must be resorted to for the purpose of bringing the

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diagnostic capability to its full potential. The key featuresof successful programs should be used in structuringfuture diagnostic maturation efforts for complex equip-ment. Unless this is done, diagnostics, and BITE in par-ticular. could be the weak link in the support chain (Ref.l0).

7-3.4

7-3.4.1

DIAGNOSTICS FOR MECHANICALSYSTEMS

Condition Monitoring of MechanicalSystems

Diagnostics, or “condition monitoring of mechanicalsystems”, combines the measurement of performance andthe detection of damage with the possibilities of prognos-tics to eliminate unnecessary and premature removal ofsystem components. A proven technology base exists anda variety of successful demonstrations have been con-ducted that indicate reliability and readiness improve-ments can be obtained by application of this technologyto current or future systems. Propulsion, transmission.and structural components are significant contributors toperformance and readiness of weapon systems and plat-forms, and they provide the focal points for the study ofcondition monitoring of mechanical systems (Ref. 11).

Unlike some of the other technologies, the issues in thecondition monitoring of mechanical systems center aroundthe development of the following specific condition-monitoring technologies:

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1. Techniques for the electrostatic monitoring ofengines

2. Ultrasonic wear particle sensors3. Advanced engine diagnostic technology4. Integrated transmission monitoring5. Structural testing6. Computer program techniques for likeliness

analysis (see par. 7-6.1).These technologies need refinement, demonstration, andintegration with automatic data processing equipment.Preliminary demonstrations have shown that each of thelisted technologies has the potential to correct some of thedefects in existing condition-monitoring technology. Forexample, the late consideration of condition monitoringin mechanical systems results in additional costs, weightpenalties, and unnecessary false alarms. If properlyincorporated into the initial design, these devices canmature with the system design, will minimize the weightpenalty, and can reduce costs of the initial testing of thesubsystem prior to production and fielding (Ref. 11).

Condition-monitoring techniques are being success-fully applied to the T700 BLACK HAWK engine in thefactory and in the field. Of particular value has been theengine history recorder for comparing the relative sever-ity of field testing of engine operation with specificationendurance test cycles. The engine chip detector hasproven to be an effective means of detecting incipientoil-wetted part failures. Borescope inspection has alsoproven to be useful and easy to do in the factory and onthe wing. Ground use of the diagnostic connector fortroubleshooting has been effective even though the cur-rently available test box is only a nonpowered resistancechecker. Condition monitoring coupled with line-replace-able unit installation and rigging with no requiredadjustments contribute to the overall mission readiness ofthe T700 engine. As a result of the demonstrated reliabil-ity of the engine. only a 10-h inspection check, which isaccomplished in 3 rein, and a periodic inspection per-formed at 500 flight-hour intervals, which can be per-formed on-wing in 1 h, are required (Ref. 12).

7-3.4.2 Nondestructive EvaluationNondestructive evaluation methods have been valuable

tools for maintenance for decades. During the past sev-eral years, however, work with these methods has shownoutstanding promise for reducing maintenance costs,maintenance time, and manpower requirements and foreliminating operational hazards. A body of knowledgenow exists of the benefits obtained from nondestructivetesting technologies that could be successfully applied totanks and rotary-wing aircraft. Examples of these tech-nologies are (Ref. 11)

1. Automated ultrasonic inspection techniques forcomposites

2. Automated nondestructive testing of software3. X-ray diffraction techniques for measuring resid-

ual stresses on torsion bars and track pins4. Advanced techniques for weld monitoring.

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7-3.5 DIAGNOSTIC TECHNIQUES

7-3.5.1 ImportanceIncorrect diagnoses of failures can result in the unneces-

sary removal of serviceable parts and repetitive mainte-nance actions, both of which reduce the efficiency ofmaintenance actions by increasing costs and decreasingsystem availability. For example (Ref. 13), repetitivemaintenance actions on helicopters were found to occurat a rate of 0.32 per flight hour and “diagnostics-including test equipment, troubleshooting, and standardmaintenance practices–-were identified as causing over50% of all repetitive maintenance actions”. If automaticmonitoring and/or alarms had been provided instead ofrelying on operator sensing, it is estimated that equipmentnonavailability due to maintenance could have beenreduced by as much as 25% per action. Obviously, thelessons learned were introduced into the design of theT700 BLACK HAWK engine (see par. 7-3.4.1). Althoughthese data are relative to helicopter maintenance, it isreasonable to assume similar data exist for other materielcategories and thus illustrate the effectiveness of improveddiagnostic techniques.

The time required to locate a fault is a function oftechniques such as integrated performance monitoringand maintenance tests. On-line monitoring of perfor-mance is designed into most Army equipment—forexample, an automotive vehicle in which the temperatureof the engine coolant, oil pressure, fuel level, and batteryor alternator output are monitored constantly. Theautomotive vehicle also serves as an example for theconduct of familiar maintenance tests such as checkingfuel, oil, coolant, battery, and other fluid levels beforedriving the vehicle. These simple performance indicationsand maintenance procedures usually localize a fault toone or several components. Diagnostic techniques takeadvantage of the gross localization aids to isolate thecomponent that requires repair or replacement.

Trying to locate a fault without proper diagnostic tech-niques usually requires more time than any other task inthe maintenance cycle. This is because trial-and-errortroubleshooting often results in an erroneous outcome;therefore, the process must be prolonged or delayed andrepeated. A simple illustration of this fact is the costassociated with the repair of a TV set—unless the picturetube is replaced, 90% of the charge is for labor to deter-mine the faulty component. Accurate automated mainte-nance provisions can significantly reduce the frequency oferrors through their consistent logic and their avoidanceof human volition and training problems. Artificial intel-ligence and expert systems (see par. 7-3.5.3) can contrib-ute to automated diagnostic processes.

7-3.5.2 Types of Diagnostic TechniquesAll diagnostic techniques can essentially be divided

into three broad categories, i.e., those employing1. Logic flow2. Automatic test equipment

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3. Built-in test equipment.Each of these categories is discussed in the paragraphsthat follow.

7-3.5.2.1 Logic FlowLogic flow–-a troubleshooting road map is an out-

line of the sequence to be followed in the event of a systemfailure; it is essentially a manual procedure. As a mainte-nance aid for fault isolation, it provides a step-by-stepdescription of the questions to be asked, tests to be con-ducted, and continued sequential localization paths to befollowed depending on test results. The “roads” lead fromgeneralized localization to specific fault localization.Specific data required by the technician to enable him tolocalize a fault will allow him to accept or reject eachcomponent tested as the source of the trouble. For exam-ple, an overheated engine can result from coolant leaks.low coolant pump pressure, and constricted passages.Measurement of fluid flow and temperature at strategicpoints in the cooling system can aid the technician ineliminating the source of the fault—radiator. hoses, andengine—and zero in on a pump leak or failure. A check-out of the system after repair usually requires a repeat ofall the checks used for localization.

In electronic systems, three data flow patterns aregenerally of diagnostic interest, i.e.,

I. Simple series pattern2. Complex—series-parallel—pattern3. Loop pattern.

In the simple series pattern the data flow goes throughone element at a time, and the element that standsbetween a good signal and a bad one is the fault source. Inthe complex pattern, places exist where the data flow fansout along parallel paths. In this case the fault may belocalized to a parallel set, and further tests must be con-ducted to locate the faulty element of the set. In the looppattern, part of the flow returns to an element as feed-back. In this case a fault in one loop element can make itmore difficult to distinguish between the element causingthe fault and elements simply reacting to the faulty signal.

Logic flow diagrams are very important in designingcomputer software used with ATE. The programmermust match the logic flow of his software with the logicflow of the hardware being tested and with various hard-ware fault modes and combinations of some of the morecommon modes. The programmer must take precautionsto code the software for all conceivable circumstances.For example, under some fault circumstances one of thetests may prematurely transfer control to the end of thesoftware loop. Adequate design of test software requiresthat the programmer strive for reliability and that hethoroughly test his program before releasing it for use.

The sequence of repair in a flowchart should recom-mend corrective actions according to

1. Probability of failure2. Accessibility and/or simplicity of repair

but not necessarily in the order given—i.e., even if it isunlikely that Part A failed, if Part A is readily accessible,it might be advantageous to check Part A first.

A logic flow for a tracking radar is discussed in par.7-5.5 and is illustrated in Fig. 7-3.

7-3.5.2.2 Automatic Test EquipmentAutomatic test equipment (ATE) is defined as equip-

ment that is designed to conduct automatically the analy-sis of functional or static parameters and to evaluate thedegree of the performance degradation of the unit undertest (UUT), and it may be used to perform fault isolationof the UUT malfunction (Ref. 14). The decision making,control. or evaluative functions are conducted with min-imum reliance on human intervention and usually aredone under computer control. ATE is used to detect andisolate a fault automatically and to check out the systemfollowing a maintenance action. ATF is separate from theunit under test and is used primarily at the intermediateand depot maintenance levels the UUT is brought to theATE or the ATE is brought to the UUT. If the use of ATFis contemplated, a trade-off analysis must be performedearly in the system acquisition process to determine thevalue of’ the automatic test features in relationship tooperational effectiveness. logistics, maintenance eflec-tiveness, and life cycle costs. The interdependent parame-ters of quantity, location, and reliability of ATE can bedetermined by using a generalized cost-effectivenessmodel. ATE is costly and. if not properly integrated intothe overall system, may introduce more reliability andmaintenance problems than it solves. However, the costsassociated with the design and procurement of ATF arenot the complete story. If the ATE results cannot beconfidenty relied upon, false alarms can result in thedisposal of serviceable equipment and in reduced equip-ment availability resulting from unnecessary mainte-nance. ATE—when it is used properly and when it can berelied upon to yield an accurate diagnosis will reducemaintenance time and the skill level and number of main-tenance personnel and will increase the availability of thesystem.

Selection of test features to be incorporated into anATE system should be based on the following considera-tions:

1. Test Function for Purpose. Is ATE to enhancemaintainability, optimize performance of the system.monitor operational readiness. improve system availabil-ity, or is it a combination of these purposes?

2. Testing Modes.. Will ATE be used on-line with thesystem operating, on-line with the system under test, oroff-line?

3. Level for Defection. At which level will ATE detecta fault or malfunction equipment, subsystem, ormodule?

4. Degree of Fault Isolation. At which level willATE isolate a fault—equipment, subsystem, groups ofreplaceable modules, modules, line-replaceable units, orindividual circuits?

System level design requirements for the automatic testfeatures should be specified quantitatively and definedquantitatively in the system specification in terms of fail-

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ure detectability; false alarm rate; degree of fault isolationdesired; fail-safe provisions; and the reliability and main-tainability of the sensors, interface hardware, and theATE itself. Some general-purpose ATE now exist, e.g.,AN/USM-410 (EQUATE) and GENRAD. Accordingly,new systems, where apropos, should be designed to becompatible with these ATE systems. Artificial intelli-gence, described in par. 7-3.5.3, may be used beneficiallyin the design of ATE.

7-3.5.2.3 Built-In Test EquipmentBuilt-in test (BIT) is defined as an integral capability of

the mission equipment that provides an on-board, auto-mated test capability to detect, diagnose, or isolate systemfailures (Ref. 14). The fault detection and, possibly, isola-tion capability are used for periodic or continuous moni-toring of the operating condition of a system and forobservation and, possibly, diagnosis as a prelude to main-tenance. BIT may be of several types (Ref. 14), i.e.,

1. Active BIT. A type that is temporarily disruptiveto the prime system operation through the introduction oftest stimuli into the system.

2. Continuous BIT. A type that continually moni-tors system operation for errors. Examples include parity—maintenance of a sameness of level or count, i.e., keepingthe same number of binary ones in a computer word andthus be able to perform a check based on an even or oddnumber for all words under examination—and othererror-detecting codes.

3. Initiated BIT. A type that is executed only afterthe occurrence of an external event such as an action byan operator.

4. Passive BIT. A type that is nondisruptive andnoninterfering to the prime system.

5. Periodic BIT. A type that is initiated at somefrequency. An example is BIT software executing duringplanned processor idle time.

6. Turn-On BIT. A specific type of initiated BIT thatis exercised each time power is applied to the unit orsystem.These definitions indicate that BIT equipment may beautomatically or manually triggered.

A BIT capability, as a means of attaining the requiredlevel of failure detection capability, must be relied upon asan automatic diagnostic tool because of the ever-increasingcomplexity of modern weapon systems. The need for BITis driven by operational availability requirements that donot permit the lengthy mean-time-to-repair associatedwith detecting and isolating failure modes in microcircuittechnology equipment. Since BIT equipment operateswithin the prime system and at the same functional speed,it has the capability to detect and isolate failures thatconventional test equipment and techniques could notprovide. A well-designed BIT system can reduce substan-tially the need for highly trained field and intermediatelevel maintenance personnel by permitting less skilledpersonnel to locate failures and send them to centralizedintermediate and depot repair facilities equipped to diag-nose and repair defective hardware.

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A BIT capability is not a comprehensive solution to allsystem maintenance problems, but rather a necessary toolto deal with the complex it y of modern electronic systems.Despite the advantages of BIT to provide a fast trouble-shooting capability, it does have disadvantages. Theprimary disadvantage is that the BIT test elements mustbe made an integral part of the prime unit. This additionincreases size, weight, complexity, and cost and imposesan extra maintenance burden when the BITE fails. If aBIT capability is to be employed, the decision must bemade in the early stage of development and the BITEmust mature together with the prime equipment. Some ofthe basic BIT decisions to be made early in system designare whether testing should be manual or automatic andwhether testing should occur without interrupting thenormal operation of the system or should be programmedat intervals when the prime system can be released fromits operational commitment to conduct the test. Otherconsiderations relative to the selection of features to beincorporated into BIT equipment are similar to thosedescribed for ATE in par. 7-3.5.2.2. Artificial intelligence,described in par. 7-3.5.3, may be used beneficially in thedesign of BITE.

Par. 7-6.2 provides an example of how BITE has beenapplied to an advanced attack helicopter to solve a criticaldiagnostic problem.

7-3.5.3 Artificial IntelligenceArtificial intelligence (AI) is a field aimed at pursuing

the possibility that a computer can be made to behave in amanner that humans recognize as intelligent behavior ineach other (Ref. 15). In a more restrictive sense AI couldbe considered a study of techniques for more effective useof digital computers through improved programmingtechniques. A further extension of the AI concept is thedevelopment of an expert system, i.e., an intelligent com-puter system that uses knowledge and inference proce-dures to solve problems that are difficult enough torequire significant human expertise in their solution (Ref.15). The consensus of experts—resulting from case stud-ies of BIT and ATE associated with existing weaponsystems—is, that despite the abundance of automateddiagnostic aids and trainers, there must be radical changesin maintainability technology in order to achieve signifi-cant improvement in maintenance procedures (Ref. 10).AI technology lends itself to the reduction of humanworkload in complex weapon systems and to the enhance-ment of training and maintenance and thus enables lowerskill level technicians to maintain complex defense sys-tems more efficiently. For example, AI technology couldbe applied to the design of very large-scale integratedcircuits (VLSIC) and very high-speed integrated circuits(VHSIC) to design in testability and fault tolerance (Ref.10). AI and expert system applications are not confinedexclusively to electronic systems. An example of theapplication of an expert system to a mechanical system isthat developed by the General Electric Company to assistrailroad maintenance personnel in the repair of GE’sdiesel-electric locomotives. The program is referred to asDELTA (Diesel-Electric Locomotive Troubleshooting

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Aid) (Ref. 15). Contracts were awarded by the Army in1985 for the development of AI technology to assist in thetroubleshooting and maintenance of helicopters andrelated aviation systems. Specifically, AI is being har-nessed to diagnostic test equipment for the AH-64APACHE attack helicopter. Software developed usingAI will be applied to a device referred to as an “intelligentfault locator”—a van-mounted computer that can servicehelicopters on a flight line or in the field. The systemenables crew chiefs to enter a fault description into acomputer, which will then take the crew chief through aseries of checks that will identify the problem.

7-4 FUNCTIONAL TESTS

7-4.1 TYPESFunctional testing performed with diagnostic equip-

ment is for the purpose of1. Fault Localization, Isolation, and Prediction.

The identification of replaceable units or componentsthat have caused a known fault and, where practicable, anindication of an impending fault

2. Verification Testing. Testing performed to insurereadiness before the start of the mission and aftermaintenance

3. Testing for Hidden Faults. Testing of diagnosticmonitors to insure readiness.Each of these functions is discussed further in the para-graphs that follow.

7-4.1.1 Fault Localization, Isolation, andPrediction

The primary test functions include fault detection, faultlocalization or isolation, and fault prediction. The desiredfunction must be defined and its resulting complexityconsidered in relation to the equipment being tested. Themaintainability plan should establish whether the testfunction is to be fault detection alone; fault detection andisolation; or fault detection, then isolation; and, whenpracticable, prediction. Neither the inability of the auto-matic test feature to predict impending failure shouldprevent it from being useful in isolating the fault once amalfunction has occurred nor should failure of the auto-matic test feature to isolate a fault prevent it from beinguseful for detecting the fault and signifying a “down-for-repair” condition. In general, fault isolation should beconsistent with the maintainability plan and should notgo below the cost-effective level of detection.

7-4.1.2 Verification TestingTesting frequently is used to verify the performance or

condition of the system when no fault is present or afterfault correction. For example, most missiles are checkedfor proper operation prior to firing; normally the sametest equipment is used to verify that the missile is operat-ing properly following the replacement of an unservice-able component. The test designer must decide whetherstimuli—the inputs or signals from the test set to the item

being tested to allow simulation of its operation undermission conditions—are required to interrogate the sys-tem. The stimuli must be realistic and controlled—e.g., ifcurrent is introduced into a system to check circuit conti-nuity, care must be exercised to insure that the magnitudeof the current will not inadvertently activate sensitivecomponents. Accordingly, stimuli application must becarefully considered relative to the functions exercised inthe item under test. The selection of the test set stimuli, ifrequired, must be made at the same time the transducersare selected.

7-4.1.3 Testing for Hidden FaultsHidden function faults are failures within the test

equipment or condition monitors, which would preventthe detection of a system component or failure for opera-tion readiness assessment or during a mission. A premis-sion test of the monitor must be designed into the testequipment if detection of the system or component failureis critical.

7-4.2 PERCENTAGE OF FAILURESDETECTABLE

Diagnostic test features must themselves be the subjectto determine that a specified percentage of component orsystem failures can be detected by the techniques and testequipment employed. The basis for this verification is thedocumentation developed during the equipment testabil-ity analysis (see par. 7-2.2). The test equipment designerinitially verifies analytically that sufficient failure modeswill be detected by the test features to meet the percentageof troubleshooting requirements. The actual testing tosubstantiate this analysis usually is done on a randomsampling basis for complex equipment. i.e., a certainfraction of the failure modes are identified randomly.These modes are then simulated in the prime equipmentto determine whether the test features detect and isolatethe source of failure. The results of the test sampling arethen used to accept or reject the hypothesis that the testfeatures meet the specified requirements. Caution must beexercised, however, because the laboratory results maynot be consistent with those experienced in the field (seeTable 7-1).

7-5 DIAGNOSTIC EQUIPMENT DESIGNCONSIDERATIONS

Design considerations for diagnostic equipment are afunction of many parameters:

1. Type of equipment to be used and factors leadingto this decision

2. Diagnostic software considerations3. Test point identification4. Test sequence considerations5. Selection of transducer or sensor6. Stimuli selection7. Output format8. Diagram use.

Each of the parameters together with their applicationis discussed in the pars, 7-5.3 through 7-5.10.

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7-5.1 CLASSIFICATIONIn this discussion it is assumed that the maintainability

plan has dictated that the test procedures and equipmentmust be automated, i.e., manual troubleshooting employ-ing off-the-shelf devices will not suffice. Thus diagnosticequipment can be classified as generalized or specialized.The decision depends on whether the test equipment willserve several subsystems of the same systems or othersimilar systems or whether it will be unique to the subsys-tem or system under design. If either a generalized orspecialized test system will satisfy the time restraints ofthe maintainability plan, then the decision should be mea-sured in relationship to

1. Newly furnished capabilities or techniques2. Efficiency of operation3. Effect on operating system(s) serviced, i.e., can

the on-line system be conducted, or must one system waitwhile another is being tested?

4. Effect on skill level, number, and type of techni-cians required

5. Total cost implications.

7-5.2 SNEAK CIRCUITS (Ref. 16)To cope with the challenge of maintaining complex

weapon systems, the maintainability engineer is forced torely on the increasing use of BITE and ATE. Their use—since the equipment itself embodies electrical hardware orsoftware systems—may introduce sneak circuits into theprime system or the sneak circuit may be indigenous tothe test equipment. A sneak circuit is an unexpected pathor logic flow within a system which, under certain condi-tions, can initiate an undesired function or inhibit adesired function. The path may consist of hardware,software, operator actions, or combinations of these fac-tors. Sneak circuits are not the result of hardware failurebut are latent conditions inadvertently designed into thesystem or coded into the software program, which cancause it to malfunction under certain conditions. Sneakcircuit analysis is the analytical technique used to identifysneak circuits in systems.

The causes of sneak circuits are system complexity,system changes, and user operations. Hardware complex-ity results in numerous interfaces between the prime sys-tem and the BITE or ATE, which may obscure theintended function or produce unintended functions. Theeffects of even a minor wiring or software change to aspecific component may result in undesired system opera-tions. Also a system that is relatively sneak free can bemade to circumvent desired functions or generate unde-sired functions as a consequence of improper test opera-tions or procedures, e.g., a test performed out of sequence.The identification of a sneak circuit, however, does notalways indicate an undesirable condition; in fact, somehave been used to accomplish tasks when other circuitryhas failed (Ref. 17). The implications of a sneak circuit,therefore, must be explored and its impact on the circuitfunction determined before any corrective action is taken.

Categories of sneak circuits are defined as1. Sneak Paths. Unexpected paths along which cur-

rent, energy, or logical sequence flow in an unintendeddirection or to an unintended destination

2. Sneak Timing. A situation in which events occurin an unexpected or conflicting sequence

3. Sneak Indication. An ambiguous or false displayof system operating condition that may result in an opera-tor’s taking an undesired action

4. Sneak Label. incorrectly labeled system function—e.g., system inputs, controls, or displays that may causean operator or technician to apply an incorrect stimulus.

Sneak circuit analysis requires a lot of computer timeand may be expensive. Therefore, the analysis should beconsidered for components—hardware and software—that are critical to mission success and safety. However, iftest equipment is yielding spurious information, i.e., anexcessive number of “cannot duplicates”, “retest OKs”,bench-checked serviceable, or false alarms—sneak cir-cuit analysis may reveal the source or cause of theanomalies.

7-5.3 BUILT-IN TEST EQUIPMENT VERSUSAUTOMATIC TEST EQUIPMENT

BITE is built-in, integral with the subsystem or systemto be tested. ATE is not integral with the subsystem orsystem; ATE must be moved to the test item or the itemmust be moved to the ATE. Some of the factors that mustbe considered in choosing between BITE and ATE are

1. Technicians’ Skill Level. In general, a technicianwill require a higher skill level to use ATE because detec-tion is not usually of the simple “go” or “no go” type. Thetechnician will have to hook up the ATE, apply selectivestimuli, and perhaps interpret readouts.

2. Physical Factors. To minimize weight and sizerequirements, it may be necessary to employ ATE; this isparticularly important in airborne systems. However, re-stricted access to the item to be tested would favor BITequipment.

3. Maintainability and Reliability. These attributesof the system must be considered in the decision process.BITE could impose another element into an alreadyoverly complex system and thus detract from the overallreliability of the system. Since it is present in the system,BITE may increase the maintenance load on the system—downtime for BITE maintenance reduces the availabilityof the system. On the other hand, since specific BITE willbe less complex than multipurpose ATE, BITE shouldrequire less maintenance and be more reliable.

4. Logistics. The numerous pieces of BITE—with itsunique purpose and function—will add to the number ofrepair parts required together with an increased need foroperational and maintenance manuals. A centralizedATE that consolidates many test functions reduces thelogistic burden.

5. Frequency of Application. If a test must be con-ducted frequently to determine the operational readinessor status of a system—particularly if the test must beconducted on-line—BITE would be the likely candidate.The immediate nonavailability, movement, and the re-quired hookup could render ATE unmanageable.

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6. Cost. Cost is always a factor in any trade-off studyof BITE versus ATE. It is generally more cost-effective toprovide specialized diagnostics for BITE and to providegeneral-purpose diagnostics for ATE.

7-5.4 DIAGNOSTIC SOFTWAREDiagnostic software designers should consider

1. Structured Programming. Structured program-ming—a set of programming principles that producessoftware modules that are easily understood and main-tained—should be used. In applying the programming,unconditional reverse transfers are to be avoided to elim-inate looping transfers.

2. Language Selection. Higher level languages,approved by the Army, may be necessary to developcomputer programs that simulate human behavior or toemploy artificial intelligence or expert systems (see par.7-3.5.3) in problem solving. The higher level languagesalso are more readily understandable, but the lower levellanguages (assembler) are often more efficient. ATLAS(Abbreviated Test Language for All Systems) is an exam-ple of a high-level directed standard test language whosemajor features are (Ref. 14)

a. Standard testing terminology to reduce er-roneous interpretations

b. UUT-oriented test statements to increase por-tability, i.e., ability to be used with different test equip-ment configurations.

3. Debugging. The debugging of diagnostic software, can be a lengthy and tedious process because of the

numerous ways in which fault sources can occur and

because some sources appear infrequently but haveserious consequences when they do occur. Thus it isimportant that the diagnostic system mature with thedevelopment of the major system, i.e., being employedthroughout the phases of system development and notwhen the prime system is released to troops.

4. Automatic Inspection, Diagnostic, and Prognos-ic System (AIDAPS) Software. AIDAPS software result-ing from various contracts have resulted in developingdiagnostic computer programs that can be transferredfrom one application to another (Ref. 18).

7-5.5 TEST POINT IDENTIFICATIONThe identification of test points involves the following

steps in the order indicated:1. List system parameters to be measured.2. List the output signals that, if present and within

tolerance, would indicate that the system is in normaloperating condition.

3. Identify internal functions that could cause thesystem output signal to fail and, therefore. should bemonitored.

4. Decide on required test points and their locutions.Each of the selected test points with its output signalshould be evaluated for its compatibility with the diag-nostic technique—manual, BIT, or ATE—to determinethose signals adaptable to direct measurement and thoserequiring signal conditioning via a transducer.

Fig. 7-1 (Ref. 19) depicts a simplified example of a test

Figure 7-1. Liquid Coolant Subsystem Showing Transducers and Parameters of Interest (Ref.19)

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point selection process for a liquid coolant subsystem.Apply the guide for test point selection previously stated:

1. Step 1. Output parameters to be measured aretemperature and flow rate.

2. Step 2. Output reading signals area. Specific temperatures plus tolerance, i.e., Ti°C-

±ti, degreesb. Specific flow rate Fi, i.e., meters per second

3. Step 3. Internal functions that could cause theouput signal to fail are

a. Coolant loss or reduced flow rate between(1) Cabinet and radiator(2) Radiator and cabinet(3) Cabinet and power amplifier

b. Coolant temperature rise due to defectiveradiator

c. Temperature rise due to defective liquid cool-ant cabinet.

4. Step 4. To monitor the parameter and fault condi-tions, test points must be located

a. Between cabinet and radiator, both inflow andoutflow, for temperature and flow rate

b. Between cabinet and power amplifier fortemperature and flow rate.To satisfy these locations and conditions, three tempera-ture transducers and two flow rate transducers werechosen for the selected test locations. The transduceroutputs were examined and found to be compatible withthe test equipment. These points will produce stable volt-age outputs with the desired time constants to provideaccurate real-time readouts of the parameters that governthe system.

7-5.6 TEST SEQUENCE CONSIDERATIONSBefore fixing the diagnostic plan and its associated test

equipment, it is necessary to determine a logical testsequence to be followed in the event of system failure. Theanalysis should reveal

1. Sequence or order of test2. System or function to be tested3. Prior tests required for measurement

DOD-HDBK-791(AM)

4. System or function required for test5. Parameters to be measured6. Processing required for interpretation.

Approaches for implementing the diagnostic analysisare

1. First. test those components or units—powersupplies, power amplifiers, and high-voltage modulators—known to exhibit the highest failure rate. Table 7-2 illus-trates this procedure.

2. Second, test the system function by function in asequence corresponding to the normal flow of signalsthrough the system. This approach, though frequentlytime-consuming, is more logical and consequently moreeasily learned by maintenance personnel.If testing is to be fully automatic, the difference in diag-nostic time between the two approaches is negligible.

The complexity of the automatic system usually can bereduced greatly if self-isolating units, i.e., those units of asystem with a BIT capability, are scanned before initiat-ing a fault isolation procedure. The diagnostics associatedwith the prime system can be further simplified by group-ing components of like functions into a self-isolatingmodule. If the module is not of the throwaway type, theBIT equipment output must be compatible with the ATEat the intermediate or depot level for further diagnosticsand repair. The self-contained concept is illustrated in thetest sequence shown in Fig. 7-2 (Ref. 19) for the liquidcoolant subsystem depicted in Fig. 7-1.

The example test sequence of Fig. 7-2 begins with adetermination of whether the coolant temperature T1 isbelow the established maximum Tmax, and whether thecoolant flow F1 exceeds the specified minimum value Fmin.When both answers are affirmative, the test equipmentwill indicate that the system is functioning properly.However, if Tl > TmaX, a test of the heat exchange radiatorinput temperature T3 and output temperature T2 must bemade. If T2 and T3 are within the specified range, the faultmust be in the coolant cabinet. If T2 and T3 are not withinspecified range and the flow rate F2 into the radiator isadequate, then the heat exchange radiator is suspect. Ifthe flow rate F1 into the power amplifier is not adequate,

TABLE 7-2. TYPICAL TEST SEQUENCE (Ref. 19)

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Figure 7-2. Test Sequence for Liquid Coolant Subsystem (Ref. 19)

the flow in the closed loop system is being blocked. Since isolating units and then to a specific failure logic, usuallythere are no pressure differentials built into the system,manual procedures must be used for fault isolation.

A possible sequence for the maintenance testing of atrack radar transmitter is shown in Fig. 7-3 (Ref. 19). Thelogic shown in Fig. 7-3 is illustrative only and is notintended to be complete. Detailed design of the sensorsand equipment to measure pulse shape, frequency spec-trum, and output products is a complex task. The sixparameters—Power Supplies 1 and 2, oscillator outputand frequency, amplifier phase, and noise output aretested in rapid succession. If a failure is detected. theself-isolating units are scanned first for probability failureindication. If none is detected, the particular failure logicis followed to isolate the failed unit. This method, i.e.,progressing from failure indication to the survey of self-

7-14

permits the design of less complex test equipment. Thetest sequence should be initiated by an internal clock tosynchronize the steps and insure that required outputs areavailable when required for further sequence decisions.The other circuitry would be conventional logic circuitry,threshold detectors, sample and hold circuits, or simpleanalog circuits.

If a centralized automatic test system is used, the testsequence is usually computer controlled and thus requiresthat the programs be developed in close coordinationwith engineers responsible for design of the authomatic testsystem. Similarly, changes to the test sequence necessi-tated by hardware changes are implemented by modifica-tion to the test program structured programming sim-plifies the required changes.

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Figure 7-3.

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DOD-HDBK-791(AM)

The system designer responsible for the automatic testfunction must consider the use environment when estab-lishing the limits and thresholds for the parameters to bemeasured; this will insure that only true malfunctions areisolated. For example, if a stabilized inertial tank plat-form is being checked in a laboratory, the parametersbeing monitored would be stable and noise free. In thetrue tank environment, however, the same stabilized plat-form would experience tank motion and gyroscopic torqu-ing, which would result in less stable parameters in anoisier background.

As a general guideline, the test designer should providefor all data manipulation within the test equipment tominimize the number of arithmetic operations required oftest personnel. For example, if power is the measuredparameter, the technician should not be required to readthe voltage and current—and then multiply the twoparameters—to determine power.

7-5.7 SELECTION OF TRANSDUCER ORSENSOR

When the parameters to be tested can be readily mea-sured without conversion, e.g., voltage, they do notrequire parameter modification by a transducer. A param-eter, such as electric current, can be transduced by meansof a transformer and a known resistance to a desiredvoltage. However, temperature, strain, vibration, pres-sure, and power parameters require more sophisticatedtransducers. The selection of transducers is a task requir-ing knowledge of measurement requirements and trans-ducer response characteristics. The five characteristics oftransducers that are of particular interest to the maintain-ability engineer are

1. Slability. The ability to produce a constant out-put for constant input within a given time constant

2. Repeatability. The ability to duplicate the outputafter a change in input and then return to the previousvalue

3. Procurability. The ability to be procured involume on the open market

4. Calibration. Should not be required except as asimple, scheduled, preventive maintenance task

5. Output. Should be amenable to multiplexingwhen the feature will simplify data transmittal; trans-ducer impedance should be low enough to prevent noisecoupling.

The electronics associated with many transducers con-sists of a bridge circuit in which the transducer is one legof the bridge. The output voltage is then the voltagerequired to balance the bridge. A useful secondary outputfrom such a circuit is the null voltage, i.e., the minimumoutput of a circuit as a function of an adjusting device.Although it is not good design practice to sense a “noreading” value, this bridge null voltage can validate theoutput as well as provide a point at which to begin faultisolation within the automatic test system. The transduc-ers required to implement the logic of Fig. 7-2, for exam-ple, may vary from simple temperature transducers tocomplex subsystems. The measurement of phase noisecould require a phase-to-amplitude modulation (P/AM)

7-16

conversion and then a determination of the AM level,which results in a multistage transducer.

A synchronized detection, sample, and hold device isrequired to monitor the radio frequency (RF) levels in theexample. A common circuit for all these functions, withappropriate attenuation to account for all stages of ampli-fication, would reduce the number of different transduc-ers and simplify calibration.

7-5.8 STIMULI SELECTIONStimuli are the inputs or signals furnished by the test

system to the UUT to allow simulation of its operation.The selection of the test set stimuli must be made simul-taneously with transducer selection to insure that the typeand magnitude of the stimulus are compatible with theprime system. For example, if the stimuli introduced is anelectric current to check circuit continuity, the magnitudeof the current must be limited to avoid the unwantedoperation or activation of a circuit component.

The application of the stimuli also must be carefullyconsidered relative to the functions exercised in the UUT.For example, in the track radar it is desirable to knowwhether the radar is operating within specified perfor-mance limits. This information can be provided by intro-ducing an RF pulse into the antenna immediately afterthe radar pulse at a time when the normal radar return isnot expected. This test pulse would then be processed andrecovered by a special gate in the video circuit and testedfor amplitude and delay. Although this example proce-dure would test a major portion of the radar, it would nottest the status or dynamic characteristic of the range-gating circuitry or transmitter synchronization, both ofwhich are vital to the satisfactory operation of the radar.A second test step should be introduced into the logic toassess these functions. In the track radar transmitterexample, normal signal generation and pulse sources areused to relieve the need for auxiliary stimulus sources.This enhances the simplicity of the automatic testcircuitry.

7-5.9 OUTPUT FORMATThe output of the test equipment may take various

forms—from the simple to the complex—e.g., “go”/“nogo” light, meter reading or indication, analog display, orcomputer-generated printout. Regardless of the form, thereadout must be both readable and understandable to theuser. Consider the elementary case of a single meter usedto display the readout from various inputs; the readershould not be required to introduce a scale factor tointerpret correctly the meter readout from differentsources. This practice can result in confusion or errors.The display or readout format may be dictated by thecriticality of the data and the sampling rate. To assurecrew safety and/ or mission success, it may be necessary tosample data at the microsecond or nanosecond rate; inthis case an analog display of the parameters indicatingsystem status would be appropriate. The contribution tothe system failure probability of latent faults. near coinci-dent faults, fault coverage, and fault recovery times allwill determine sample rate and output form.

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If a printout or permanent record is generated by thetest equipment, the record should be easily readable witha minimum of interpretation. Structure the software sothat the parameters or components are identified byname, rather than a numerical code. The output alsoshould be easy to repproduce i.e., avoid nonreproducibleink colors, odd-sized output forms, and punched cards ortape. Where the output data from one test systembecomes the input data to a second test system, insure thatthe software programs and language are compatible. Adata translation process is costly and subject to error.Moreover, data translation becomes impractical if thedata must be translated in real time.

As indicated in Fig. 7-3, the output consists of severaldisplays. A printout of the unit failures, with time anddate markings, may be desirable if the size of the systemwere large enough to justify the added complexity. Theprintouts would provide audit trails of system and com-ponent reliability and would serve as a basis for inventorycontrol and to support warranty claims. Power outputand noise level in this example would be recorded or readfrom a meter at regular intervals.

7-5.10 DIAGRAMSPrevious paragraphs have provided guidance to the

designer of test equipment to assist him in establishingdiagnostic features that achieve the maintainability objec-tives that are compatible with the system being tested. Thefact that an automatic test system might be fashionableshould not be the deciding factor in pursuing thisapproach the complexity of the prime system, reliabilityof the system components and test equipment, and costmust enter into the decision. To facilitate the technician’stask regardless of whether the test system selected ismanual, BIT, and/or ATE—maximum use of block dia-grams, flow diagrams, schematics, logic diagrams, andfront panel layouts should be developed to assist with thenecessary measuring, switching, and control circuits.

7-6 EXAMPLESThree examples are presented

1. likelihood Analysis Computer Program2. Advanced Attack Helicopter3. T700 Gas Turbine Engine

which illustrate the applications of the design considera-tions presented in par. 7-5.

7-6.1 LIKELIHOOD ANALYSIS COMPUTERPROGRAM

The application of the state-of-the-art diagnostics isnot limited exclusively to electronic components andassociated circuitry; techniques have been developed formechanical systems (see par. 7-3.4). one of the importantdiagnostic techniques resulting from Ref. 20 was the Like-lihood Analysis Computer Program which detects differ-ences in vibration signatures between rotating parts thatare operating satisfactorily and those that have a faultthat signals impending failure. Under the AIDAPS Pro-gram, vibration signature data for gearboxes, transmis-

DOD-HDBK-791(AM)

sions, and similar rotating items were developed. The testdata were gathered for helicopters, but the principles,diagnostic techniques, and software can be adapted toArmy vehicles in general. To assist in applying the tech-niques, computer programs for diagnostic evaluationwere developed. The program uses Fast Fourier Trans-form (FFT) a mathematical tool for analyzing a peri-odic function—to reduce the analog data to digital form.The raw data are also categorized in power spectral-density bands. A program for likelihood analysis com-putes the statistical means and variances for referencedata signatures and for test data signatures; it then com-putes probabilities that the signature populations in var-ious frequency bands are identical within a prescribednumber of standard deviations. By this procedure themechanical replaceable unit is classified as “satisfactoryoperation” or “impending failure”. Signatures for theimpending failure classes for specific components aredeveloped by sensing and processing specific failure-related data. Fig. 7-4 illustrates the diagnostic logic usedfor the analysis of vibration signatures of rotating com-ponents.

7-6.2 ADVANCED ATTACK HELICOPTERSThe Advanced Attack Helicopter (A AH) was designed

to surpass any existing Army aircraft in weaponry sophis-tication (Ref. 21). This aircraft provides an excellentexample of how BITE was applied to solve critical diag-nostic problems.

The Fault Detection Location Subsystem (FD/LS)for the AAH employs a combination of BITE ATE,ground test equipment, technical manuals, and manualdiagnostic procedures to accomplish its function. Table7-3 provides a list of the systems that can be tested on theA AH by BITE. This list is part of the operator’s checklist.

The on-aircraft fault isolation is provided for replace-able units in the following subsystems:

1.2.3.4.5.6.7.8.9.

10.11.12.13.14.15.16.17.18.

In the

FD/LS itselfArmamentFlight controlAuxiliary powerPilot’s night visionIntegrated helmet sight and displayFuelEnvironmental controlAntiice and deiceFire controlInstrumentsNavigationAvionicsMultiplexElectric powerFire detection and extinguishingIntermediate gearboxTail rotor gearbox.area of electronics, the FD/LS detects and iso-

lates faults to the repairable module level. The replaceableunits have sufficient test provisions to detect and isolate98% or more of all failures to the repairable module. Fig.7-5 shows the data entry keyboard used by the crew to

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Figure 7-4. Data Analysis Methodology for Vibration signals (Ref. 20)

TABLE 7-3. DIAGNOSTIC PORTION OF OPERATOR’S CHECKLIST

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Figure 7-5.

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DOD-HDBK-791(AM)

locate faults during preflight checkout and by the mainte-nance personnel to locate and diagnose faults duringpostflight operations. The diagnostic data supplied by theBITE are presented on the alphanumeric display on thecopilot, gunner’s panel.

A critical diagnostic problem on the AAH relates to theautomatic control of the stabilitor, i.e., a movable, hori-zontal stabilizer located near the tail of the helicopter.The leading edge of the stabilitor moves up or down tomaintain an even pitch as the helicopter makes transitionsin speed. A predecessor to the AAH crashed because itsairspeed indicators were not connected to the positionsensors, which caused the helicopter to pitch over whenmaking the transition from hover to forward flight.Accordingly, specifications for stabilitor control werewritten to guard against instability hazards. Fig. 7-6 illus-trates the redundant control system and diagnosticsemployed to insure a positive connection between theairspeed indicator and the position sensor. The BITequipment circuitry simulated failure signals for everycritical fault mode so that the monitor could be automati-cally checked out in preflight simulation.

7-6.3 T700 GAS TURBINEThe diagnostic analysis of the T700 gas turbine is typi-

cal of analysis results for gas turbine engines used in Armytrucks and aircraft; the data are excerpted from Ref. 23.Table 7-4 summarizes the results of diagnostic effective-

ness analysis, i.e.. ratio of the failures not detected by thediagnostic equipment and procedures to the total failures,for engine modules. Table 7-5 is a typical page from theFailure Mode and Effect Analysis backup for Table 7-4for the Cold Section Module (CSM) of the T700 engine.The numerically coded fault isolation elements of Table7-5 are identified in Table 7-6. The failure rates in Table7-5 Categories II and IV—for the various engine com-ponents, Column 1, that can be isolated within the CSMare shown in Columns 4 and 5, respectively; the faultisolation elements are shown in Column 6. These failurerates for Categories II and IV—Columns 4, 5, 7, and8—are summed for insertion into the summary of diag-nostic effectiveness data in Table 7-4. These analysis sam-plings are typical of those performed to validate thedesign effectiveness of the diagnostic equipment and thetestability features built into a gas turbine engine.

7-7 CHECKLISTSTable 7-7 provides a checklist to aid in the design of

diagnostic networks and function grouping in the primeend-item for optimal testing. Table 7-8 lists questions tobe answered by the designer to assist in the determinationof the required diagnostic and preferred test equipment. Ifthe answer is “no" for any answer on the checklists, thedesign should be restudied to determine whether correc-tion is required.

Figure 7-6. Redundant FBW Stabilator

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TABLE 7-4. SUMMARY OF DIAGNOSTIC EFFECTIVENESSFOR GAS TURBINE ENGINE (Ref. 23)

Nonisolated Percent ofTotal Events Events Nonisolated to

(per 106 EFH) (per 106 EFH) Total Events

Class II Class IV Class II Class IV Class II Class IV TotalLRUs 0.0924 44.686 0.0009 2.1948 0.97 4.91 4.90

CSM 4.892 27.977 0.6612 5.7692 13.52 20.62 19.56

HSM 0.228 8.958 0.0023 1.7916 1.01 20.0 19.53

PTM 0.298 12.1 0.0090 0.802 3.02 6.63 6.54

ACC 0.0 3.698 0.0 0.0739 0.0 2.0 2.0

ENG 12.88 0.0 0.188 0.0 10.0 0.0 10.0

Totals 7.390 97.419 0.8614 10.6315 11.66 10.91

Combined Totals 104.809 11.4929 10.97EFH—Engine Flight HoursLRU—Line-Replaceable UnitCSM—Cold Section ModuleHSM—Hot Section ModulePTM—Power Turbine ModuleACC—Accessory ModuleENG—Engine, Assembly

Class II—Failures that result in in-flight shutdowns, i.e., unrecoverable power loss.Class IV—Failures that result in power loss or no start.

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TABLE 7-6. ELEMENTS FORFAULT ISOLATION (Ref. 23)

QUALIFICATION ITEMS1.2.3.4.5.6.7.8.9.

10.11.12.13.14.15.16.17.18.19.20.21.22.23.

24.25.

History RecorderMaster Chip DetectorGas Generator Bearing AccelerometerPower Turbine Bearing AccelerometerErosion IndicatorOil Level Sight GlassOil Temperature SensorOil Pressure SensorOil Filter Actual Bypass SensorOil Filter Impending Bypass SensorOil Screen—A Sump AftOil Screen—A Sump ForwardOil Screen—B SumpOil Screen—C Sump AftOil Screen—C Sump ForwardOil Screen—Accessory GearboxGas Generator Speed SensorPower Turbine Speed SensorTorque SensorFuel Filter Actual Bypass SensorFuel Filter Impending Bypass SensorGas Temperature IndicatorSOAP Sample Accessibility (Spectrographic OilAnalysis Procedure)FOD Signal (Foreign Object Damage)Airframe Fire Warning System

SUPPORT ITEMS26.27.28.29.30.31.32.

Health MonitorOil Quality AnalyzerBearing MonitorBorescope SetPilot/Flight CrewPreventive Maintenance CrewCorrective Maintenance Crew

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TABLE 7-7. TESTABILITY CHECKLIST

1. Was the testability design concurrent with prime equipment design?2. Did test point selection, and design and testing partitioning play a major role in the layout and packaging of the

system?3. Was a failure modes and effects analysis available and used as a part of the testability analysis?4. Were BIT/BITE and ATE analyses and fault simulation used to evaluate the coverage and effectiveness of the test

equipment design?5. Did the testability design approach evolve as information was obtained from analyses and test experience?6. Was a level of repair analysis used as part of the testability analysis?7. Are test points located on the front panel wherever possible?8. Are system test points accessible without removing modules and/or components?9. Is accessibility of external test points assured?

10. Are test points conveniently grouped for accessibility and sequential arrangement of testing?11. Is each test point labeled with a name or symbol appropriate to that point?12. Is each test point labeled with the in-tolerance signal or limits that should be measured?13. Are test points labeled with the designation of what output is available?14. Are all test points color coded with distinctive colors?15. Are test points provided in accordance with the system test plan?16. Are test lead connectors used that require no more than a fraction of a turn to connect?17. Are test points located close to the controls and displays with which they are associated?18. Is the test point used in an adjustment control?19. Are means provided for an unambiguous signal indication at the test point when the associated control has been

moved?20. Are test points located so the technician operating the associated control can read the signal on display?21. Are test points provided to isolate a failure to replaceable units or modules?22. Are fan-out cables in junction boxes used for testing if isolated test points are not conveniently provided?23. Are test points planned for compatibility with the maintenance skill levels involved?24. Are test points coded or cross-referenced with the associated units to indicate the location of faulty circuits?25. Are test points provided to reduce the number of steps required—i.e., split-half isolation of trouble, automatic

self-check sequencing, minimizing of step retracing or multiple concurrent tests?26. Are test points located to reduce search time—i.e., near main access openings, in groups, properly labeled, near

primary display to be observed from working position?27. Are test points that require test probe retention provided with fixtures so that the technician will not have to hold the

probe?28. Are built-in test features provided wherever standard portable test equipment cannot be used?29. Are test points adequately protected and illuminated?30. Are routine test points provided that are available to the technician without removal of the chassis from the cabinet?

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TABLE 7-8. DIAGNOSTIC CHECKLIST

1.

2.

3.

4.

5.6.

7.8.

9.

10.

11.

12.

13.

14.15.16.

17.

18.19.20.21.22.

23.24.25.26.27.28.29.

30.31.

Since BITE manual or automatic—usually will increase the complexity and cost of a system, is this requirement theresult of careful study?Was the progress of the BITE and ATE design monitored and evaluated? Were adequate time and funds allocated inthe development plan to prove the effectiveness of test equipment? Was the test equipment used to verify items inproduction?Were adequate demonstrations, as required contractually, performed to determine BITE or ATE performance? Wasthe laboratory performance—achieved by inserting faults or malfunctions—consistent with that observed in a realenvironment?Were the number of false alarms, could not duplicates, and retests satisfactory consistent with the contractuallimitations specified?Are the instructions for using test equipment in a step-by-step format?IS a signal provided that shows when the test equipment is warmed up? If it is not feasible to present such a signal, isthe warm-up time required clearly indicated near the warm-up switch or is a lockout provided until equipment haswarmed up?Is a simple check provided to indicate when the test equipment is out of calibration or is otherwise not functioning?Do test equipment displays that require conversion of observed values have conversion tables attached to theequipment with the scale factor by each individual switch position or display scale?IS adequate support provided for UUT test equipment that must be taken into the work area so the technician doesnot have to support the test equipment or to take separate support devices to the work area for this purpose?Does portable test equipment have a mass of under 23 kg (50 lb) if it is to be carried by one person? It not, can it beseparated into two or more assemblies?Are display lights, automatic power switches, or printed warnings provided to insure that test equipment is turnedoff when testing is completed?Is the purpose of test equipment and special precautions displayed in a conspicuous place on the outer surface of thetest equipment?Are units that are not self-checking designed to be checked in the operating condition without the aid of special rigsand harnesses wherever possible?Are selector switches provided in lieu of a number of plug-in connectors?Is test equipment designed to be capable of connection to prime equipment within 2 min or less?Does continuing analysis of automatic test hardware design provide assurance of greater maintainability improve-ment potential than is possible by system design changes?Does error analysis of the checkout capability of the test system verify conformance to specified detectabilityrequirements?Does the test system provide fault isolation to the desired replacement level?Is test hardware design compatible with system environrnental requirements?Has the use of commercial test equipment or Government-furnished test equipment been considered?Was the design of test equipment based on an adequate failure prediction baseline?Are self-test features and procedures adequate to insure proper automatic test operation over sustained periods ofsystem test?Has test equipment been built with sufficient ruggedness to reduce the need for frequent calibration?Are all parameter and measurement limits established?Is the output format satisfactory to the user; can the output as formatted be used for data collection or interpretation?Is the automatic test function operable without difficulty by all levels of maintenance personnel?Do the sensors operate without disturbing or loading the system under test?Is the automatic test hardware failsafe, i.e., will its failure not result in system failure?Has the availability of necessary inputs to and the conditioning of ATE in the installed position been verified—e.g.,power, cooling, external references?Are untestable parameters or units identified; can the system design be changed to eliminate them?Has a closed cycle heating or cooling system, if necessary, been considered to maintain the proper test setenvironment, and if used, has the source of coolant or the source of heat been ascertained and coordinated withsystem design?

(cont’d on next page)

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TABLE 7-8 (cont’d)32. Have transducers been selected that give quick, consistent, and repeatable responses?33. Are test stimuli—introduced into a system or component to determine its status safe. i.e.. will not damage the

system or component or cause inadvertent functioning?34. Has a sneak circuit analysis been performed on critical mission components that interface with test equipment,

particularly where circuit changes have been introduced?35. Are the selection and acquisition of test equipment in accordance with AR 750-43—e.g., is DA PAM 700-21-1,

preferred items list (PIL), used for priority selection of Army standard test equipment?36. Is the test equipment hardware and/or software designed for logistic supportability?37. Do the ATE and/or test program sets use a DOD-approved automated test language, e.g., C/ATLAS or IEEE-7 16?38. Is the ATE/test equipment selected compatible with the UUT design?39. Was the ATE/test equipment selected for commonality and/or compatibility with ATE, test equipment at other

maintenance levels?

REFERENCES1. IDA/OSD Reliability and Maintainability Study,

Vol. II: Core Group Report, IDA Report R-272,November 1983.

2. IDA/OSD Reliability and Maintainability Study,F-16, APG-66 Fire Control Radar Case StudyReport, IDA Record Document D-21, August 1983.

3. Jane's Defence Weekly, 5, No. 8, 352 (1 March 1986),London, England.

4. MIL-STD-2165, Testability Program for ElectronicSystems and Equipments, 26 January 1985.

5. IDA/OSD Reliability and Maintainability Study,R&M Program Review Elements Document, IDARecord Document D-26, August 1983.

6. AMC Pamphlet 70-2, AMC-TRADOC MaterielAcquisition Handbook, 26 March 1987.

7. V. Tasar and Henry L. Ohlef, “A Method for Deter-mining the Statistically Weighted Percent FaultDetection Coverage of a Self-Test System”, Proceed-ings of the 1979 Annual Reliability and Maintainabil-ity Symposium, p. 39.

8. DOD 3235.1-H, Test and Evaluation of System Reli-ability, Availability, and Maintainability: A Primer,p. 3-8, March 1982.

9. IDA/OSD Reliability and Maintainability Study,Vol. II: Core Group Report, IDA Report R-272, pp.IV-15 to IV-21, November 1983.

10. IDA/OSD Reliability and Maintainability Study,Vol. II: Core Group Report, IDA Report R-272, p.11-12, November 1983.

11. IDA/OSD Reliability and Maintainability Study,Vol. IV: Technology Steering Group Report, IDAReport R-272, p. IV-34, November 1983,

12. IDA/OSD Reliability and Maintainability Study,7700 Engine Case Study, IDA Record DocumentD-22, p. IIC-95, August 1983.

13. Calvin Holbert and Gary Newport, Helicopter Main-tenance Effectiveness Analysis, USAAMRDL-TR-75-14, Eustis Directorate, US Army Air MobilityResearch and Development Laboratory, Fort Eustis,VA, May 1975.

14. MIL-STD- 1309, Definitions of Terms for Test, Mea-surement, and Diagnostic Equiment, 18 November1983.

15. Paul Harmon and David King, Expert Systems,John Wiley and Sons, Inc., New York, NY, 1985.

16. The General Dynamics System for Sneak CircuitAnalysis, General Dynamics Convair Division, SanDiego, CA, June 1983.

17. MIL-STD-785, Reliability Program for Systems andEquipment Development and Production, 15 Sep-tember 1980.

18. AIDAPS Program, First Interim TechnologyReport, Report 74010011, Garrett Air Research Manu-facturing Company, Los Angeles, CA. 30 December1973.

19. NAVORD OD 39223, Maintainability EngineeringHandbook, 1 February 1970.

20. D. B. Board, “Incipient Failure Detection in High-Speed Rotating Machinery”, Boeing Vertol Co., Phil-adelphia, PA, 10th Symposium on NondestructiveEvaluation held 23-25 April 1975.

21. A MC-SS-AA H-H 10000A, System Specification forYAH-64 Advanced Attack Helicopter, 30 July 1976.

22. S. W. Ferguson and K. E. Builta, “Development of aFly-by-Wire Elevator for the Bell Helicopter Textron214ST”, 35th Annual National Forum of the Ameri-can Helicopter Society, Washington, DC, May 1979.

23. Diagnostic System Effectiveness Report for GeneralElectric T700-GE-700 Engines, 18 December 1972.

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CHAPTER 8PREVENTIVE MAINTENANCE

This chapter describes the types of maintenance that are classified as “preventive’; the means for determin-ing the cost-effectiveness of preventive maintenance, and the trade-of between the inherent downtime forpreventive maintenance and the desired level of equipment performance and reliability. Reliability CenteredMaintenance (R CM) is discussed, and an example of its application is given. The Army Oil Analysis Program(AOAP) is presented as a major contributor to more efficient maintenance. Design considerations for ease oflubrication, servicing, and cleaning and preservation of equipment for use and for longtime storage areoutlined. A servicing checklist is provided.

8-0 LIST OF SYMBOLS

8-1 INTRODUCTIONMaintenance—actions necessary for retaining materiel

in, or restoring it to, a serviceable condition is precipi-tated by various causes and can occur in different loca-tions. Maintenance actions can be categorized as fallinginto three types—preventive, corrective, and servicing—defined as follows:

1. Preventive Maintenance. Performed to retain anitem in satisfactory operational condition by providingsystematic inspection, detection, and prevention of incip-ient failures. Detection and prevention may take placeeither before failures occur or before they develop intomajor defects.

2. Corrective Maintenance. Performed to restore anitem to a satisfactory condition by correcting a malfunc-tion that has caused degradation of the item below thespecified performance level.

3. Servicing Maintenance. Performance of any act—other than prevention or correction-–required to retainan item of equipment in operating condition. Suchactions include lubricating, fueling, oiling, cleaning, etc.Servicing does not include periodic part replacements orany corrective maintenance tasks.

The basic maintenance actions—preventive and correc-tive—can occur while the equipment is in or out of ser-vice. Thus in maintenance planning evaluations it isnecessary to recognize not only the type and criticality ofaction required but also the operational status and role ofthe equipment. This relationship is illustrated by Fig. 8-1,which shows that noncritical repairs, even though neces-

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Figure 8-1. Relation of Equipment Status on Availability

sary, can be deferred until the equipment is idle (betweenmissions) or until scheduled maintenance; this results inincreased equipment availability.

Preventive maintenance and servicing are more appli-cable to mechanical systems and basic structural elementsthan to electronic or electrical equipment. The routinecheckout of electronic or electrical equipment indicatesthat the equipment is performing satisfactorily or unsatis-factorily. Physical inspection remains the primary meansfor detecting degradation of structures and of manymechanical and electromechanical devices.

8-2 RELIABILITY CENTEREDMAINTENANCE

8-2.1 GENERALThe Reliability and Maintainability Subcommittee of

the Air Transport Association published a MaintenanceSteering Group document, Airline Manufacturer's Main-tenance Program and Planning Document (MSG-2)(Ref.1), which described the reliability centered maintenance(RCM) concept for new aircraft. This concept was sosuccessful in its initial application that the airlines appliedit to revise maintenance programs for older aircraft. TheUS Navy tailored the concept and applied it to the P-3aircraft. Through the issuance of Program ObjectiveMemorandum (POM) 78-82 (Ref. 2), the Army estab-lished the requirement that the MSG-2 concept, under thetitle “Reliability Centered Maintenance (RCM)”, beincorporated on all Army weapon systems and equipmentby the end of fiscal year 1979. DA Pam 750-40 (Ref. 3)implements Ref. 2. The US Army Materiel Command

(AMC) integrated the provisions of Ref. 3 into AMC-P750-2, Guide to Reliability Centered Maintenance (Ref.4). AMC-P 750-2 provides assistance in the preparationand implementation of the RCM program as directed inthe policies of DOD Directive 4151.16 (Ref. 5). AR 750-1(Ref. 6), AR 700-127 (Ref. 7), and MIL-STDS-1388 (Refs.8 and 9). Through the proper use of RCM procedures, aviable, realistic scheduled maintenance program can bedeveloped. Ref. 4 describes in detail how to use the RCMlogic and the failure mode, effects, and criticality analysis(FMECA) to develop a scheduled maintenance plan thatincludes the maintenance task and the maintenance inter-val for preventive maintenance checks and services(PMCS) and provides information for overhaul, ageexploration, economic analysis, and redesign.

8-2.2 OBJECTIVEMaintenance planning one of nine principal elements

of integrated logistics support (ILS)—includes develop-ment of the maintenance concept, reliability and main-tainability parameters, repair level determinations, main-tenance requirements. and supply support essential toadequate and economical support of the system orequipment (Ref. 4). As such, maintenance planning isnormally integrated into equipment design during theconcept exploration phase of the acquisition process aspart of the logistical support analyses. RCM is an essen-tial input to this planning.

RCM is based on the premise that maintenance cannotimprove upon reliability inherent in the design of hard-ware; good maintenance can only preserve this character-

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istic. The object of RCM is to preserve the inherent designlevels of reliability and accomplish it at minimum cost.The RCM concept uses decision logic to evaluate andconstruct maintenance tasks which are based on theequipment functions and failure modes. Evaluation ofequipment designs in accordance with RCM techniqueswill also determine when it is cost-effective to employpreventive maintenance and when it is not cost-effective.It is essential that the RCM analysis be integrated with themaintainability concepts for testability and diagnostictechniques.

8-2.3 RELIABILITY CENTEREDMAINTENANCE LOGIC

RCM logic is intended for application when the failuremode, effect, and criticality of component failure hasbeen identified. Fig. 8-2 (Ref. 4) illustrates the RCManalysis process—a process that is applied to each repair-able item in the equipment or system. The logic isdesigned to accomplish the following (Ref. 4):

1. By using data from the system safety and reliabil-ity programs, identify components in the system orequipment that are critical in terms of mission or oper-ating safety.

2. Provide a logical analysis process to determinethe feasibility and desirability of scheduled maintenancetask alternatives.

3. Highlight maintenance problem areas for designreview consideration.

4. Provide the supporting justification for scheduledmaintenance task requirements.Par. 4-9, Ref. 4, presents a detailed discussion on theapplication of the logic steps identified in Fig. 8-2.

The functional failure identified by the application ofRCM logic can be assessed for consequence of failure andis processed according to its severity category—i.e., cata-strophic, critical, marginal, or minor. The logic process isbased on the following (Ref. 4):

1. Minor or Marginal Severity Categories. Sched-uled maintenance tasks should be performed only whenperformance of the scheduled task will reduce the lifecycle cost of the equipment or system.

2. Critical and Catastrophic Severity Categories.Scheduled maintenance tasks should be performed whensuch tasks will prevent a decrease in reliability or deterio-ration of safety to unacceptable levels or when the taskwill reduce the life cycle cost of ownership of the equip-ment or system.

8-2.4 DISPOSITION OF RELIABILITYCENTERED MAINTENANCEANALYSIS

Each failure analyzed by the application of the RCMlogic leads the analysts to make a decision on the disposi-tion of each failure mode. The Data Record B: Item

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Reliability (R) and Maintainability (M) Characteristicsform, Fig. 8-3, is used to record the disposition. Instruc-tions for locating the data on the form are contained inpar. 4-8 (Ref. 4).

8-3 PREVENTIVE MAINTENANCEEVALUATION PARAMETERSAND TRADE-OFFS

The relative values of preventive maintenance actionsmay be determined as a function of several performanceand cost parameters. The value should be quantifiedwhenever possible to remove the decision from subjectivejudgment. However, performance and cost must never betraded off with safety.

Preventive maintenance actions incur cost in terms ofmaterials, man-hours, and equipment downtime; hencethe detailed analysis discussed in par. 8-2. The cost ofman-hours and repair parts and materials can be deter-mined rather easily. The cost of equipment downtime,however, is a function of the operational value of theparticular equipment in the situation that exists when it isdown. Accordingly, maintenance is scheduled into theoperations when there is the minimum risk that theequipment will be required for an operational assign-ment. The two chief parameters—availability and cost ofownership—relative to an optimum preventive mainte-nance program are discussed in the paragraphs thatfollow.

8-3.1 AVAILABILITYAchieved availability is the probability that a system or

equipment, when used under stated conditions in an idealsupport environment—i.e., available tools, parts, man-power, manuals, etc.—shall operate satisfactorily at agiven time. Note that supply downtime and waiting oradministrative downtime are excluded. Two equationsfor achieved availability are given

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Figure 8-3.

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MTBDE = mean time between downtime events, hMTTRS = mean time to restore system, h.

In Eq. 8-2, MTBDE includes both preventive and correc-tive maintenance; similarly, MTTRS includes downtimefor both preventive and corrective maintenance.

It is evident from Eq. 8-1 that an increase in operatingtime OT for the same total maintenance downtime willresult in a greater availability Aa1. Similarly from Eq. 8-2,a higher MTBDE for the same MTTRS will result ingreater availability Aa2. Likewise, a lower TPM (Eq. 8-l)or MTTRS (Eq. 8-2) will result in an increased availabil-ity. The trade-off decisions involve both the determina-tion of the most cost-effective method for achieving animprovement in reliability (MTBDE) or maintainability(MTTRS) and the determination of the cost-effectiveness.or value, of the resulting improvement in terms of theincreased life of the equipment.

The value of increased availability must be determinedon the basis of the planned use of each type of equipment.In simplest terms. increased availability can be equated tofewer required items. For example. if a requirement forhelicopters stated that 200 helicopters are required and anexpected availability prediction was 80%, it would beexpected that only 160 helicopters would be available foruse at any given time. Therefore, to be reasonably certainof having 200 helicopters in an operational state, 250helicopters would have to be procured. In this example a10% improvement in availability, i.e., 90%, would reducethe required procurement to 222 or by a value equal to theacquisition cost of 28 helicopters plus their operating andsupport costs.

8-3.2 COST OF OWNERSHIPPreventive maintenance versus repairing after condi-

tion monitoring is one of the major trade-offs for deter-mining when to perform preventive maintenance. Theadvantage of scheduled, on-condition maintenance is thatdamage would be detected when the equipment is alreadyout of service and in a condition to permit the repair to bemade. If the unserviceability occurs at a random time, thecost of repair must include the cost of providing therequired maintenance capability—e. g., travel time of per-sonnel, towing cost, and travel costs as well as the costrelated to the unavailability of the equipment.

The value of performing preventive maintenance on anitem in lieu of allowing the item to operate without main-tenance until failure can be measured in terms of relativecost. The performance of preventive maintenance is cost-effective if the reduction in repair cost plus the equipmentout-of-service cost exceeds the accrued preventive main-tenance cost. The following equations illustrate thishypothesis: From the standpoint of dollars only, Eq. 8-3 indicates

that for a positive value of Vpm i.e., Cpm > C cm, it is betterto adopt a policy of no preventive maintenance: a nega-tive value of V p m, i.e., C pm < C c m, indicates preventivemaintenance is the better policy. Cost considerations,however. cannot be considered in isolation. Consider a

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fan or alternalor belt; wear to failure without preventivemaintenance may be the less expensive option rather thanreplacement at a fixed number of’ miles. However, whenthe belt breaks even though a replacement belt andpersonnel are immediately available to make the repairthe equipment is down for a finite time period. Dependingon the circumstances, the interruption of function pro-vided by the disabled equipment may be intolerable.

Cost-effectiveness also is a factor in evaluating a pro-posed change to improve the maintenance capabilityand or reduce the preventive maintenance requirements.The cost-saving ratio, i.e., the cost saving divided by theoriginal cost, is a measure of maintenance cost-effective-ness. The cost of improvement includes the cost of designchanges, cost of new parts and perhaps new test equip-ment and technical manuals, and cost of incorporation.As previously indicated, cost may not be the sole measureof effectiveness. In the example in par. 8-2.4 the questionwas “to redesign the equipment to provide a test or inspec-tion capability or to live with the inherent reliability char-acteristics and risks”. Hovever, unless the improvementis introduced early in design, the cost of redesign andretrofit may be greater than the dollars saved by requiringless maintenance effort. Increased availability, and itsvalue in terms of reduced equipment procurement, isoften a more significant cost-saying factor than a reduc-tion in maintenance man-hours.

8-4 DESIGN CONSIDERATIONSDesign considerations for preventive maintenance are

generally the same as those for corrective maintenance.Access for replacement will usually provide sufficientaccess for periodic inspection, cleaning, and adjustmentas well. Instrumentation for fault isolation can also pro-vide for condition monitoring. Periodic servicing ofequipment imposes other criteria that are discussed in theparagraphs that follow.

Servicing of mechanical equipment is important toassure that the equipment achieves the expected usefullife. Ease of servicing is important in reducing downtimesand cost of ownership. A major reason for the high cost ofmaintenance is the repetitive frequency—some of whichmay be unnecessary—of service tasks. Ease of servicing isprovided by ease of access to servicing points and by useof standard servicing features for a large population ofitems—e.g., lubrication fittings, servicing locations, com-mon lubricants, and fuels.

Periodic servicing is important because of the possibil-ity and danger of impairing overall weapon system effec-tiveness if the servicing is not performed. All militarymateriel is serviced on a systematic schedule. One or moreof the following service operations are usually performed:

1. Lubrication—Oiling and greasing, and filling anddraining

2. Cleaning and preserving3. Adjusting and aligning.

Different types of equipment require different predom-inant types of service. For example, motor vehiclesrequire frequent lubrication and occasional cleaning and

adjusting. Gun systems and missiles are more prone torequire preserving. at least during peacetime. Ground testequipment requires adjusting and aligning to maintaincalibration. Peacetime operations place a strong empha-sis on cleaning and preserving stored equipment to pre-vent degradation.

8-4.1 LUBRICATION

8-4.1.1 Oiling and GreasingOiling and greasing (lubrication) of equipment is of

vital importance. The best designed mechanical and elec-tric mechanical equipment can and does fail completelydue to inadequate and/or improper lubrication. Lubrica-tion is often the only maintenance required for long,maintenance-free service. Equipment designs often evolvewith little thought given to the vast number of mainte-nance hours required in the field for periodic checking ofoil levels and lubrication. Lubricant-free designs and, orrapid lubrication capability should be built into theequipment and given equal design importance with theproper functioning of the equipment. Lubrication require-ments for mechanical items—bearings, gears, shafts—isusually recognized. There are particular lubrication re-quirements for electronic and electrical equipment. Syn-chronous switch shafts, generators, motors, and relayarms have been a serious source of malfunction and ofsubsequent destruction of the equipment.

Working surfaces subject to wear or deteriorationshould be provided with the appropriate means of lubri-cation. Lubricants, fluids, and associated products shouldbe selected in accordance with the provisions of US ArmyMateriel Development and Readiness Command (DAR-COM) Regulation 750-11 (Ref. 1l). This regulation estab-lishes the Belvoir RD&E Center as the AMC focal pointon the proper selection and use of the packaged productsit governs. In this role the center’s Fuels and LubricationDivision provides the coordination and approval neces-sary to insure that lubricant orders and technical manualscontain only current standardized product specifications.The regulation

1. Explicitly prohibits the random introduction ofproprietary products

2. Requires compelling justification for the use ofnonstandard products as opposed to those qualified inaccordance with military and, or federal specifications, orpurchase descriptions

3. Imposes MIL-STD-838 (Ref. 12) on all designs,developments, and acquisitions

4. Insists that all procurement requests, solicita-tions, and contracts have lubricant order or technicalmanual approval before acceptance of the first unit.

From the maintainability standpoint equipment shouldbe designed to use only one type of oil and one type ofgrease. The types should be the same as those for otherequipment to be used in the same operational locations.Where a special lubricant is required, such as high or lowtemperature operational requirements, each lubricationfitting should be clearly labeled with the grease or oil

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specification letters 6.4 mm (0.25 in.) high; the labelshould be placed as close to the fitting as is suitable.

In applications requiring a high load-carrying capacitywith minimum space requirements, bearings containingtheir own supply of lubricants are highly desirable. How-ever, these bearings should be provided with some meansof external relubrication, e.g., the oil hole maybe througha synthetic seal pierced by a hypodermic needle or anentrance may be drilled in the bearing and lead out to aneasily accessible point on the housing. The loss of lubri-cant back through an oil hole so provided or the entranceof contaminants into the lubricant must be avoided.

The lubrication of aircraft equipment is most impor-tant due to possible wide variations in temperature withina short time.

Equipment should be designed to use the minimumnumber of lubrication fittings, and they should be of thesame size and of a standard type. Typical shapes of lubri-cation fittings are illustrated in Fig. 8-4. All grease fittingsshould conform to specification MIL-F-3541 (Ref. 13).and all electronic lubrication designs should conform toMIL-STD-454 (Ref. 14).

Grease fittings should be readily and easily accessibleand positioned to provide positive mating with the greasepressure gun. Where a grease location is not easily accessi-ble, extension lines should be built into the equipment tobring the grease fitting to an accessible location on theoutside of the equipment. The fitting end of the lineshould be securely anchored to withstand rough use. Theuse of grease cups, exposed oil holes, and oil cups shouldbe avoided.

The following design features relating to lubricants andlubrication fittings are recommended:

1. Consider the use of a central mechanism forapplying lubricant.

2. Provide lubrication fittings and reservoirs for alltypes of plain annular and plain self-aligning bearinginstallations as shown in Fig. 8-5. It is not necessary toprovide a means for lubricating plain bearings fabricatedof oil-impregnated sintered metal (bronze or iron), pro-vided the bearings are not expected to maintain lubricitybeyond the life of the lubricant with which they areimpregnated. Where the amount of lubricant contained inthe bearing is not sufficient to last for the life required,provide lubrication fittings or reservoirs to contact theouter surface of the sintered bearing. Incorporate sealingprovisions in all plain bearing installations to prevent theprogress of contamination between the moving surfacesand into the lubricant.

3. Oil seals should be easy to replace. Design toavoid blind fitting. Seal seatings and lands should beprovided with adequate openings for driving the seals out.Oil seals should retain their elasticity during long periodsof storage.

4. Dipsticks should be provided for measuring oillevels. They should be graduated to show the amountrequired for filling. Contrast between the finish of thegage and clear, thin oil should be provided by specifying aroughened surface or metal with a dark. dull finish.Locate oil dipsticks and other level indicators so that they

Figure 8-4. Typical Lubrication Fittings

may be fully withdrawn without touching other pieces ofequipment and away from hot areas of engines.

5. Provide magnetic chip detectors equipped withwarning lights, rather than electrical detectors, in lubri-cating systems. Most electrical chip detectors requirecomplete oil drainage: however, it is unnecesary to drainthe oil when inspecting magnetic chip detectors. In elec-trical detectors particles similar to carbon sludge orgraphite although harmless to engine operation willproduce an indication of the test light. i.e., falsely indicatea maintenance problem. Magnetic plugs should conformto Military Standard MS-35844 (Ref. 15) and should beused as practical throughout all fluid handling systems.Particular attention should be given to sumps or crank-cases, gearboxes, positive displacement pump inlets, andwherever iron or steel chips may endanger the life oroperation of equipment.

6. Design equipment to operate on as few differenttypes and grades of standard lubricants as possible. Oilfilters should be standard in range and sizt, and filterelements should be easily removed and replaced.

7. Avoid designs that require high-pressure lubri-cants.

8. A built-in, automatic lubrication system is desir-able for a piece of equipment that relist operate continu-ously for long periods of time, especially in dusty condi-tions, or when its lubricants tend to be forced frombearing surfaces by heavy impact or vibration loads.

9. Provide a schedule for all lubrication require-ments that shows the frequency of lubrication, the type oflubricant, specific points requiring lubrictaion, methods,and the cautions to be observed.

10. Provide adapters to permit the use of equipmentwith conventional filler parts when using pressure oilinginstead of the gravity-fill method.

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Figure 8-5. Bearing Lubrication Methods

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11. Select a lubricant, when possible, that can beused in a given piece of equipment for both operationaland storage purposes. Consider the use of prelubricatedgears and bearings, packaged in plastic. to meet thisrequirement.

12. Provide easy access to the equipment for directlubrication if lubrication points are not feasible.

13. Design lubrication points with a reservoir areato reduce the frequency of required lubrication.

14. Provide guards around lubrication points thatmay require servicing while equipment is operating toprevent accidental insertion of hands or tools into operat-ing equipment.

Lubrication charts should be provided with each pieceof equipment. The material of the chart should be water-proof and oilproof, and the chart should be printed onmaterial suitable for rough handling. All necessary in-formation for lubrication, including specification num-bers, should be included on the charts in the largest lettersthat the size of the chart will permit. Suggested lubrica-tion time intervals should also be included. Fig. 8-6 is anexample of a lubrication order.

8-4.1.2 Army Oil Analysis Program (AOAP)The policies and procedures governing the Army oil

Analysis Program (AOAP) are described in AR 750-22(Ref. 16). The objectives of the AOAP are

1. Improve operational readiness of Army equip-ment

2. Promote safety3. Detect impending component failures in time to

avoid more costly and extensive repairs4. Conserve lubricating and hydraulic oils by apply-

ing on-conditional oil changes.All Army aircraft and those nonaeronautical systems

specified in TB 43-0210 (Ref. 17) are enrolled in theAOAP. Each month thousands of lubricating andhydraulic oils are analyzed at AOAP laboratories at spec-ified intervals for contamination and wear-metal concen-trations. By means of these tests the technicians candetermine equipment parts that show signs of possiblemalfunction. Water seepage. rust particles. and oil dilu-tion are additional problems that surface. Detailed oper-ating procedures for the AOAP are contained in TB43-0106 (Ref. 18) and TB 43-0210 (Ref. 17). The AOAP isa major contribution for improving preventive mainte-nance procedures.

The US Army Materiel Command is charged withexercising staff supervision over the AOAP.

8-4.2 FILLING AND DRAINING

8-4.2.1 GeneralNo repeated operation of maintenance should be given

more attention than the details relating to the ease andrapidity of refueling and reoiling. Fuel, exotic fluids andgases, oil, hydraulic fluids, water, and compressed airsystems should redesigned to permit the most rapid, totaloverall inspection. The necessity for opening doors and

hatches for inspection or to gain access to service pointsor filler caps should be reduced. Servicing points forchecking, filling, and draining fuel, lubricant. hydraulicfluid, and coolant should be readily accessible, but pro-tected. The need for special tools should also be elimi-nated wherever possible.

Vehicular fuel tanks should be designed and fabricatedto eliminate or minimize internal corrosion when in use orin storage (empty), and the interiors of tanks should beaccessible for inspection and cleaning.

8-4.2.2 Filling RequirementsThe design recommendations that follow should be

considered:1. The fuel outlet should be located at least 19 mm

(0.75 in.) above the bottom of the sump or tank or at thebottom of the sump or tank with a 19-mm (0.75-in.)stand pipe.

2. The fuel tanks should be capable of acceptingfuel at 198 min (50 gpm) when tank capacity exceeds189 (50 gal). Tanks having capacities of less than 189(50 gal) should be capable of being filled at the 189- min(50-gpm) rate when using a standard 35-mm (1.375-in.)diameter nozzle that is 406 mm (16 in.) long. The fillerneck should have a flexible seal to fit such a nozzle and anopening other than the filler neck itself for venting dis-placed air.

3. The tank filler should be located to prevent tothe maximum extent possible entrance of dirt, water,and foreign matter into the fuel tank.

4. For gravity-filled tanks the tank filler should be

well as forced-nozzle filling. Filler necks should also belocated to permit use of rigid spout fuel nozzles conform-ing to MS-17967 (Ref. 19).

5. Fuel filler neck dimension should be a minimum

ity or less, and 63.5 mm (2.5 in.) for larger fuel tanks.Water and coolant filler necks should have a minimumdiameter of 38 mm (1.5 in.) and should be located so they

MIL-C-13984 (Ref. 20).6. The type of fuel to be used and the tank capacity

should be stenciled or marked adjacent to the filler open-ing of the equipment.

7. Fuel, oil, and coolant filler caps should be colorcoded as shown in Table 8-1.

8. Locate the hydraulic reservoir so that it is visu-ally accessible for refilling. Also the fluid level should bevisible while filling.

9. Level indicators of the type indicated by Fig. 8-7for fuels, oils, water, or other liquids should be locatedoutside of the vehicle or tank, when possible, and shouldbe easilu viewed without removal of doors or covers. Forlevel indication of infrequently checkd units, such asgearboxes, dipsticks with integral sealing caps should beprovided. The use of threaded plugs for level indicationshould be confined to such locations as automotive dif-ferentials because at such locations other types should betorn off by rocks, mud, or stumps. When such plugs are

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Figure 8-6. Example of a Lubrication Order (cont’d on next page)

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8-12

Figure 8-6 (cont’d)

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DOD-HDBK-791(AM)

TABLE 8-1. COLOR CODE FORFILLER CAPS

FED-STD-595 No.Cap Function Color (Ref. 21)Fuel Insignia Red 11136

Oil Orange Yellow 13538

Coolant White 17875

used, they should have the wrench projection accuratelymade to fit standard end socket wrenches and should besufficiently long to obtain full bearing area on a wrench.

10. Design fuel and oil tank filler caps for aircraft sothey cannot be improperly secured to filler necks andcannot come off in flight. If the cap comes off in flight,fuel or oil can be siphoned overboard. Use of spring-loaded locking caps will reduce the risk of cap loss.

8-4.2.3 Draining RequirementsConsider the following design recommendations for

the draining of tanks:1. The fuel system should be easily drained for stor-

age and cleaning. Fuel filters should be located so thatthey can be cleaned and replaced without disassembly ofother parts.

2. Provide each fuel tank with a sump located at thelowest portion of the tank when the equipment is in itsnormal position. The sump. used for collecting sedimentand water, may be combined with the fuel tank outlet.Provide a machine-threaded drain plug or self-lockingdrain valve at the lowest point of the tank. The sumpdrain should be made accessible to personnel wearingheavy, winter gloves and should not require the use ofspecial tools.

3. When it is essential to avoid overfilling, accessiblelevel plugs as shown in Fig. 8-7 maybe used. External-hexagonal plugs (Fig. 8-8(A)) are desirable because thehexagonal head provides ample surfaces on which to getsufficient purchase to maneuver the plug, particularly in aconfined space. The internal-square plug (Fig. 8-8(B))should be used where the plug must be flush with thesurrounding surface. Drain cocks that provide a high rateof drainage should be fitted to all air receivers and oilreservoirs. All drain cocks should be designed to be closedwhen the handle is in the down position to prevent acci-dental opening.

4. Drain holes large enough to facilitate cleaningshould be provided wherever water is likely to collect.Provide drainage facilities in enclosed equipment thatmight be subject to accumulated moisture resulting fromcondensation or other causes. Provide adequate draintubes or channels to carry the liquids to a safe distanceoutside the unit. An appropriate drain valve may beselected from conventional sizes shown in Table 8-2.When selecting drain valves, consider the galvanic seriesof metals to reduce the effects of corrosion between thedrain valve and the metal to which it is attached. Theelectromotive series shown in Fig. 8-9 (Ref. 22) reveals therelative electromotive potential of variations of various

Figure 8-7. Oil Level Sight Plugs

Figure 8-8. Drain Plug Heads

TABLE 8-2. DRAIN VALVES

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Figure 8-9. Electromotive Series (Ref. 22)

metals. Table 8-3 (Ref. 22) lists the galvanic series for avariety of metals and alloys in sea water. Usually thegreater the separation of two bare metals of this series incontact with each other, the greater will be the corrosionproblem.

5. Tank and reservoir drain valves should be locatedso that they maybe removed from the outside to eliminatethe need to enter the tank or reservoir to remove ormaintain the valves.

Aircraft drainage presents a particular problem.Drainage should be provided in wings, bodies. and con-trol surfaces to prevent the collection of unwanted fluids,including water, from condensation within the aircraft.Consider the following design recommendations:

1. Use drain holes in the skin and limberholes in

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1.2.3.4.5.6.7.8.9.

10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.

TABLE 8-3. GALVANIC SERIES INSEA WATER (Ref. 22)

MagnesiumMagnesium alloysZincGalvanized steelAluminum (52SH, 61S, 3S, 2S, 53ST in this order)Aluminum clad, 24 ST, 17STCadmiumAluminum (75 ST, A 17 ST, 17 ST, 24ST in this order)Mild steelWrought ironCast ironNi-Resist13% chromium stainless steel, type 410 (active)50-50 lead-tin solder18-8 stainless steel, type 304 (active)18-8-3 stainless steel, type 316 (active)LeadTinMuntz metalManganese bronzeNaval brassNickel (active)Inconel (active)Yellow brassAdmiralty brassAluminum bronzeRed brassCopperSilicon bronzeAmbrac70-30 copper nickelComp. G-bronzeComp. M-bronzeNickel (passive)Inconel (passive)Monel18-8 stainless steel, type 396 (passive)18-8-3 stainless steel, type 316 (passive)

bulkheads or stiffeners to permit the unwanted fluids torun to low points when the aircraft is resting in a normalposition.

2. Locate drain holes judiciously so that only afew are required and a scavenging suction is produced inflight.

3. Where fabric covering is used, install grom-mets for drainage purposes.

4. It is sometimes desirable to group a numberof drain lines into a common outlet to alleviate congestionand to obtain the most direct and efficient routing of lines.To reduce fire hazards, adhere to the following require-ments:

a. Do not interconnect drain lines for electri-cal accessories with lines draining fuel, oil, hydraulicfluid, water-alcohol solution, etc.

b. Other accessory drains unless return ofone fluid may damage any of the components of these

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drains—may be interconnected provided line sizes ade-quately insure proper drainage.

5. Provide adequate ventilation and drainageof all interior areas to prevent the accumulation of toxicor irritating gases, liquids, and explosive mixtures.

8-4.3 CLEANING AND PRESERVINGVehicles, weapons, and weapon systems will require

cleaning at some time during their service life. Therefore,equipment should be designed to require a minimum ofmanpower, supplies, and equipment for cleaning, pre-serving, and refinishing.

Equipment ordinarily should not require protectiveprocessing more often than once each six months while instorage. Eliminate requirements for special protection orprocessing for storage by using designs featuring built-incorrosion deterioration protection.

8-4.3.1 CleaningExposed surfaces should be shaped to avoid recesses

that collect and retain dirt, water, servicing fluids (spilledin servicing or lost in operation), cleaning solutions, andother foreign materials. Where such recesses cannot beavoided, suitable deflectors and drains should be pro-vided.

Where feasible, design equipment to permit the use ofultrasonic cleaning of parts. In addition to speed of opera-tion, ultrasonic equipment has the advantage of eliminat-ing the toxic cleaning fluids and the soaking in cleaningtanks containing alkaline solutions. To reduce firehazards, provide tight-fitting metal covers for the petro-leum solvent tanks when they are not in use. Nonflamma-ble solvents should be used wherever possible.

It should be possible to steam clean the external partsof all vehicles, weapons, or weapon systems. This appliesalso to tanks, pumps, valves, filter bodies, accumulators,and cylinders used with common fuels, exotic fluids, oils,water, air, and gases. Materials not suitable for steamcleaning, such as upholstery and soft linings of vehicles,should be washable with strong detergents and water orwith nonflammable solvents. Consideration should alsobe given to the cleaning, and to the resistance to cleaning,required when, for example, a hydraulic component leaksand coats everything in a compartment with oil.

Engine parts are commonly cleaned in alkaline clean-ing solutions compounded for steel components. Brassparts, such as block core plugs (freeze plugs), are notseriously affected by alkaline solutions. Aluminum com-ponents, however, are affected by alkaline solutions and,therefore, should not be used as spacers or brackets.

If delicate equipment is located in an area that will besubjected to steam cleaning, the housings should bedesigned to close tightly to insure that steam cleaning orsprayed solvents will not damage the internal components.

DOD-HDBK-791(AM)

8-4.3.2 PreservingConsider the use of preservative materials for the fol-

lowing items:1. Surfaces subject to rubbing and chipping2. Fastenings and small parts in hidden locations3. Hidden surfaces whose complex shape or inac-

cessible location make them difficult to prepare andrefinish

4. Small, light parts, such as those made of sheetmetal and other thin-gage materials.

Specify ozone-resistant compounding for all rubbercomponents that are exposed to the atmosphere. Other-wise, these components must be specially preserved dur-ing storage to prevent deterioration.

There is a definite correlation between climate anddeterioration of materials; an unfriendly environmentincreases the maintenance burden. The full range of mil-itary environments is contained in MIL-STD-210 (Ref.23). Tropical climates—characterized by high ambienttemperature and high humidity—present the most severetest for preservation measures. A tropical climate isdefined as one in which the mean monthly temperaturenever goes below 18.2°C (64.9°F). The following majorproblems are associated with tropical areas:

1. Corrosion of steel and copper alloys caused byelectrolytic action

2. Fungous growth on organic materials such ascanvas, felt, gasket materials, sealing compounds, andeven on the optical elements of fire control equipment

3. Deterioration through corrosion and fungousgrowth in insulation, generating and charging sets, demo-lition and mine detection equipment, meters, dry cellbatteries, storage batteries, cables, and a variety of lessercomponents.

Since fungi can cause corrosion, rotting, and weaken-ing of materials, materials inert to the growth of fungishould be used whenever possible in the design of USArmy equipment. In general, synthetic resins—such asmelamine, silicone, phenolic, and fluorinated ethylenicpolymers with inert fillers, such as glass, mica, asbestos,and certain metallic oxides—provide good resistance tofungous growth. Not all rubber is fungous resistant, andantifungous coatings generally are impractical for thismaterial. When fungous-resistant rubber is needed, itshould be so specified to insure that the manufacturerfurnishes a suitable compound.

Termites attack all wooden parts not impregnated witha repellent agent, and they are especially attracted toplywood bonded with a vegetable glue.

Selected materials should be corrosion resistant, orthey should be protected by plating, painting, anodizing,or by some other surface treatment to resist corrosion.Surfaces required to be acid proof should be given addi-tional surface treatment. Treatment should be selected inaccordance with MIL-S-5002 (Ref. 24). The use of anyprotective coating that will crack, chip, or scale with ageor extremes of climatic and environmental conditionsshould be avoided.

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DOD-HDBK-791(AM)

It is difficult to make definite comparisons of thecorrosion-resistant properties of metals because the resis-tance of each varies with the chemical environments.However, in vehicle design the metals most commonlyused for their corrosion-resistant properties are—not inorder of resistance—titanium, molybdenum alloys, stain-less steel, pure aluminum, cadmium, chromium, zinc,nickel, tin, and copper alloys. The aluminum and magne-sium alloys are seriously degraded by corrosion andshould be avoided. The automobile manufacturers haveintroduced zinc plating—either galvanizing or electroplat-ing—of steel body panels and mufflers to increase corro-sion resistance.

Dissimilar metals that are far apart in the galvanicseries—see Fig. 8-8 and Table 8-3—-should not be directlyjoined together, but if they must be used together, theirjoining surfaces should be separated by an insulatingmaterial or both surfaces must be covered with the sameprotective coating.

For more detailed coverage of corrosion and the corro-sion protection of metals, see MIL-E-5400 (Ref. 25) andRefs. 26, 27, 28, and 29 for listings of acceptablecorrosion-resistant materials.

8-4.4 MOISTURE PROTECTIONThe exclusion of moisture from equipment. particu-

larly in the tropics, considerably eases maintenance prob-lems. To help minimize the effects of moisture on insulat-ing and other materials, the guidelines that follow shouldbe considered:

1. Choose materials with low moisture absorptionqualities.

2. Use hermetic sealing whenever possible.3. Use gaskets and other sealing devices to keep

moisture out.4. Impregnate or encapsulate materials with fungous-

resistant hydrocarbon waxes and varnishes.5. Do not place bare metal parts in contact with

materials that have been waterproofed; metal may sup-port fungous growth and deposit corrosive waste pro-ducts on the treated material.

6. When waterproofed materials are used, be surethey do not contribute to corrosion or alter electrical orphysical properties.

If these methods are not practical, drain holes shouldbe provided, and chassis and racks should be channeled toprevent moisture traps. Additional information on mois-ture protection can be supplied by the Prevention ofDeterioration Center, National Research Council, 2101Constitution Avenue, Washington, DC 20037. Refer alsoto MIL-E-5400 (Ref. 25) for listings of acceptablemoisture-resistant materials.

8-4.5 ADJUSTMENT AND ALIGNMENTAlthough many types of equipment may require adjust-

ment and alignment during their useful life, tank automo-tive equipment and aircraft are the most affected by thisrequirement. Items that require adjustments include tanktreads, engine timing (gasoline engines), alternator belt

drives, and cam drive belts. Aligning applies to steeringand headlights on automotive equipment and to firingcontrols and sights on guns. Some alignment and adjust-ment requirements are associated with initial installation;however, because many components have a long, usefullife before wear-out, periodic inspections and adjust-ments are necessary to assure proper functioning. Whenmechanical components have actuating linkages, e.g.,throttle controls and flight controls on aircraft, the con-trol at the component and the component itself shouldincorporate alignment positioning pins to assist in therapid attachment and proper positioning of the controlwithout two-person cooperation.

Equipment should redesigned to require the minimumnumber of periodic maintenance adjustments. However,maintenance adjustments that cannot feasibly be elimi-nated should be simplified enough to permit theiraccomplishment at the lowest practicable maintenancelevel. The use of built-in, self-adjusting devices should beconsidered provided their addition does not present amaintenance burden more difficult than manual adjust-ment.

If adjustments are to be made manually. insure thatdisassembly of the components is not required for theiraccomplishment. Wherever practicable, the effect ofmanipulating the service adjustments should be clearlyand easily discernible by reference to appropriatc gages orother displays. Avoid critical adjustments, i.e., a slightmanipulation of the device (or slight variation duringnormal operation) that will cause a very large change ofthe affected parameter. Adjustment devices should havean adequate range of adjustment without being undulycritical or dependent on other adjustments. Maladjust-ments that may occur during a servicing procedure shouldnot result in damage to any parts when the equipment isoperated under the maladjusted conditions for a period ofup to 5 min. In general, the range of control of serviceadjustments should be such as to prevent damage bymaladjustment.

Alignment and adjustment devices should be neither sofine that a number of turns is required to obtain a peakvalue nor so coarse that a peak is quickly passed, whichwould necessitate delicate adjustment. Part selection andsystem design should provide a straightforward align-ment procedure. It should be unnecessary to go back toreadjust or realign earlier stages after alignments oradjustments are made to later stages. Also it should bepossible to make all alignment adjustments without re-moval of any case, cover, or shield that would affect theaccuracy of alignment upon replacement. and no specialtools should be required for alignment or adjustment.

Alignment and adjustment devices should be located sothey can be readily operated while the technician isobserving the displays associated with the function beingadjusted. It should be possible to check and adjust eachunit of system separately and then connect the units intoa total, functioning sytem with little or no additionaladjustment required.

Components should be designed with the minimumnumber of pivots and bearing surfaces that wear and

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require periodic adjustment. Any alignment or adjust-ment devices that are susceptible to vibration or shockshould have a positive locking device to assure retentionof settings. The locking device should be easy to apply andrelease, and the application and release of the lock shouldnot affect the setting of the adjustment. Traveling clampsand locking devices should be designed to avoid inadver-tent release.

Spindles for adjustments may be slotted, but the headshould be strong enough to withstand many manipula-tions with a screwdriver. A method of locating and hold-ing the adjusting screwdriver while in use is desirable. Ifadjustments must be made blindly, design the head toaccommodate a wrench to facilitate adjustment.

Consider the use of locked-nut-and-thread adjustmentsinstead of shims. Avoid shim-type adjustments that per-form the dual function of adjusting bearings and position-ing units. Where corrosion of nuts and threads may be afactor in the adjustment of large components, use cor-rosion-resistant materials.

DOD-HDBK-791(AM)

Where applicable, use variable pitch, V-belt drivers forhigh-speed applications. Use spring-loaded idler sprocketson chain drives to avoid frequent adjustments. Eliminateadjustment of hose fasteners used in low-pressure appli-cations (air ducts) by using spring-type fasteners thatmaintain a constant peripheral pressure. When enclosedchain drives require periodic adjustment, automatic ad-justers should be provided.

8-5 PREVENTIVE MAINTENANCE ANDSERVICING DESIGN CHECKLIST

Table 8-4 is a checklist summarizing the design recom-mendations presented in this chapter. The checklist con-tains several items that were not discussed separately inthe text. These items are included here because theirnecessity in the design is so obvious that they might beinadvertently overlooked. If the answer to any question inthe checklist is “no”, the design should be restudied toascertain the need for correction.

TABLE 8-4. PREVENTIVE MAINTENANCE AND SERVICING DESIGN CHECKLIST

1.2.3.

4.

5.

6.

7.8.

9.

10.

11.

12.

13.

14.15.16.17.

18.19.20.

Are standard lubrication fittings used so that no special extensions or fittings are required?Are standard lubricants that are already in the federal supply system specified?Are adequate lubrication instructions (lubrication orders) provided that identify the frequency and type of lubricantsrequired?Are filler areas for combustible materials located away from sources of heat or sparking, and are spark-resistant fillercaps and nozzles used on such equipment?Are fluid-replenishing points located so there is little chance of spillage during servicing, especially on easily damagedequipment?Are filler openings located where they are readily accessible and do not require special filling adapters or workstands?Are air reservoir safety valves easily accessible and located where pop-off action will not injure personnel?Are fuel tank filler necks, brake air cocks, flexible lines or cables, pipe runs, fragile components, and like itemspositioned so they are not likely to be used as convenient footholds or handholds, thereby sustaining damage?If bleeds are required to remove entrapped air or gases from a fluid system, are they located in an easily operable andaccessible position? Are bleed valves labeled with the proper operating instructions?Are drains provided on all fluid tanks and systems, fluid-filled cases or pans, filter systems, float chambers, and otheritems designed to or likely to contain fluid that would otherwise be difficult to remove?Are drain fittings of few types and sizes used, and are they standardized according to application throughout thesystem?Are valves or petcocks used in preference to drain plugs? Where drain plugs are used, do they require only commonhand tools for operation, and does the design insure adequate tool and work clearance for operation?Are drain cocks or valves clearly labeled to indicate open and closed positions and the direction of movementrequired to open them?Do drain cocks always close with clockwise motion and open with counterclockwise motion?When drain cocks are closed, is the handle designed to be in the down position?Are drain points placed so that fluid will not drain on the technician or on sensitive equipment?Are drain points located at the lowest point when complete drainage is required or when separation of fluids is desired(as when water is drained out of fuel tanks)?Are drain points located to permit fluid drainage directly into a waste container without use of adapters or piping?Are drain points placed where they are readily operable by the technician?Are instruction plates provided as necessary to insure that the system is properly prepared prior to draining?

(cont’d on next page)

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DOD-HDBK-791(AM)

21.

22.

23.

24.

25.26.27.

28.

29.

30.31.32.33.

34.

35.36.

37.38.39.40.41.

42,

43.44.

TABLE 8-4 (cont’d)

Are drain points located so that fuel of other combustible fluids cannot run down to or collect in starters, exhausts, orother hazardous areas?Are lubrication requirements reduced to two types, if possible, one for engine lubrication and one for gearlubrication?Are the same fuels and lubricants used in auxiliary or mounted equipment as in the prime unit where practical’? Arethe fuels and lubricants used common with other commodities that would be assigned to the same combat units(tanks, trucks, ground power units, etc.)?Are easily distinguished or different types of fittings used for points or systems requiring different or incompatiblelubricants?Are pressure fittings provided for the application of grease to bearings that are shielded from oil?Is ample grease reservoir space provided for bearings in gear unit?Is provision made for a central lubrication or filler point, or a minimum number of points, to all areas requiringlubrication with a given system component?Are service points provided, as necessary, to insure adequate adjustment, lubrication, filling, changing, charging, andother services to all points requiring such servicing?Are oil filler caps designed so that theya. Snap, then remain open or closed?b. Provide a large, round opening for oil filling?c. Permit application of breather vents, dipsticks, and strainers?d. Use hinges rather than dangerous chains for attaching the lid?e. Are located outside of enclosure, where possible, to eliminate necessity for access doors, plates or hatches?Are materials properly protected against moisture, fungus, and corrosion for storage and use?Are items designed to be compatible with the standard military cleaning methods and materials?Are parts subject to galvanic action properly separated and protected?Are components subject to steam or solvent cleaning (or random contact during equipment cleaning) properly sealedto prevent interior damage?Are mechanisms subject to wear and mechanical damage in use equipped with adjustments for aligning andrepositioning?Are filler locations for tanks and reservoirs labeled to indicate the type of fluid and maximum quantity?Are preventive inspection and maintenance procedures based on detailed evaluation of actual preventivemaintenance requirements for safety and reliability?Does the equipment provide for adjustment and alignment without disassembly?Does alignment or adjustment require no special tools or equipment?Are alignment and adjustment controls located to permit observation or associated displays?Are adjustments for all controls for a mechanism in a single location, e.g., engine controls?Are control cable breakpoints separated sufficiently to assure that they will not be disconnected, which results incrossed controls? Are adjustment turnbuckles separated by sufficient distance to prevent interference within therange of full travel?Will equipment tolerate a small maladjustment of controls for a reasonable period of test operation (5 rein) duringthe adjustment period without sustaining damage?Are lubrication charts adequate?Are group inspection features that are applicable to a particular technical skill held to a minimum of locations tominimize the need for technicians moving around the equipment and personnel interference during scheduledinspections?

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DOD-HDBK-791(AM)

REFERENCES

1. Airline Manufacturer's Maintenance Program andPlanning Document MSG-2, Air Transport Associa-tion of America, March 1970.

2. Program Objective Memorandum. (POM) 78-82.3. DA PAM 750-40, Guide to Reliability Centered

Maintenance (RCM) for Fielded Equipment, 1 5February, 1980.

4. AMC-P 750-2, Maintenance Supply and EquipmentGuide to Reliability Centered Maintenance, 14 June1985.

5. DOD Directive 4/51.16, DoD Equipment Mainte-nance Program, 23 August 1984.

6. AR 750-1, Army Materiel Maintenance Conceptsand Policies, 15 March 1983.

7. AR 700-127, Integrated Logistics Support, 1 6December 1986.

8. MIL-STD-1388/1A, Logistic Support Analysis, 11April 1983.

9. MIL-STD-1388/2A, A Logistical Support AnalysisRecord, DoD Requirements for, 20 July 1984.

10. DOD Directive 5000-40, Reliability and Maintain-ability, February 1982.

11. DARCOM Regulation 750-11, Maintenance andSupplies—Use of Lubricants, Fluids, and AssociatedProducts, 20 November 1981.

12. MIL-STD-838, Lubrication of Military Equipment,December 1983.

13. MIL-F-3541, Fitting, Lubrication, January 1982.14. MIL-STD-454, Standard General Requirements for

Electronic Equipment, January 1983.

15. MS-35844, Plug, Machine Thread, Magnetic (Drain),February 1978.

16. AR 750-22, Army Oil Analysis Program, April 1977.17. TB 43-0210, Nonaeronautical Equipment Army Oil

Analysis Program (AOAP), December 1984.18. TB 43-0106, Army Aeronautical Equipment Army

Oil Analysis Program (AOAP), April 1981.19. MS-17967, Nozzle Assembly Ventilation and Ex-

haust, April 1965.20. MIL-C-13984, Can, Water, Military 5-gallon,

October 1981.21. Federal Standard 595, Color, February 1980.22. MIL-HDBK-772, Military Packaging Engineering,

30 March 1981.23. MIL-STD-210, Climatic Extremes for Military

Equipment, December 1973.24. MIL-S-5002, Surface Treatments and Inorganic

Coatings for Metal Surfaces of Weapon Systems,August 1978.

25. MIL-E-5400, Electronic Equipment, Airborne,General Specifications For, September 1980.

26. C. P. Dillon, Ed., Forms of Corrosion, Recognitionand Prevention, National Association of CorrosionEngineers, Houston, TX, 1982, pp. 1 and 2.

27. M. G. Fontana and N. D. Greene, Corrosion Engi-neering, 2nd Ed., McGraw-Hill Book Company,New York, NY, 1978.

28. MIL-HDBK-721(MR), Corrosion and CorrosionProtection of Metals, November 1965.

29. B. Chalmers, Physical Metallurgy, John Wiley andSons, Inc., New York, NY, 1959, p. 445.

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CHAPTER 9HUMAN FACTORS

The concept of human factors and the relationship between human factors and maintainability arediscussed. This includes consideration of body measurements in designing for maintainability, with a goal ofdesigning equipment for use by at least 90% of the user population (male and female). The major human sensesand the relationship of each to maintainability design are also presented, as is the need to consider thepsychology, motivation, and training of personnel. Other topics included are human error, stress, andstrength.

9-1 INTRODUCTIONIn order for equipment to be used most efficiently, it

must be designed for the specific user population. Thisconstraint upon design is obviously, although perhapsunconsciously, one of the designer’s primary considera-tions. Designers must design for personnel who, in tacti-cal situations, will be under conditions of stress andfatigue from many causes. A performance decrement mayarise in the tactical situation not so much because troopsare basically unable to perform but because the individualsoldier is overloaded both physically and mentally.Accordingly, equipment must be designed so that theprocedures for using and maintaining it are as simple aspossible. Equipment should not require intellectual tasksthat will detract from the user’s primary mission.Machines work well only if the personnel operating themperform their tasks satisfactorily. Therefore, the systemengineering concept must be one of a person/machinesystem, i.e., a system in which operator and machineinteract efficiently to perform a function. Training canimprove operator or crew proficiency; however, trainingshould not be considered a substitute for good design.

Human factors engineering is the science of applyingtechnical knowledge to the design of equipment toachieve effective person machine integration, operation,and ease of maintenance. Human factors also are anessential element of the Manpower and Personnel Inte-gration (MANPRINT) program, whose purpose is toimpose human factors, manpower, personnel, training,system safety, and health hazard assessment considera-tions across the entire materiel acquisition process. Thesaying, “People are our most important resource”, hasbeen uttered many times. ‘The application of human fac-tors engineering offers the prospect of moving beyondrhetoric and into action.

The human factors engineer relates human factors suchas size, strength, and human sensory perceptions to thetask to be accomplished. Failure to consider these factorsresults in increased problems of operability and main-tainability. To avoid these problems, human factors engi-neers consider complex military systems as person/ ma-chine systems, including the capabilities and limitationsof personnel under various conditions. Specifically, humanfactors engineering is concerned with

1. Persons, their characteristics—biomedical andpsychological and their capabilities

2. Persons and their environment3. Persons as an integral component of the system4. Person/machine interfaces.

Because maintenance economy and efficiency are signifi-cantly affected by how well the human factors engineeringfunction is implemented, the human factors engineer isresponsible for insuring that an optimum interface existsbetween human capabilities and materiel design features.Each of the AMC commands have human factorsengineers—representatives of the Human EngineeringLaboratory—assigned to assist them in this importantarea.

Important sources of information to guide the main-tainability engineer in assuring that human factors engi-neering is integrated into the design process follow:

1. MIL-H-468S5 (Ref. 1). Establishes human engi-neering principles and procedures for acquiring anddeveloping military systems and equipment. This militaryspecification integrates personnel into the design andprovides the Army with management control of the con-tractor’s effort. For major acquisition programs this isusually accomplished with a Human Engineering Pro-gram Plan.

2. MIL-STD-1472 (Ref. 2). Applicable to the designof all military systems and equipment. Ref. 2 includeshuman engineering design requirements for maintainabil-ity, labeling, work space design, and displays; and humanengineering criteria, principles, and practices necessary toachieve mission success through the integration of thehuman into the system and to achieve effectiveness, sim-plicity, reliability, and safety of operation.

3. MIL-HDBK-759 (Ref. 3). Establishes, in hand-book form, general data and detailed criteria for humanfactors engineering application in the design and devel-opment of Army materiel for ease of maintenance. Areascovered in Ref. 3 of particular interest are

a. Anthropometric data for men and womenb. Features relating to military hardware—e.g.,

ammunition, missiles, tank gun control systems, andoptical instruments

c. Effects of environmental factors on humanperformance

d. Physical limitations of the human body—strength and movement

e. Illumination requirements.Anthropometric data, body strength and movement

9-1

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limitations, human sensory capabilities, environmentalfactors as they affect the soldier/ machine interface, psy-chological factors, and human error quantification areaddressed in the paragraphs that follow. The informationand data contained in MIL-HDBK-759 (Ref. 3) are notrepeated in detail in this handbook; MIL-HDBK-759 willbe referenced, and examples of the types of data will bepresented. The reader is encouraged to refer to MIL-HDBK-759 for detailed data and criteria for human fac-tors engineering applications.

9-2 ANTHROPOMETRY (BODYMEASUREMENTS)

Anthropometry—the study of human body measure-ments—is an important consideration in designing formaintainability because this information is necessary todesign equipment that will accommodate operators andmaintenance personnel of various sizes and shapes. Themeasurements relate to body dimensions together withthe range of motion of body members and musclestrength. The data usually are presented in terms of upperand lower percentiles. The designer should always striveto accommodate the full range of personnel designated inMIL-STD-1472 (Ref. 2), i.e., 5th through 95th percentilemale and female. When this does not appear to be feasi-ble, the procuring activity must be notified. The reverse,designing work space and then adding the person, isusually inefficient and costly.

TABLE 9-1. SOURCES OFANTHROPOMETRIC DATA

Name Location

A. Primary: laboratories of anthropometry (whichspecialize in anthropometric research as well as gather a

library of data)

Aerospace Medical ResearchLaboratory, Wright-Pat-terson Air Force Base

US Army Natick Laboratories

Anthropology project. WebbAssociates

Department of Human Anat-omy, University of Newcastle

Centre d’Anthropologie Appli-quee, University de Paris

Department of Anthropology,Harvard University

School of Public Health, Har-vard University

Dayton, OH

Natick, MA

Yellow Springs, OH

Newcastle-on-Tyne,England

Paris, France

Cambridge, MA

Boston, MA

B. Secondary: repositories where anthropometric datamay be found (where actual anthropornetric services

may or may not be obtainable)

US Naval Training DevicesCenter

Aerospace Crew EquipmentLaboratory, Naval Air Engi-neering Center

Human EngineeringLaboratory

Guggenheim Center for Avia-tion and Safety, HarvardUniversity

Institute for PsychologicalResearch, Tufts University

Biotechnology Laboratory,University of California. LosAngeles

Furniture Institute ResearchAssociation

Unit for Research for HumanPerformance in Industry,Welsh College of AdvancedTechnology

Department of Ergonomics andCybernetics, LoughboroughCollege of Technology

Institute of Engineering Pro-

Orlando, FL

Philadelphia, PA

Aberdeen ProvingGround, MD

Boston, MA

Medford, MA

Los Angeles, CA

Stevenage, Hertford-shire, England

Cardiff, Wales

Leicestershire,England

Birmingham, England

Geneva, Switzerland

9-2.1 SOURCES OF INFORMATIONSection 5.6 of MIL-STD-1472 (Ref. 2) and par. 2.2 of

MIL-HDBK-759 (Ref. 3) are devoted to anthropometricdata together with illustrations to explain the data. Anexample of the type of data contained in Ref. 3 is shown inFig. 9-1. (Similar data are presented for women.) Addi-tional anthropometric data are contained in Refs. 4through 11. Table 9-1 provides a list of laboratories thatspecialize in anthropometric research and a list of reposi-tories of anthropometric data.

9-2.2 MEASUREMENTSDesign and sizing measurements must insure accom-

modation, compatibility, operability, and maintainabil-ity by at least 90% of the user population (Ref. 3). Gener-ally, design limits should be based upon a range from the5th to the 95th percentile values for critical body dimen-sions. For any dimension, the 5th percentile value indi-cates that 5% of the population will be equal to or smallerthan that value and 95% will be larger. Conversely, the95th percentile value indicates that 95% of the populationwill be equal to or smaller than that value and 5% will belarger. Therefore, the use of a design range from the 5th tothe 95th percentile values will theoretically accommodate90% of the required user population for that dimension. Itshould be noted that designing to accommodate the 5thfemale through the 95th percentile male accommodates90% of the combined populations (male and female)regardless of the male and female mix. This results—

duction, University ofBirmingham

Bureau International duTravail

9-2

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Figure 9-1. US Army Basic Trainees (1966): Breadth& Circumference Measurements (Ref. 3)

9-3

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because the male and female unthropometric dimensionsoverlap so that all males equal to or below the 95thpercentile and all females equal to or above the 5th per-centile are accommodated.

Some of the measurements important to maintainabil-ity (Ref. 12) are

1. Basic body dimensions:a.b.c.d.e.f.g.h.

StatureEye heightShoulder heightArm reachElbow-hand lengthKnee height and leg lengthHand sizeBody breadth

2. Body mobility3. Dexterity4. Field of vision.

Anthropometric data are usually given as nude bodydimensions; however, MIL-STD-1472 (Ref. 2) and MIL-HDBK-759 (Ref. 3) provide data that take into accountarctic clothing, which usually is the worst case from thestandpoint of maintenance accessibility. Nuclear, biolog-ical, and chemical (NBC) protective garments are also aproblem because they produce heat stress, reduced vision,reduced tactile sense, and increased breathing effort. Fig.4-15 shows the increased dimensions for a gloved hand.Figs. 4-13 and 4-14 and Table 4-2 illustrate application ofanthropometric data to facilitate maintenance operations.

In the application of anthropometric data, both staticand dynamic body measurements must be addressed.Static measurements range from the gross aspects of bodysize, such as stature, to the distance between the pupils ofthe eyes and are measured with the subject in rigid, stan-dardized positions. Dynamic body measurements, on theother hand, usually vary with body movement and relatemore to human performance than to human “fit’’ (Ref. 1).Par. 2.3.1 of MIL-HDBK-759 (Ref. 3) gives the ranges forall voluntary movements the joints of the body can make.

In summary, the following should be considered wheninterpreting and applying anthropometric data:

1. Nature, frequency, and difficulty of related tasks2. Position of body during performance of tasks3. Mobility or flexibility requirements imposed by

tasks4. Increments in the design-critical dimensions im-

posed by protective garments, packages, lines, padding,etc.

9-3 HUMAN STRENGTH ANDHANDLING CAPACITY

Equipment that is designed to be consistent with aperson’s capabilities permits more force to be exertedwith less fatigue. However, if the demands placed onhuman strength are too high, inefficient and unsafe per-formance will result. Conversely, if the designer underes-timates strength, unnecessary design effort and expensemay be incurred. The proper strength value to use indesigning equipment is the maximum force that can be

9-4

exerted by a 5th percentile member of the user population.The maximum force that can be applied by a person

depends on many factors such as the position of the body,the body member(s) applying the force, the direction ofapplication, the object to which the force is applied, andwhether or not support is provided. The following con-clusions regarding the application of force should be ofvalue to the designer (Ref. 12):

1. The greatest force is developed in pulling towardthe body. An individual using his leg and back musclescan exert a stronger pulling force than a seated individual.

2. The maximum force that can be exerted increaseswith the use of the whole arm and shoulder, but use of thefingers alone requires the least energy per unit of forceapplied.

3. Push exerts a greater force than pull in side-to-side motion.

Fig. 9-2 (Ref. 3) shows the arm, hand, and fingerstrength of 95% of male personnel for various directionsof movement and body members.

Whenever possible, equipment parts should be designedso that one person can lift them. If this is not possible. theparts should be clearly labeled with weight and lift limita-tions. Table 9-2 (Ref. 3) shows the lifting capacity of 95%of male users. These weight limits should be reduced whendifficult conditions exist, such as

1. When the object is very difficult to handle i.e.,bulky. slippery, etc.

2. When access and work space conditions arc lessthan optimal

3. When the required force must be exerted contin-uously for more than 1 min

4. When the object relist be positioned exactly orhandled delicately

5. When the task must be repeated frequently.Par. 2.4.2 of MIL-HDBK-759 (Ref. 3) provided addi-

tional figures and tables for determining human strengthsand handling capacities.

9-4 HUMAN SENSORY CAPABILITIESIndividuals, as part of a total system, possess many

useful sensors. Of the five major senses sght, hearing,taste, smell, and touch only sight, hearing, and touchwill be addressed since they are the major factors affectingmaintainability engineering.

9-4.1 SIGHTIt is estimated that humans acquire 80% of their knowl-

edge visually (Ref. 7). Thus the maintenance technician'ssight capabilities are imporatnt to the maintainabilityengineer. The typical maintenance technician

1. Can distingiush 10 colors, five sizes of figure, fivebrightnesses of light, and two flicker rates

2. Can easily read 6-point type with adequate lighting3. Has a visual field of about 130 deg vertically and

208 deg horizontally with maximum acuity at the center4. Requires about 0.6 s to change visual fixation

from near to far5. Takes about 30 min to adapt completely from

daylight to darkness

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Figure 9-2. Arm, Hand, and Thumb-Finger Strength (5th Percentile Man) (Ref. 3)

9-5

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TABLE 9-2. MAXIMUM WEIGHT LIMITS (Ref. 3)

lThese weight limits should be used as maximum values in establishing the weights of items that must be lifted.These limits apply to items up to 380 mm (15 in.) long and up to 305 mm (12 in.) high, with handles or grasp areasas shown in Fig. 4-20. These limits should not be used for larger items or for items which must be liftedrepetitively.

2These weight limits should not be used if personnel must carry the item more than five steps.3When an item weighs more than the limit for one-man lifting, it should be prorminently labelled with weight andlift limitations, e.g., two-person or mechanical lift. Items to be lifted mechanically should have prominentlylabelled hoist and lift points.

6. Suffers discomfort and impaired vision if brightlights or reflections are located within 60 deg of his line ofsight.

Sight is stimulated by radiation of certain wavelengths—430 to 690 m (4300 to 6900 A)—commonly called thevisible portion of the electromagnetic spectrum. Themaintenance technician can see all colors of the spectrumviolet through red—while looking straight ahead. How-ever, color perception decreases as the viewing angleincreases. Consequently, if the equipment has color-banded meters or warning lights of different colors thatare near the maximum viewing angle limits, the mainte-nance technician may not be able to distinguish one colorfrom another. The color of the illuminating light source isalso an important factor when viewing color-codedobjects. At night, or in any poorly illuminated area, colormakes little difference. Similarly, if the source is distant orsmall—such as a small warning light—blue, green, yel-low, and orange are indistinguishable; they all appear tobe white. Another phenomenon of color perception isapparent reversal of color. When an individual stares at ared or green light, for instance, and then glances away, thesignal to the brain may reverse the color. This pheno-menon has caused many accidents. Therefore, colorshould not be relied upon solely when critical operationsmay be performed by fatigued personnel or under circum-stances where color perception may not be good. In addi-tion to problems regarding color perception, maintain-ability design is concerned with insuring that visual dis-plays are placed properly for effective use. Fig. 9-3 pro-vides design guidance for the horizontal and verticalvisual fields.

A technician needs sufficient light to perform tasksproperly; accuracy, speed, and safety suffer if he cannot

see well. Adequate illumination, however, will not alwaysbe available. Accordingly, equipment designers shouldendeavor to develop their designs to permit effectivemaintenance under even the poorest anticipated lightingconditions. To this end, designers should acquaint them-selves with all of the possible circumstances that mayreduce available illumination. For example, if a flashlightis necessary to illuminate the areai to be accessed, theequipment shoud bed designed so that maintenance can beperformed by the light of a flashlight. This assumes thatthe person performing the maintenance has a free hand tohold the flashlight and that there is space available tostand while holding the flashlight.

Basic factors that should be considered in the design oflighting systems are

1. Suitable brightness for the tash at hand2 Uniform lighting3, Suitable brightness contract between task and

background4. Lack of glare from light source or work surface.

It is difficult to specity exact illumination levels fordesigning an efficient Iighting system, but guidelines areshown in Tables 9-3 and 9-4 (Ref. 3). The examples ofillumination levels expressed in Table 9-3 are presented sothe designer may estimate the illumination levels for thesetasks within the system that are not directly related to thetasks of Table 9-4.

The ability of personnel to see cathode-ray tube signals,such as those on oscilloscopes, depends on five mainvisual factors (Ref. 3). namely.

1. Size, in visual angle, of the signal2. Brightness of the background (The ambient illumi-

nance should not contribute more than 25% of the screenbrightness.)

9-6

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Figure 9-3. Visual Field (Ref. 3)

TABLE 9-3. ILLUMINATION REQUIREMENT FOR REPRESENTATIVE TASKS (Ref. 3)

9-7

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TABLE 9-4. SPECIFIC TASK ILLUMINATION REQUIREMENTS (Ref. 3)

Illumination Levels*Work Area or Type of Task Recommended Minimum

Assembly, missile component

Assembly, generalcoarsemediumfineprecise

Bench workroughmediumfineextra fine

Business machine operation(calculator, digital, input, etc.)

Console surface

Corridors

Circuit diagram

Dials

Electrical equipment testing

Emergency lighting

Gages

Hallways

Inspection tasks, generalroughmediumfineextra fine

Machine operation, automatic

Meters

Missilesrepair and servicingstorage areasgeneral inspection

Office work, general

Ordinary seeing tasks

Panelsfrontrear

Passageways

lux

1075

540810

10753230

540810

16153230

1075

540

215

1075

540

540

540

215

540107521553230

540

540

1075215540

755

540

540325

215

fc

100

5075

100300

5075

150300

100

50

20

100

50

50

50

20

50100200300

50

50

1002050

70

50

5030

20

lux

540

325540810

2155

325540

10752155

540

325

1l0

540

325

325

32

325

110

325540

10752155

325

325

5401l0325

540

325

325110

110

fc

50

305075

200

3050

100200

50

30

10

50

30

30

3

30

10

3050

I 00200

30

30

501030

50

30

3010

10

(cont’d on next page)

9-8

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TABLE 9-4 (cont’d)

Illumination Levels*Work Area or Type of Task Recommended Minimum

lux fc lux fc

Readinglarge printnewsprinthandwritten report, in pencilsmall typeprolonged reading

3050707070

325540755755755

755

110325540540540

1030505050

Recording 70 540 50

Repair workgeneralinstrument

5402155

540

540

215

50200

3251075

30100

Scales 50 325 30

Screw fastening 50 325 30

Service areas, general 20 110 10

Storageinactive or deadgeneral warehouselive, rough or bulklive, mediumlive, fine

54110110325540

540

215

510103050

325454

215325

355

2030

Switchboards 50 325 30

Tanks, container 20 110 10

Testingroughfineextra fine

54010752155

1075

50100200

325540

1075

3050

100

Transcribing and tabulation 100 540 50

*As measured at the task object or 760 mm (30 in. ) above the floor

NOTE 1:

NOTE 2:

NOTE 3:

Some unusual inspection tasks may require up to 10,800 lux ( 1000 fc)

As a guide in determining illumination requirements, the use of a steel scale with 0.38-mm (0.01 -in.) divisionsrequires 1950 lux (180 fc) of light for optimum visibility.

The brightness of transilluminated indicators should be compatible with the expected ambient illuminationlevel and should be at least 10% greater than the surrounding brightness; however, the indicator brightnessshould not exceed 300% of the surrounding brightness.

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3. Brightness of the signal4. Length of time the signal is present5. State of adaptation of the eye.

The level of luminance recommended for characters on avisual display terminal is 170 cd m2 (50 fc) (Ref. 13).

9-4.2 HEARINGHearing is an important sense in terms of information

gathering. Sound waves reaching the ear vary in fre-quency, amplitude, and complexity. Sound is usually de-scribed in terms of its two major characteristics, i.e.,frequency and intensity. Frequency is measured in cyclesper second expressed as hertz (Hz). Generally, the humanear responds to frequencies between 20 and 20,000 Hz,though few adults hear near the 20,000-Hz limit. Fre-quencies above the 20,000-Hz limit cannot normally beheard by humans, but they do produce some biologicaleffects.

The intensity of sound, or loudness, is usually mea-sured in decibels (d B). Weighting networks consist ofthree alternate frequency response characteristics desig-nated A-, B-, and C-weighting. Whenever one of these

networks is used, the reading obtained must be identifiedproperly. For example, if an A-weighted sound pressurelevel of 90 is obtained, it would be reptorted as a 90-dB(A).The A-weighted network is particularly valuable if aquick estimate of the interference of noise upon speech isrequired (par. B.1.1.2.1.2. Ret. 3). Areas requiring occa-sional telephone use or occasional direct communicationat distances up to 1.5 m (5 ft) should not exceed a 75-dB(A) level (par. 10-5.1. Ret. 13).

The intensity and source location of audio alarms andsignals should be selected for compatibility with theacoustic environmenl of the intended receiver (Ref. 14).When discrimination of warning signals is critical to per-sonnel safety or system performance, audio signals shoulddeselected to signify different conditions requiring difier-ent operator responses i.e. one signal for maintenance,another for emergencies, etc. For the purpose of thishandbook, auditory signals will be considered to be ofthree basic types i.e., tones, complex sounds, andspeech. A comparison of the strengths and weaknesses ofthese types with respect to their use for various functionsis presented in Table 9-5 (Ref. 3).

TABLE 9-5. FUNCTIONAL EVALUATION OF AUDIO SIGNALS (Ref. 3)

FUNCTION

QUANTITATIVEINDICATION

QUALITATIVEINDICATION

STATUSINDICATION

TRACKING

GENERAL

9-10

TONES(Periodic)

POOR

Maximum of 5 to 6 tonesabsolutely recognizable.

POOR-TO-FAIR

Difficult to judge approximatevalue and direction of devia-tion from null setting unlesspresented in close temporalsequence.

GOODStart and stop timing. Con-tinuous information whererate of change of input is.

FAIRNull position easily moni-tored; problem of signal-response compatibility.

Good for automatic commun-ication of limited information.Meaning must be learned.Easily generated.

TYPE OF SIGNAL

COMPLEX SOUNDS(Nonperiodic)

POOR

Interpolation between sig-nals inaccurate.

POOR

Difficult to judge approxi-mate deviation fromdesired value.

GOODEspecially suitable forirregularly occurring sig-nals. e.g., alarm signals.

POORRequired qualitative indica-tions difficult to provide.

Some sounds available withcommon meaning, e.g., firebell. Easily generated.

SPEECH

GOOD

Minimum time and error inobtaining exact value in termscompatible with response.

GOOD

Information concerning dis-placement, direction, and ratepresented in form compatiblewith required response.

POORInefficient: more easilymasked; problem of repeata-bility.

GOODMeaning intrinsic in signal.

Most effective for rapid (butnot automatic) communica-tion of complex, multidimen-sional information. Meaningintrinsic in signal and contextwhen standardized. Minimumof new learning required.

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Occasionally in maintainability design, auditory sig-nals are preferable to visual ones, and vice versa. Table9-6 summarizes the situations in which one form is pre-ferred over the other. However, when the maintenancetechnician must monitor several audio signals (channels)which must be recognizable above the sound of theequipment being tested or maintained, the signals shouldbe of different frequencies—e.g., high, medium, andlow—to avoid confusion.

Noise is defined as any undesirable sound. Excessivenoise in a maintenance or operations area reduces theefficiency of the personnel and thus may impair or reduceoverall system effectiveness. Excessive noise also affectspersonnel psychologically; they fatigue more rapidly,their ability to concentrate decreases, and their annoy-ance increases. In addition to affecting the performanceof maintenance technicians, excessive noise can renderoral communications ineffectual or impossible. Fig. 9-4(Ref. 2) shows the difficulty in communicating at variousnoise levels and speaker-to-listener distances.

Exposure to high noise levels can also produce physio-logical effects—exposure to noise levels exceeding 85 dBmay result in a temporary or permanent hearing loss; theextent of damage depends on the length of the exposure.Table 9-7 lists the intensity of common sounds and theireffect on exposed personnel. TB MED 501 (Ref. 15)defines the noise hazards and the hearing conservationprogram of the US Army. Appendix B of Ref. 3 presents adetailed discussion of hearing loss resulting from bothsteady state and impulse noise together with the factorsinfluencing hearing loss and recovery; the resultant effectson performance are also discussed. Personnel in acousticenvironments where the risk criteria are exceeded shouldbe protected to avoid damage to their hearing. The mosteffective method of protection is to control the noise at itssource. If noise control is not practical, personnel must beprotected with ear protectors such as earmuffs or earplugs.

9-4.3 TOUCHAs equipment becomes more complex, it is necessary

for maintenance workers to employ all their senses to thefullest. Man’s ability to interpret visual and auditory stim-uli is closely associated with the sense of touch. Thesensory cues received by the skin and muscles can be used,to some degree, to convey messages to the brain, whichrelieves the eyes and ears of part of the load they wouldotherwise bear. For example, the control knob shapesillustrated in Fig. 9-5 (Ref. 3) can be recognized easily bytouch alone. These knobs have been specifically designedand experimentally validated for tactile recognition (Ref.3). Many of these knob shapes could be used when themaintenance technician must rely completely on his senseof touch, for example, when the user is busily lookingelsewhere when that control must be moved.

It is an Army tradition that personnel must be able tofield strip weapons based on touch alone. Therefore, it isincumbent upon weapon designers to make the shape andsize of each part unique to facilitate field stripping.

Figure. 9-5. Easily Recognizable KnobShapes (Ref. 3)

TABLE 9-6. WHEN TO USE AUDITORY OR VISUAL FORM OF PRESENTATIONUse auditory presentation if Use visual presentation if

1.

2.

3.

4.

5.

6.

7.

8.

The message is simple. 1. The message is complex.

The message is short. 2. The message is long.

The message will not be needed later. 3. The message will be needed later.

The message deals with events in time. 4. The message deals with location in space.

The message calls for immediate action. 5. The message does not call for immediate action.

The visual system of the person is overburdened. 6. The auditory system of the person is overburdened.

The receiving location is too bright or dark; adapta-tion integrity is necessary.

The person’s job requires him to be moving com- position.tinually.

7. The receiving location is too noisy.

8. The person's job allows him to remain in one

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Figure 9-4. Permissible Distance Between Speaker and Listener for Specified Voice Levels andAmbient Noise Levels (The levels in parentheses refer to voice levels measured one meter fromthe mouth.) (Ref. 2)

TABLE 9-7. SOUND INTENSITY LEVELS

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The sense of touch is greatly diminished by the protec-tive gloves required when working in cold or contami-nated environments or on cryogenic systems such as liq-uid oxygen. In these cases, it is inappropriate to rely onthe sense of touch to the same degree as when a task isperformed with bare hands.

9-5 PSYCHOLOGICAL FACTORSPsychological factors include adaptability, aptitude,

attitude, motivation, and behavior. Since all individualshave different physical attributes, experiences, abilities,intellect, and emotions, each person is psychologicallyunique. Accordingly, designers must keep in mind thelack of psychological uniformity among users and main-tenance workers.

Although errors and inefficiencies introduced by psy-chological factors will inevitably occur, methods havebeen developed to minimize their frequency and severity.Work environments should be designed for the mentalwell-being of personnel, and the capabilities and normalreactions of personnel should be considered in the designprocess. Many of these design features are defined inMIL-STD-1472 (Ref. 2).

9-5.1 MOTIVATION AND TRAININGPeople are motivated by numerous things esprit de

corps, patriotism, love, hate, revenge. sex, competition,prestige, hunger, fear, pride of accomplishment, financialrewards, etc. In most cases, motivation is a positive factorbecause it creates a desire to accomplish a mission suc-cessfully. Some of the factors that can be controlled by thedesigner to enhance motivation include comfort, security,and safety. Lack of these factors will adversely affectmotivation.

Motivational factors promoted by the Army that areimportant to the maintenance technicians are achieve-ment, recognition of achievement, the nature of the workitself, responsibility, and growth or advancement. Armycareer planning provides for optimal personnel develop-ment by providing (1) opportunity for training and aprogression of selected assignments, (2) counseling toassist in career goal setting, and (3) incentive recognitionfor accomplishments (Ref. 16).

Skill level is an indicator of capability, not of drive.Motivational training for the maintenance technicianhelps to maintain an adequate level of personnel compe-tency. This is especially true of training designed to influ-ence attitudes, as in safety training. Appeals to loyalty,interest in work. prestige, and other such factors willincrease motivation.

9-5.2 CAPABILITY

The design engineer should consider the skills requiredand the personnel available to operate and maintainequipment. Equipment that requires skill levels higherthan those available cannot be successfully maintained,Care must be exercised to insure that the equipment doesnot outstrip the abilities of the soldier.

It is difficult to obtain and retain skilled military main-

tenance personnel. Therefore, the equipment designermust build in maintenance features that would be unneces-sary if highly skilled technicians were available.

As the complexity of the maintenance task increases,the time required to train maintenance specialists alsoincreases. Maintenance actions—relying on modulariza-tion and the use of built-in test equipment—should, there-fore, be as simple as possible to permit the shortest train-ing time so that a technician’s effective service aftertraining can be proportionately increased.

In the design of Army equipment, the user skill levelshould be considered throughout the life cycle of theproduct. The optimal design goal should be equipmentthat can be repaired effectively by the least experiencedpersonnel using pertinent manuals. For developmentpurposes the “typical” Army technician should be assumedto possess the following characteristics (Ref. 17):

1. Average age: 22.2 yr2. Average civilian education: 12 yr (90.8% are high

school graduates)3. Average reading comprehension: ninth grade

(based on US Army Infantry School 1978 report.)4. Average service education: 19 weeks, including

basic training, specialty training, and weapon systemtraining

5. Applicable civilian experience: minimum6. Applicable Army experience: overall average is

approximately 6.7 yr, but 50.3% of the technicians whowill perform most of the work (nonsupervisory) will be intheir first term of service.

Terms used to identify and establish specific tasks andskills follow:

1. Military Occupational Specialty (MOS). This is aterm used to identify a group of closely related dutypositions. The skills required in each of these duty posi-tions are similar. Therefore, an individual qualified toperform in one of these positions can, with adequateon-the-job training, perform satisfactorily in others of thesame complexity or difficulty (Ref. 16). The MOS identi-fies type of skill rather than level of skill. For example, theMOS for infantrymen (11 B) encompasses all positionsranging from a rifleman to a battalion operationssergeant.

2. Military Occupational Specialty Code (MOSC).This is a more specific operational identification. It notonly defines the type of skill but also the level of skill, thelevel of proficiency, and the scope of responsibility.

3. Career Management Fields (CMF). These aremanageable groups of related MOSS. As an example, theCMF for Air Defense missile maintenance contains 28MOSS. They are used in the selection of personnel for AirDefense units and in unit and intermediate support mis-sile maintenance units. Qualifications for the Air Defensemissile maintenance CMF include

a. Educational:(1) Basic mechanical, electrical, and mathemati-

cal abilities and interests(2) A high verbal ability for comprehension and

communication of complex technical data(3) A high degree of reasoning ability for rapid

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diagnosis of equipment malfunctionsb. Physical Requirements. Normal color vision,

night vision, near vision, auditory acuity. hand-eye coor-dination, manual dexterity, and clarity of speech

c. Occupational Qualifications. Knowledge of thefunctioning, assembly, testing, and maintenance of a vari-ety of mechanical, electronic, electrical, hydraulic, andpneumatic components of missile weapon systems. Thesesystems include missile guidance, acquisition radar, targettrack radar, missile track radar, digital and analog com-puters, and fire distribution systems.

9-5.3 HUMAN ERROR

9-5.3.1 GeneralHuman error can be defined as any personnel action

that is inconsistent with behavioral patterns considered tobe normal or any action that differs from prescribedprocedures. Human error includes (Ref. 18)

1. Failing to perform a task (omission)2. Incorrectly performing a task3. Performing a task not required4. Performing a task out of sequence5. Failing to perform a task within the allocated time6. Responding inadequately to a contingency.

Human errors always have been made and will con-tinue to be made, as stated by the following variations ofMurphy’s Law (“Anything that can go wrong will gowrong.”) (Ref. 18):

1. “Any task that can be done incorrectly, no matterhow remote the possibility is, will someday be doneincorrectly. ”

2. “No matter how difficult it is to damage equip-ment, a way will be found to do so.”

3. “At some time, instructions will be ignored whenthe most complicated task is being performed. ”

The need to avoid human error increases with the size,complexity, and yield of the weapon system. The greaterthe size and complexity, the greater the number of main-tenance tasks, and the more chances there are for humanerror. The larger the weapon system yield, the greater theaccident potential in the event of a human error.

Complex equipment does not of necessity requiregreater skill to operate nor is it more difficult to service;complex equipment can be designed for simplicity ofoperation and maintenance. However, the more complexthese actions are, the more vulnerable they are to humanerror—particularly when the user is under tension oremotional stress. This can be a critical problem in combator emergency situations. An Army study (Ref. 19) sub-jected recruits to simulated emergencies, such as theincreasing proximity of falling mortar projectiles to theircommand posts, in a manner that they believed the situa-tions to be real. As many as one third of the new recruitsfled in panic; they did not perform the assigned task thatwould have ended the mortar attack. These studies indi-cate the devastating effects that very high stress levels canhave on the performance of even thoroughly trained,reliable personnel.

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9-5.3.2 CausesRegardless of thorough training and high skill levels, a

technician will make mistakes, and errors frequentlycause equipment malfunction with varying consequences.For example, a driver fails to fill the radiator of a truck,the engine overheats, and the truck stops on the roadinconvenient but not serious. A technician fails to put acotter pin in a castellate nut in the flight control linkage ofan aircraft, control of the plane is lost in flight, the planecrashes, and all aboard are killed very serious.

Maintenance requirements are so demanding that theyoften leave no room for human error, yet mistakes will bemade. For example, a report by one of the military ser-vices revealed that in a 15-month period errors made inaircraft maintenance contributed to 475 accidents andincidents in flight and ground operations. Ninety-six air-craft were seriously damaged or destroyed, and 14 liveswere lost (Ref. 14). A study of these accidents revealedthat many of the failures that caused the accidentsoccurred shortly after periodic inspections. The reportconcluded that these human failures were caused by

1. Inadequate basic training in the relevant mainte-nance practices, policies, and procedures

2. Lack of training in maintenance of the types andmodules of equipment being maintained

3. Inadequate or improper supervision4. Inadequate inspection.

9-5.3.3 Contributing FactorsKnowledge about human error can reduce the proba-

bility of damaged equipment or personnel injury byimposing human factors constraints on the equipmentdesign. The characteristics that follow contribute tohuman errors and diminish the safety of person machinerelationships (Ref. 7):

1. Population Stereotypes. A population stereotypeis “the way most people in the population expect some-thing to be”. People expect, for example, that when acontrol is turned clockwise (except for flow controlvalves), the controlled function should increase. and viceversa.

2. Performance Requirements in Excess of HumanCapablility for the Full Range of Maintainers. Equipmentdesign that exceeds the physical and psychological limitsof human capability creates a high likelihood of acci-dents. For example, a design may require pitch or visualdiscrimination beyond the capability of human senses.

3. Designs That Promote Fatigue. Any design thatmakes personnel work harder than normally expected islikely to promote fatigue and increase error. For example,inadequate lighting produces eyestrain and fatigue. andexcessive noise in the work environment increases the rateof fatigue.

4. Inadequate Facilities or Information. When per-sonnel must perform tasks in inadequate facilities orwithout proper information, errors are likely to occur.For example, if the tolerances for instrument readings arenot provided, personnel tend to assume tolerances theybelieve to be reasonable. Operator action based on

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instrument indication may be too late or too inaccurate tobe of value.

5. Unnecessarily Difficult or Unpleasant Tasks.When design results in tasks that are unpleasant or com-plex, personnel may not devote the proper amount oftime and attention to attaining satisfactory performance.For example, if two adjustments of equipment interrelateso that precise setting and resetting are required to attainthe proper value, personnel making the adjustments aremore likely to stop short of the proper value than if theadjustments were independent. Also tasks that may getthe technician excessively dirty or wet, such as crawlingunder a vehicle in a muddy field, will frequently be“overlooked”.

6. Necessarily Dangerous Tasks. Motivationalcharacteristics, rather than performance capabilities, mayintercede when personnel perform dangerous tasks. Forexample, personnel exposed to high voltages during pre-ventive maintenance operations are less likely to performtasks thoroughly, and as frequently as they would if thedanger did not exist.

7. Unpleasant Environments. Proper environmen-tal conditions must be maintained as much as possible sothat the capability of the body’s regulatory mechanisms tosustain a constant internal environment is not strained.Optimum environmental conditions make minimal de-mands on the body’s self-regulatory mechanisms. (Thisrelates to performance of unpleasant tasks discussed inItem 5.)

To minimize the possibility of human error in accom-plishing any procedure involving a nuclear device, theArmy has developed the “two-person concept”. Two ormore persons, each capable of undertaking the prescribedtasks and of detecting incorrect or unauthorized proce-dures are involved. One person performs the task whilethe other checks to make sure the task has been performedcorrectly.

9-5.3.4 QuantificationThere is a need to understand and predict the contribu-

tion of human error to reliability and maintainabilityparameters such as mean time between failure and meantime to repair. Both of these are characteristics of thehardware, but both are also influenced by human perfor-mance. In fact, there are some analyses that indicate thatthe majority of system failures are attributable to humansand not to hardware.

Much work has been done in human performance reli-ability (HPR). Yet a basic problem remains, i.e., lack of agood data base of human error and performance (Ref.20). The models available are fairly sophisticated, butconsidering the poor quality of the data input to themodels, the output should be used with caution. There areseveral HPR indices that differ both in scope and in typeof model used. Two types––the technique for humanerror rate (THERP) and the Siegel-Wolf model—will bediscussed.

The analytical or simulation THERP can be used topredict the total system or subsystem failure rate resultingfrom human errors (Ref. 21). The THERP methodology

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begins with a task analysis that breaks the system into aseries of personnel-equipment functional (PEF) units.The system being analyzed is then described by a func-tional flow diagram. Prediction data are assigned to eachPEF. A computer program calculates the reliability oftask accomplishment and performance completion timeand takes into account dependent and redundant relation-ships.

The Siegel-Wolf digital simulation model is orientedtoward the effects of time stress on the successful comple-tion of the task. The model outputs are (Ref. 21)

1. Average time expended2. Average peak stress3. Average final stress4. Probability of task success5. Average waiting time6. Sum of subtasks ignored7. Sum of subtasks failed.

A good start on a data bank of human error rates hasbeen made by the American Institute of Research (AIR)(Ref. 22). The AIR estimates of error rates are for aver-age, trained military personnel with average motivationwho are operating under normal conditions. However,very little work has been done to quantify the degradationof human performance under operational stress.

A second source of data results from an analysis of themaintenance data in the Army maintenance managementsystem (TAMMS). Maintenance actions reported throughTAM MS are analyzed and supplemented with indepen-dent judgments and arrive at quantitative values forhuman error data. Table 9-8 presents the results of a studythat used this method, Human error rate estimates for alarge system were derived from existing data of poorquality by modifying the data with the independentjudgments of human reliability analysts. These judgmentswere made after reviewing information on personnel skilllevels; previous jobs held by these personnel; procedures;and design of the control, displays, and other equipmentread or manipulated by the personnel.

To date, the primary source of HPR information issubjective data based on expert opinion or objective datasupplemented as necessary with subjective judgments.Techniques for developing expert estimates include theDelphi technique (Ref. 21).

9-6 PHYSICAL FACTORSPhysical factors are relevant in designing for maintain-

ability; it is important that maintenance personnel bephysically capable of performing required tasks. If theycannot, maintenance efficiency suffers.

Maintenance proficiency is directly affected by a widevariety of natural and induced environmental elementsthat degrade performance by interfering with a sensoryprocess or by creating physiological or psychologicalstress. Stress is a function of many factors—e.g., fatigue,lack of training, worry, fear—that is exacerbated by theirritants of noise, vibration, and inclement temperature.The stress level rises to a point at which the person’sability to perform in a satisfactory manner declines sud-denly and markedly. Logically, the more severe the stress,

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TABLE 9-8. HUMAN ERROR RATE ESTIMATE DATA (Ref. 22)Estimated

Rates Activity10-4

Selection of a key-operated switch, rather than a nonkey switch. (The value does not include theerror of decision where the operator misinterprets the situation and believes the key switch is thecorrect choice. )

10-3

Selection of a switch (or pair of switches) dissimilar in shape or location to the desired switch (orpair of switches) assuming no decision error. For example, operator actuates a large-handledswitch, rather than a small switch.

3 X 10-3 General human error of commission, e.g., misreading label and thereby selecting wrong switch.10-2

General human error of omission where there is no display in the control room of the status ofthe item omitted, e.g., failure to return a manually operated test value to proper configurationafter maintenance.

3 x 10-3 Errors of omission, where the items being omitted are embedded in procedure, rather than at theend as in the previous activity.

3 x 10-2Simple arithmetic errors with self-checking but without repeating the calculation by redoing it onanother piece of paper.

0.2-0.3 General error rate given very high stress levels where dangerous activities are occurring rapidly.

the sooner this point is reached. Accordingly, some of the 2. Noise (discussed in par. 9-4.2)environmental elements that interfere with the normal 3. Temperature and humidity (see Chapter 10)sensory process or heighten stress require study and mea- 4. Acceleration. shock, and vibration (see Chaptersurement during engineering test (Ref. 3). Contributing 10)environmental elements are 5. Toxicological, radiological, and electromagnetic

1. Illumination (discussed in par. 9-4.1) hazards (see Chapter 10).

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1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

A

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REFERENCESMIL-H-46855B, Human Engineering Requirementsfor Military Systems, Equipment, and Facilities, 5April 1984.MIL-STD-1472C, Human Engineering DesignCrileria for Military Systems, Equipment and Facili-ties, 10 May 1984.MIL-HDBK-759A, Human Factors in Design ofArmy Materiel, June 1981.MIL-HDBK-743, Anthropometry of US MilitaryPersonnel, 3 October 1980.Report TR EPT-2, Reference Anthropometry of theArctic-Equipped Soldier, US Army Natick Labora-tories, Natick, MA.Report TR 72-51-CE, The Body Size of Soldiers: USArmy Anthropomtetr, 1966.DH 1-3, Design Handbook, Human Factors Engi-neering, AFSC, Wright-Patterson AFB, OH, ThirdEdition, March 1977.J. W. Garett and K. W. Kennedy, A Collection ofAnthropometry, Aerospace Medical Research Labora-tory, Wright-Patterson AFB, OH, 1971.Harold P. VanCott and Robert G. Kinkade, HumanEngineering Guide to Equipment Designs, Superin-tendent of Documents, US Government PrintingOffice, Washington, DC, 1972. (Revision of 1963McGraw-Hill edition)Wood son and Conover, Human Engineering Guidefor Equipment Designers, University of CaliforniaPress, Berkeley, CA, 1964.Harold P. VanCott and Robert G. Kinkade, Eds.,Human Engineering Guide to Equipment Design,McGraw-Hill Book Company, New York, NY 1963.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

J. R. Mertney, et al., Human Factors in Design,Energy Research and Development AdministrationReport, ERDA 76-45-2.DOD-HDBK-761, Human Engineering Guidelines

for Management Information Systems, June 1985.AMCP 706-133, Engineering Design Handbook,Maintainability Engineering Theory and Practice,January 1976.TB ME D 501, Occupational and EnvironmentalHealth: Hearing Conservation, March 1980.AR 611-201, Enlisted Career Management Fieldsand Military Occupational Specialties, 25 October1985.Field Circular 21-451, I am the American Soldier, USArmy Soldier Support Center, Fort Benjamin Harri-son, IN, 31 March 1985.W. Hammer, Handbook of System and ProductSafety, Prentice-Hall, Englewood Cliffs, NJ, 1972.M. M. Berkun, “Performance Decrement UnderPsychological Stress”, Human Factors 6, 21-30( 1964).David Meister, Subjective Data in Human Reliabil-ily Estimates, Reliability and Maintainability Con-ference, 1978.Human Reliability Prediction System User's Man-ual, Department of Navy, Sea Command, December1977.A. D. Swain, A Method for Performing a HumanFactors Reliability Analysis, Report SCR-685, San-dia Corporation, August 1963.

BIBLIOGRAPHYA Collection of Anthropometry, J. W. Garett and K. W. Engineering Anthropometry Methods, J. A. Roebuck, H.Kennedy, Aerospace Medical Research Laboratory, H. E. Kroemer, and W. G. Thomson, John Wiley andWright-Patterson AFB, OH, 1971. Sons, Inc., New York, NY, 1975.

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CHAPTER 10ENVIRONMENTAL FACTORS

This chapter emphasizes the importance of the environment in which equipment will be operated or storedas a factor in establishing design criteria and evaluating maintainability features. The environmental factors—natural and induced—and combined environ mental factors as they affect maintenance personnel and materielare discussed.

10-1 INTRODUCTIONEnvironment is defined in MIL-STD-1165 (Ref. 1) as

“the totality of natural and induced conditions occurringor encountered at any one time and place”. The naturaland induced conditions (factors) are presented in par.10-2. The description of the environment must be tailoredfor particular consideration according to the particularmateriel type and to the challenge the materiel presents topersonnel who must operate and maintain it. In differentclimates, the environmental factors vary in importance.For example. solid precipitant comprise an importantfactor in Alaska, but this factor is of no importance in thePanama Canal Zone. Similarly, rain is an important fac-tor in the outdoor envrironment in the temperate zone butis unimportant inside a warehouse. The interior of a

warehouse, however, is an important region of the envir-onment for materiel. Table 10-1 (Ref. 2) indicates therelationship of environmental factors to climatic types.

Certain combinations of environmental factors andclimates exert various levels of effects on materiel andpersonnel—some more severe, others less severe. In thepast it was considered impractical to design equipment tomeet a specific climatic condition, i.e., equipment wasdesigned to be operated and maintained in a worldwideenvironment. This philosophy is being challenged (Ref. 3)when nondevelopmental items (NDI) could satisfy anexisting requirement. It is necessary to examine the prac-tice of designing to the “worst case” scenario for allequipment. The design for all environmental conditions isa necessary approach for front-line combat materiel butmay not make sense for materiel used in rear echelons or

TABLE 10-1. RELATIONSHIP OF ENVIRONMENTAL FACTORS TO CLIMATE (Ref. 2)Factor Intermediate Arctic Hot-Dry Hot-Wet

TerrainLow temperatureHigh temperatureLow humidityHigh humidityPressureSolar radiationRainFogSolid precipitationWhiteout and ice fogSalt, salt fog, salt waterWindOzoneMicrobiological organismsMicrobiological organismsAtmospheric pollutantsSand & dustShockVibrationAccelerationAcousticsElectromagnetic radiationNuclear radiation

+ + ++ ++0

+ +0+

+ ++ ++ +

0++*

++*+******

+ + ++ + +

00+0

+ ++

+ ++ ++ ++

+ +*

00*0******

+ + +0

+ + ++ +

00

+ + ++000++*

00*

+ + +******

+ + +0

+ +0

+ + ++

+ ++ +

000

+ ++*+

+ + +*+******

+++ Key factor++ Important factor

+ Active factor0 Unimportant or absent factor* Little or no climatic relationship

10-1

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stateside. Let us take climatic hardening as an example.Do we need to harden the entire Army inventory ofequipment to withstand an arctic climate when only 10%of the equipment is ever used there? Why not harden the10% that is supposed to endure the environment or pro-vide supplemental environmental protection in the formof kits or shelters? Accordingly, the maintainabilityengineer—before developing a maintainability plan --should ascertain the decision of the Materiel AcquisitionReview Board, Army Materiel Command, and the ArmyTraining and Doctrine Command relative to the degree ofhardness required.

Of the natural environmental factors, temperature andhumidity represent the most severe environment formaintenance in the field. Extreme cold affects the techni-cians’ ability to handle parts, and the heavy gloves makesmall parts virtually impossible to handle or manipulate.Heavy clothing interferes with access and impairs visi-bility. Cold temperatures also have a deleterious effect onmany materials.

Hot climates also create poor working conditions.Extremely hot temperatures, particularly if associatedwith high relative humidity, have a debilitating effect onpersonnel. Hot climates also may create difficult workingconditions. Hot, dry areas usually produce dust, whichpenetrates mechanical equipment and causes prematurewear in moving parts. Where dust accumulates, it canabsorb water; this may result in corrosion and electricalproblems. Hot, wet areas—such as the tropics causefungous growth in and on equipment.

Warfare, although not an induced environmental fac-tor in the classical sense, is an important factor. Warfareneglecting increased materiel damage—causes severemaintenance problems because of the mental stress on thetechnician; he is concerned with both his position and theurgency to return equipment to a serviceable condition asrapidly as possible. The conditions of war may also makeit difficult to obtain required repair parts and thusaggrevate the situation.

Of the induced environmental factors, shock and/orvibration represent the most severe environment.Personnel performance—operator and maintenance—isadversely affected by the degree of the shock and/orvibration. These factors also can produce mechanicaldamage to materiel. Materiel experiences a variety ofdynamic mechanical loads during movement i.e., fromthe origin of manufacture to the stockpile and from thestockpile to target. Some of these loads are intrinsic to thetype of transport and handling; others are characteristicof the system itself or associated with inadvertentmishandling.

10-2 ENVIRONMENTS10-2.1 NATURAL, INDUCED, AND

COMBINEDThe components, or descriptors, of the environment

are referred to as factors. The factors (see Table 10-2 (Ref.4)) are divided into natural and induced, defined asfollows:

1. Natural. Those factors primarily natural in origin.It is notable that the importance of each of these factorsmay be altered by man and. in fact, of often is whenprotection is provided.

2. Induced. Those factors for which man’s activitiesconstitute the major contribution. These factors, resultingfrom man’s activity, may be controlled to any extentdeemed necessary and practical.Table 10-2 also identifies the environmental factors as toclass.

The term “combined environmental factors" is used insituations to identify combinations of environrnental fac-tors that frequently are observed arid that are associatedby natural coupling. In their effects on materiel, manyenvironmental factors act in conjunction or in synergism.In the conjunctive case are found examples of factors inpairs or in multiple combinations that are characteristicof geographic regions or other circumstances. Thus hightemperature and high humidity often cooccur its do hightemperature and airborne sand and dust. In the synergis-tic case two or more environmental factors act together toproduce effects that are more important than the separateeffects of either constituent. An example of synergism isthe effect obtained with low temperature and vibration.With this combination of factors, rubber shock mountsthat can survive either the severe cold or the severe vibri-tions readily are destroyed by the combined action of thetwo envronmental factors. In similar fashion, the ap-pearance of one environmental factor may inhibit theaction of another e.g., high temperature inhibits solidprecipitants, and low temperature inhibits attack bymicrobiological organisms. Time of exposure is an impor-tant factor when considering the effects of combinedenvironmental factors. Table 10-3 (Ref. 2) summarizesthe qualitative relationships between pairs of environ-mental factors.

For a detailed and sophisticated treatment of environ-mental factors definitions, concentrations or severityworldwide, methods of measurement, and effects on per-sonnel and materiel refer to Refs. 2, 4, 5. and 6.

10-2.2 WARTIME ENVIRONMENTSWarfare—ignoring a worst case scenario of a “nuclear

winter”—will have no effect on the naturally occurringenvironmental factors presented in par. 10-2.1. Theinduced environmental factors may be increased in inten-sity and effects due to the employment of nuclear and orchemical munitions. Enemy and friendly fire with theattendant noise although not classical envrironrnentalfactors– must be considered in the total environrnent ofwarfare.

Maintenance operations. following a nuclear or chemi-cal attack. will include decontamination. The guidance inthe previous chapters as to permanent labels and thedesign of doors and external fixtures so as to drain readilyand not act as liquid reservoirs will facilitate decontami-nation. Radiation that has been induced in materiel by anuclear explosion cannot be removed by decontamina-tion. Accordingly, to avoid possible overexposure of per-sonnel, speed of repair is of the essence. The guidance in

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TABLE 10-2. MAJOR ENVIRONMENTAL FACTORS (Ref. 4)

Type

Natural

Induced

Class

Terrain

Climatic

Biological

Airborne

Mechanical

Energy

Factor

TopographyHydrologySoilsVegetation

TemperatureHumidityPressureSolar radiationRainSolid precipitantFogWindSaltOzone

Microbiological organismsMicrobiological organisms

Sand and dustPollutants

VibrationShockAcceleration

the previous chapters relative to simplicity, accessibility,and module replacement is, therefore, apropos.

Decontamination and working in a contaminated areawill require a protective ensemble. The wearing of theprotective mask, hood. overgarment, and gloves posesserious problems in functional efficiency and degrada-tion. The overall ensemble imposes the problems of heatstress, loss of visual field and visual acuity, lens foggingwith attendant loss of vision, and reduced tactility. Thebulk of the protective ensemble must be evaluated as afunction of the operators’ and technicians’ task (Ref. 7).

10-2.2.1 ChemicalA chemical (pollutant) environment can be created by

the introduction of lethal and nonlethal chemical agentsand riot control chemical agents. The chemical environ-ment may result in the deposition of corrosive materialson materiel; if inhaled or deposited on bare skin, physio-logical and psychological changes to personnel will result.

10-2.2.2 NuclearThe nature and severity of the contamination resulting

from a nuclear detonation are a function of weapon yieldand type of burst i.e.. air, ground, or underwater. Thefire and blast effects, except for magnitude, are identicalwith those associated with conventional weapons; theeffects of the nuclear radiation are unique. Salvageable

equipment immediately after the event will be radioactivedue both to induced nuclear radiation and fallout. Thefallout can be removed by decontamination techniques,but the induced radiation will persist. The persistence ofthe radiation is a function of the material in which theradiation was induced. An electromagnetic pulse (EMP)—which has the potential for severely damaging electroniccomponents and circuits—is associated with a nucleardetonation. Nuclear effects on materiel are presented inpar. 10-3.2.1.7.

10-2.2.3 Electromagnetic RadiationRef. 6 is a thorough, scholarly presentation of electro-

magnetic radiation—sources, detection, measurement,and effects on man and materiel. Electromagnetic radia-tion is present in many maintenance environments andmay be introduced by equipment related to the functionsbeing performed or under the control of the techniciansubject to exposure. Basically, the electronic environmentconsists of two categories of radiation (Ref. 6), i.e., natu-rally occurring radiation and radiation generated by man-made equipment. The very low frequency band, i.e., lessthan 104 Hz, is not considered to be environmentallyimportant because (1) the radiated power densities arerelatively low and (2) at the long wavelengths, the energyabsorption by materiel is negligible.

Although the electromagnetic environment is com-posed of emanations from a multiplicity of sources, only a

10-3

AcousticsElectromagnetic radiationNuclear radiation

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TABLE 10-3. COMBINATION OF ENVIRONMENTAL FACTORS (Ref. 2)

few sources produce radiation of sufficient intensity tomerit consideration. These include (Ref. 6)

1. Near-field radiation from communication andtelevision transmitting antennas

2. Radiation in the immediate vicinity of diathermyequipment, microwave ovens, and induction heatingapparatus

3. Focused microwave beams associated with alltypes of radar

4. Laser-generated coherent light beams5. Medical and industrial X-ray apparatus6. High intensity and high frequency light generated

by nuclear events, ultraviolet lamps, and similar sources

7. Electromagnetic pulse effect associated withnuclear events

8. Electromagnetic field accompanying lightning.Except for the radiations originating with the enormous

energy releases of nuclear events or the highly focusedenergy beams of radar and lasers, intensities sufficient toproduce observable effects occur only in close proximityto the sources. The multiplicity of sources in the environ-ment, however, causes the probability of such exposure tobe relatively high. In military operations, for example,both materiel and personnel are readily exposed to radarbeams (Ref. 6).

The effects of electromagnetic radiation within the

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frequency spectrum allotted to the communications bandgenerally are not believed to cause significant effects onpersnnnnel. On the other hand, it is documented and well-known that the effects of X rays, lightning strikes, highintensity light pulses, and even microwaves under theright combination of conditions can produce hazardouseffects on personnel and can affect materiel (Ref. 6).Accordingly, the electromagnetic effects on personnelmust be considered in the design of electronic equipment.Basically, the effects produced by electromagnetic fieldson personnel are classified into thermal and nonthermal.Certain parts of the human physiology are particularlysusceptible to certain frequencies of electromagneticenergy. One of the prime areas of concern involves theeffects of microwaves. As an example, it is well docu-mented that microwaves produce cataracts in the eyes ofpersons who are subjected to strong microwave fields forlong periods of time. This is believed to be a thermal effectbecause there is no blood supply to the lens of the eye tocarry away the heat of absorption. Other effects occur inhumans. but cataracts seem to appear first. The im-portance of nonthermal effects is a source of scientificdiscussions; sufficient evidence is not yet present tospecify the nonthermal effects that are both importantand originate within the electromagnetic environment.Accordingly, since there is no consensus on the effects ofelectromagnetic radiation on personnel, the maintain-ability engineer should seek the expert advice of healthphysicists and medical personnel. Areas in which electro-magnetic energy is present should be posted as indicatedin Chapter 8.

10-2.2.4 Enemy and Friendly FireEnemy and friendly fire produce detrimental effects

that are both psychological and acoustic. Psychologicaleffects are related solely to personnel; acoustic effects canaffect both personnel and materiel. By far the mostsignificant aspect of sound to personnel is its relationshipto communication by speech and hearing. Speech and itsaccurate perception are absolutely essential to the normalexistence of humans; the fact that speech perception isaffected adversely by excessive noise or by hearing loss isobvious. Consequently, every reasonable effort must bemade to insure that the inadvertent hearing loss fromfriendly fire is minimized. Above certain sound intensitylimits, exposure to sound has physical and physiologicaleffects in addition to its effects on hearing. Sufficientlyintense airborne sound can destroy materiel and killexposed personnel (Ref. 6).

Sound pressure levels (SPL) of interest in the Army’sacoustic environment cover roughly 20 decibel (dB)orders of magnitude, i.e., a 200-dB range. Any commonArmy small arms—all weapons up to and including cal. 50and shotguns—produce impulse noise levels in excess of140 dB. Most, if not all, of the mechanized equipment inthe Army produces steady state noise environments thatcan interfere with direct person-to-person communication(Ref. 6). Table 10-4 (Ref. 6) relates sound levels to weaponsources. A detailed discussion of noise can be found inAppendix B, Ref. 7. Ref. 8 is the basic regulatorydocument governing maximum noise levels in Armyequipment. Ref. 9 officially defines dangerous noise levelsfor the Army and specifies methods for controlling noiseexposures and for conserving hearing in high-noiseenvironments.

10-3 EFFECTS OF ENVIRONMENTS

10-3.1 EFFECTS ON PERSONNELEach of the modified environments described in par.

10-2.2 has an adverse effect on personnel performingmaintenance operations. Some affect comfort, someincrease the time required to perform a maintenanceaction, and all may introduce a higher level of error thanwould an optimum maintenance environment. Successfulmaintainability design must also consider the normalenvironmental factors that affect the ability of personnelto perform at an optimum level of proficiency. Humanerror, comfort level, and some of the psychological effectsof noise were discussed in Chapter 9.

10-3.1.1 TemperatureAlthough the effects of temperature on human per-

formance are not completely understood, it is known thatcertain temperature extremes are detrimental to workefficiency. As the temperature increases above the comfortzone, mental processes slow down, motor responses areslower, motivation is reduced, and the likelihood of errorincreases. Fig. 10-1 illustrates the increased error ratewith increasing effective temperature. Dry-bulb tempera-ture is not the sole criterion in relating temperature tocomfort or efficiency; humidity and airspeed are im-portant considerations. The discussion of effectivetemperature, Wet-Bulb Global Index, and WindchillIndex in the paragraphs that follow presents this inter-relationship.

TABLE 10-4. SOUND LEVELS ASSOCIATED WITH WEAPON TYPES (Ref. 6)

Sound Level Typical Source Significance or Actiondb bar Required

140-170 0.010-0.012 Small arms Hearing protection required forrepeated exposure

175-190 0.0125-0.014 Artillery Hearing protection essential; bodyprotection desirable

10-5

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Figure 10-1. Error Increase Due to Rise inEffective Temperature

10-3.1.1.1 Effective TemperatureThe effective temperature (ET) of an environment is an

empirical thermal index that considers how combinationsof dry-bulb air temperature, humidity, air movement, and

clothing affect people (Ref. 7), Numerically, the ET isequal to the temperature of still saturated air, whichwould give the same sensation. The effective temperaturemay be read from Fig. 10-2 (Ref. 7).

The optimum temperatures for personnel vary accord-ing to the nature of the tasks performed, the conditionsunder which the tasks are performed, and the clothing thepersonnel are wearing. For maximum physical comfortwhile normally dressed appropriate to the season orclimate, the optimum range of effective temperature foraccomplishing light work is (Ref. 7)

1. 21° to 27°C (70° to 81°F) in a warm climate orduring summer

2. 18° to 24°C (64° to 75°F) in a colder climate orduring winter.Fig. 10-3 (Ref. 7) indicates summer and winter comfortzones and thermal tolerances in hours for inhabited areas.

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10-3.1.1.2 Wet-Bulb Global TemperatureFor military personnel who must work outside in hot

climates in ranges beyond the comfort and discomfortzone of heat stress—the Wet-Bulb Global Temperature(WBGT) Index is more applicable than the ET Index. TheWBGT Index takes into consideration dry-bulb tem-perature; relative humidity calculated with ambient airmovement, rather than at a standardized rate; and thesolar, or radiant, heat load. This index and its use aredescribed in Ref. 11. The WBGT is calculated as follows(Ref. 7):

Generally, the activities of unacclimatized individuals arerestricted when the WBGI exceeds 25°C (77°F) (Ref. 7).See Refs. 7 and 12 relative to the WBGT Index and limitsof heat exposure.

10-3.1.1.3 Windchill IndexNo general index, such as effective temperature, is

available for expressing all the factors involved in coldexposure, but the Windchill Index commonly is used toexpress the severity of cold environments. Althoughwindchill is not based on physical cooling and is probablynot very accurate as an expression of human cooling, ithas come into use as a single-value, practical guide to theseverity of temperature-wind combinations. Fig. 10-4(Ref. 7) portrays a windchill chart. Note, for example.-40°C (-40° F) with air movement of 0.1 m/s (0.3 ft/s) (seeLine A) has the same windchill value and, therefore, thessme sensation on exposed skin—as-24°C (-13°F) with a0.5-m/s (1.5-ft/s) wind (see Line B). The Windchill Indexdoes not account for physiological adaptations or adjust-ments, however, and should not be used rigorously. Aqualitative description of human reaction to windchillvalues on exposed skin is shown in Table 10-5 (Ref. 7).

10-3.1.1.4 Guidelines for MaintenanceOperations in Extreme Climates

Guidelines for operations in a hot climate follow:1. Where possible, provide air conditioning if tem-

peratures exceed 32°C (90°F). Proper ventilation shouldbe provided in equipment trailers or other locationswhere personnel are monitoring, servicing, or performingother maintenance tasks.

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Figure 10-2. Deriving Effective Temperature

10-7

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Figure 10-3. Summer and Winter Comfort Zones and Thermal Tolerances for InhabitedCompartments

10-8

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Figure 10-4. Windchill Chart

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10-9

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TABLE 10-5. HUMAN REACTION TOWINDCHILL VALUES (Ref. 7)

Windchill Values(See Fig. 10-4) Human Reaction

100 Warm400 Pleasant800 Cold

1000 Very cold1200 Bitterly cold1400 Exposed flesh freezes

2. When it is necessary for maintenance techniciansto work for extended periods inside equipment exposedto the sun, provide either permanent or portable airconditioning.

3. When maintenance technicians will be required towork for extended periods on equipment in open air,provide canopies for shade from direct sun.

4. Where feasible, employ reflective or absorbentsurfaces, as appropriate, on equipment that must bemaintained while exposed to the sun.

5. Where excessive temperatures interfere with fre-quent maintenance, redesign the equipment so that thecomponent needing checking or adjustment is in a coolerarea. If this is not possible, it may be feasible to providefor cooling of the component in place.

Guidelines for operating in a cold climate follow:1. Provide heated working areas for maintenance

personnel in arctic environments. For unit maintenanceactivities, specify procedures and design equipment thatrequire a minimum sustained amount of working time.For example, use quick-disconnect servicing equipment.

2. Provide for drying of equipment that is to bereturned to outdoor arctic temperatures after shop main-tenance. Moisture that condenses on or in such equip-ment will freeze and possibly cause damage.

3. Design for maintenance accessibility of winter-ized equipment in arctic zones. Consider the following:

a. Winterization equipment, such as preheater,should be placed where it does not interfere with accessi-bility for inspection, servicing, or other maintenancetasks.

b. When locating access doors and panels, con-sider the formation of ice and the presence of snow orrain.

c. Provide access openings and work space largeenough to accommodate personnel wearing arctic clothing.

d. Provide drains that can be operated by person-nel wearing heavy gloves. Drains should be easily accessi-ble and properly located to insure draining of liquids toprevent damage caused by freezing.

4. Where a technician’s bare hands can freeze to coldmetal or other cold surfaces when performing mainte-nance operations, provide sufficient access and internalwork space to permit wearing of protective gloves.

10-3.1.2 Other Climatic FactorsThe other climatic environmental factors listed in

Table 10-2 are of less concern than the temperatureextremes previously discussed. The protective measuresthat minimize the efforts of high temperature will alsooffer protection against solar radiation. Precipitation,blowing sand, and blowing dust will usually require pro-tective clothing; unfortunately, this reduces the effective-ness of the technician and results in increased mainte-nance time.

10-3.1.3 Whole-Body VibrationIS0 DIS 2631, Guide to the Evaluation of Human

Exposure to Whole-Body Vibration (Ref. 12), providesdata to assess the effects of vibration and motion onhumans. Equipment design should limit whole-bodyvibration to levels that permit safe operation andmaintenance.

Fig. 10-5 (Ref. 7) shows the maximum allowable expo-sure times that will maintain proficiency for differentcombinations of acceleration and frequency levels. Forinstance, the maximum exposure time for an accelerationof 0.5 m/s2 and a frequency of 1.6 H7 in the longitudinaldirection is 8 h (Point A). For acceleration and frequencylevel combinations above the 8-h curve, exposure timemust be reduced accordingly to maintain proficiency. Incase of multidirectional vibration, each direction is to beevaluated independently with respect to limits presentedin Fig. 10-5.

Limits on whole-body vibration to accommodate thehuman body follow (Ref. 7):

1. Safely Limits. To protect the human body, whole-body vibration should not exceed twice the accelerationvalues in Fig. 10-5 for the times and frequencies indicated.

2. Proficiency Levels. When proficiency is requiredfor operational and maintenance tasks, whole-body vibra-tion should not exceed the acceleration values in Fig. 10-5for the times and frequencies indicated.

3. Comfort Level. Where comfort is to be main-tained, the acceleration values in Fig. 10-5 for a givenfrequency should be divided by 3.15.

4. Motion Sickness. Very low frequency verticalvibration should not exceed the limits given in Fig. 10-6(Ref. 7) to protect 90% of the unadapted males fromvomiting in the exposure time indicated.

Where both whole- and part-body vibrations are not afactor, equipment should be designed so that oscillationswill not impair human performance with respect to con-trol manipulations or the readability of numerals andletters. Such equipment should be designed to precludevibrations in the shaded area of Fig. 10-5(A).

Vibration may be detrimental to the maintenance tech-nician’s performance of both mental and physical tasks.Large amplitude, low frequency vibrations contribute tomotion sickness, headaches, fatigue, and eyestrain, andthey interfere with depth perception and the ability toread and interpret instruments. Exposure to continual,high-speed vibrations promotes worker fatigue and de-creases worker proficiency. The designer can reduce and

10-10

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Figure 10-5. Vibration Criteria for Longitudinal and Transverse Directions With Respect toBody Axes

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Figure 10-6. 90% Motion Sickness Protection Limits for Human Exposure to Very LowFrequency Vibration

control vibration by isolation, proper balancing of rotat-ing machinery, and provision of damping materials orcushioned seats for personnel.

10-3.1.4 Terrain FactorsThe terrain factors listed in Table 10-2 do not affect the

performance of maintenance personnel directly. Thesefactors may make it more difficult to arrive at the scenewhere maintenance is required or to recover materiel thatrequires maintenance. The terrain factors may also makeit more difficult to locate a suitable shop area.

10-3.2 EFFECTS ON EQUIPMENTEquipment must be designed for ease of maintenance in

the environments identified in Table 10-2 and discussed inpar. 10-2. The risk of failure due to natural and inducedenvironmental factors and their combined effects must beminimized to limit the maintenance demand. Accord-

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ingly, environmental design criteria should be included inmaintainability considerations. The maintainability fac-tors of simplicity, ease of access, location of drains, per-manent identification of parts, etc., have been discussedin previous chapters; it is important that they be appliedwith consideration to the expected environment. Themaintainability engineer must be aware of the specificenvironment in which materiel will be operated and main-tained in order to develop a comprehensive maintainabil-ity plan.

The current philosophy that equipment must bedesigned to operate anywhere in the world has addedclimatic and terrain factors to the performance efficiencyevaluation criteria. The destructive effects of inducedenvironmental factors—e.g., shock, vibration, and chem-ical agents -and natural environmental factors e.g.,wind, sand, dust, humidity, and solar radiation—are notto be ignored. However, they play a minor role whencompared to the relentless attack of induced moisture-

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either directly or in support of biological and galvanicactions. The combined effects of moisture and highambient temperatures are more destructive than all otherfactor combinations. The next most destructive factor islow ambient temperature, a condition which makes mate-rials more sensitive to rapidly applied loads. The destruc-tive actions resulting from moisture and temperature canbe guarded against by a combination of design, materials,protective finishes, and packaging. For example, avoid-ing dissimilar bare metal contact—i. e., metals far apart inthe galvanic series indicated in Fig. 8-9 will preventcorrosion resulting from galvanic action.

MIL-HDBK-721 (Ref. 13) provides detailed coverageof corrosion and the protection of metals. MIL-E-5400(Ref. 14) and MIL-E-16400 (Ref. 15) list acceptablecorrosion-resistant materials.

10-3.2.1 General Considerations

10-3 .2.1.1 Moisture ProtectionThe exclusion of moisture from equipment in the trop-

ics considerably eases maintenance problems. For exam-ple, Fig. 10-7 illustrates the effect of moisture in loweringthe resistance of insulating materials. To help minimizesuch effects in insulation and other materials, the follow-ing guidelines should be considered:

1. Choose materials with low moisture-absorption

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qualities wherever possible.2. Use hermetic sealing whenever possible. Make

sure the sealing area is kept to a minimum to reducedanger of leakage.

3. Where hermetic sealing is not possible, considerthe use of gaskets and other sealing devices to keep mois-ture out. Insure that the sealing devices do not contributeto fungal activity, and allow for detection and eliminationof any “breathing” that may admit moisture.

4. Consider impregnating or encapsulating mate-rials with fungus-resistant hydrocarbon waxes and var-nishes. This also will prevent wicking.

5. Do not place corrodible metal parts in contactwith treated materials. Glass and metal parts might sup-port fungal growth and deposit corrosive waste productson the treated materials.

6. When treated materials are used, make sure theydo not contribute to corrosion or alter electrical or physi-cal properties.

Where these methods are not practical, drain holesshould be provided, and chassis and racks should bechanneled to prevent moisture traps. Additional informa-tion on moisture protection can be supplied by the Pre-vention of Deterioration Center, National ResearchCouncil, 2101 Constitution Avenue, Washington, DC20037. Refer also to MIL-E-5400 (Ref. 14) and MIL-E-16400 (Ref. 15) for listings of acceptable moisture-resistant materials.

Figure 10-7. Reduction in Insulation Resistance of Typical Electronic Components Exposed ForFive Months in Tropical Jungle

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10-3.2.1.2 Desert RegionsDesert regions occupy approximately 19% of the land

surface of the earth. The outstanding attribute of alldeserts is dryness. A widely accepted definition of ’’desert”is an area with an annual rainfall of less than 254 mm (10in.). Hot deserts are further characterized by a clearatmosphere and intense solar radiation. both of whichresult in temperatures as high as 52°C (126°F) andambient illumination levels as high as 10,280 cd/m2 (3000ftL) (Ref. 16). This intense solar radiation combined withterrain that has a high reflectance can create high levels ofglare. Other characteristic phenomena associated withdeserts are atmospheric boil and mirages. Design fordesert areas should also consider sand and dust, whichnearly always accompany dryness.

The high daytime temperatures, solar radiation, dust,and sand combined with sudden violent winds and largedaily temperature fluctuations may create many of thefollowing maintenance problems:

1. Heat can lead to difficulties with electronic andelectrical equipment, especially if they have been designedfor moderate climates.

2. Materials—such as waxes—soften, lose strength,and melt.

3. Materials may lose mechanical or electrical prop-erties because of prolonged exposure.

4. Fluids may lose viscosity.5. Joints that would be adequate under most other

conditions may leak.Heat can also cause the progressive deterioration of manytypes of seals found in transformers and capacitors.Capacitors of some types develop large and permanentchanges in capacity when exposed to temperatures above49°C (120°F).

The temperature extremes for electronic equipmentoperating in a desert environment are shown in Table10-6. The following factors also should be considered:

1. Dry cells have a short life in hot environments anddeteriorate rapidly at temperatures above 35° C (95° F).

2. Wet batteries lose their charge readily.3. Tires wear out rapidly.4. Paint, varnish, and lacquer crack and blister.

In the desert, relays, all types of switching equipment,and gasoline engines are susceptible to damage by sand

and dust. Sand and dust hazards present severe problemsto finely machined or lubricated moving parts of light andheavy equipment. Sand and dust get into almost everynook and cranny and in engines, instruments, and arma-ment. Desert dust becomes airborne with only slight agi-tation and can remain suspended for hours so that per-sonnel have difficulty seeing and breathing. The mostinjurious effects of sand and dust result from their adher-ence to lubricated surfaces, but glass or plastic windowsand goggles can be etched by sand particles driven by highwinds.

10-3.2.1.3 Arctic RegionsIn arctic regions the mean temperature for the warmest

summer month is below 10°C (50°F), and for the coldestmonth, it is below -32°C (-25°F). The extremely lowtemperatures of these regions change the physical proper-ties of materials. Blowing snow, snow and ice loads, icefog, and windchill cause additional problems.

Problems associated with the operation and mainte-nance of equipment seem to be more numerous in arcticregions than elsewhere and are caused mainly by driftingsnow and extremely low temperatures. The temperatureextremes to which electronic equipment may be exposedare shown in Table 10-7.

With the exception of inhabited areas, vehicle trans-portation is uncertain and hazardous because of theabsence of roads. Travel from base to base is over rugged.snow-and-ice or tundra-covered terrain. Drifting snowcan enter a piece of equipment and either impede its

TABLE 10-7. ARCTIC TEMPERATUREEXTREMES FOR ELECTRONIC

EQUIPMENT

Conditions Temperature

Low temperature, driv-ing snow, ice dust

Exposed arctic:-70°C (-94°F), extreme-40°C (-40°F), common

Subarctic:-25°C (-13°F), common

TABLE 10-6. TEMPERATURE EXTREMES FOR ELECTRONIC EQUIPMENTOPERATING IN A DESERT ENVIRONMENT

Conditions

Dry heat, intense sunlight,sand dust, destructive insects

Temperature

Day high:+60°C (+140°F), air+75°C (167°F), exposed ground

Night low: Relative humidity-l0°C (+14°F) 5%

Large daily variation:22°C (72°F), average

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operation or melt and then refreeze inside. Then, whenthe unit generates heat, the melted ice can cause shortcircuits, form rust, or rot organic materials.

The subzero temperatures may produce the effects thatfollow:

1. Volatility of fuels is reduced.2. Waxes and protective compounds stiffen and

crack.3. Rubber, rubber compounds, plastics, and metals

lose their flexibility, become hard and brittle, and are lessresistant to shock.At a temperature of -34°C (-30°F) batteries are reduced incurrent capacity by 90% and will not take an adequatecharge until warmed to 2°C (35°F). The variations in thecapacitance, inductance, and resistance of electrical com-ponents and parts can become great enough to requirereadjustment of critical circuits.

10-3.2.1.4 Vibration (Ref. 6)Vibration in the environment can degrade materiel in

several ways, i.e.,1. Malfunction of sensitive, electric, electronic, and

mechanical devices2. Mechanical and or structural damage to struc-

tures both stationary and mobile3. Excessive wear in rotating parts4. Frothing or sloshing of fluids in containers.

Table 10-8 (Ref. 6) indicates the effects of vibration onelectrical and electronic equipment.

When vibration becomes severe enough to cause mal-function or failure, measures must be taken to permitmateriel to survive in such an environment. The processof reducing the effects of the vibration environment isknown as vibration control and consists of varying struc-tural properties such as inertial, stiffness, and dampingproperties of mechanical systems to attenuate theamount of vibration transmitted to the materiel or toreduce the effects of the transmitted vibration.

A variety of techniques can attenuate vibration.Obviously, a very effective method is to remove the vibra-tion at its source. Damping, i.e., a process of producing acontinuing decrease in the amplitude of the vibration,also may be employed, This is accomplished by employ-ing frictional losses that dissipate the energy of the sys-tem, i.e., an energy-absorbing mechanism. Detuning ordecoupling a member from a resonant frequent) can alsobe used.

10-3.2.1.5 Shock (Ref. 6)Equipment subjected to shock loads responds in a

complex manner. The shock load can overstress anddeform the basic equipment structure or damage fragilecomponents attached to the structure. Both responsesexist together, and their relative intensities are a functionof the shape, duration, and intensity of the shock pulse;the geometrical configuration; total mass; internal massdistribution; stiffness distribution; and damping of theitem or equipment. The effects of shock include breakageof brittle or fragile components, displacement of massive

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components, and change in geometrical relationshipamong components.

To protect against shock, it is necessary to1. Isolate the equipment from the shock forces

through proper packaging and stowing techniques2. Design equipment in a way that will make it

unsusceptible to the shock environment.Though not a shock in the classical sense, damage that

can be introduced by electrostatic discharge during nor-mal handling of modern electronic devices mandates thatthe potential for damage be controlled. By proper packag-ing and methods of discharging static electricity fromworkers and tools, the sensitive components handled dur-ing manufacture, test, and repair can be protected (Ref.17).

Fundamentals of package design, barrier, cushioning,and container material are discussed in Refs. 18, 19, and20.

10-3.2.1.6 Acceleration (Ref. 6)Most items of materiel are designed to operate within a

narrow band of accelerating forces centered on the nor-mal gravitational force of 1G. When accelerations differappreciably from 1G, items fail to operate properly. Theeffects of large accelerations on equipment include struc-tural and mechanical failures, abnormal operation ofelectron tubes, characteristic changes in vibration isola-tors, and malfunctions due to deformation of parts. Typi-cal effects of acceleration on various types of equipmentand components are listed in Table 10-9 (Ref. 6).

The primary means of protecting materiel againstdamage from the acceleration environment is throughproper packaging. Other techniques include the use ofshock mounts; the selection and use of the correct types ofmaterials in terms of weight, strength, and flexibility; andproper structural mounting of component parts.

10-3 .2.1.7 Nuclear RadiationThe effects of nuclear radiation are derived from the

amount of energy deposited within a material by theradiation and the form that the energy depositionassumes. Thus if a material absorbs little radiation, it maybe unaffected in many applications. If the energy deposi-tion is large, however, a material may lose its structuralintegrity. Most effects fall inbetween these two extremes.Solid-state electronic devices, for example, are extremelysensitive to nuclear radiation effects because their opera-tion is very sensitive to the structure of the material.Absorbed radiation that ionizes an atom or displaces anatom in a semiconductor will affect the operation of adevice that uses the semiconductor.

Damage to materials is classified into two categories,i.e., transient radiation damage and permanent radiationdamage. This categorization is derived from the fact thatmany incidents of nuclear radiation in the environmentare transient and produce transient effects in material.The magnitude of such effects decays with time, and theperformance of materiel exposed to a transient radiationevent often will return to its initial state. Transient effectsusually are associated with low radiation doses. Rela-

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TABLE 10-8. VIBRATION-INDUCED DAMAGE TO ELECTRICALAND ELECTRONIC EQUIPMENT (Ref. 6)

Component Category

Cabinet and framestructures

Chassis

Cathode-ray tubes

Meters and indicators

Relays

Wiring

Transformers

Damage Observations

Among some 200 equipment cabinet and frame structures subjected to shock andvibration, damage included 30 permanent deformations, 17 fractures in areas ofstress concentration, two fractures at no apparent stress concentrations, 23 fracturesin or near welds, and 26 miscellaneous undefined failures.

Nearly 300 chassis subjected to shock and vibration experienced 18 permanentdeformations, eight fractures in or near welds, nine fractures at no apparent stressconcentrations, 46 fractures at points of stress concentration, and 12 miscellaneousfailures.

Cathode-ray (CR) tubes are susceptible to vibration damage if they are improperlymounted and supported. CR tubes with screens larger than 5 in. are particularlysusceptible. Of 31 cathode-ray tubes subjected to shock and vibration, thedeflection plates of one tube became deformed, another had a filament failure, fivesuffered envelope fractures, and one had a glass-socket seal break.

Although the moving coil type of meter comprises the majority of units in thiscategory, other indicators such as Bourdon tubes and drive-type synchros were alsotested. Of the latter group, most of the failures were either erratic performance orzero shift difficulties. Nearly 200 units were subjected to shock and vibration. Twosuffered permanent deformation of the case, one had elements loosened, 12 gaveerratic readings, one had the glass face fractured, two developed internal opencircuits, two had loose or damaged pivots, three had deformed pointers, and 10others failed from miscellaneous causes.

Relays present a particularly difficult problem for dynamic conditions because of thedifficulty in balancing all of the mechanical moments. Shock generally causesfailure in the form of the armature failing to hold during the shock. A total of 300relays were subjected to shock and vibration. Armature difficulties accounted for 29defects, four relays had contacts fuse or burn because of arcing, one had the coilloosened on the pivots, two had the springs disengaged from the armature, and foursustained miscellaneous defects.

Wiring failure from shock and vibration is a serious problem. A defect not onlyresults in malfunctioning of the equipment, but presents a difficult troubleshootingjob in locating the wire break. In a number of equipments subjected to shock andvibration, the failures were as follows: 10 cold solder joints opened, 14 lead-supported components had the leads fail, insufficient clearance caused three casesor arcing, and insufficient slack caused nine lead failures. In addition. three plasticcable clamps fractured, 14 solder joints or connections failed, 16 solid conductorwires broke, and 92 sustained miscellaneous failures.

In electronic equipments transformers are probably the heaviest and densestcomponents found on an electronic chassis. Because of the weight and size oftransformers, shock and vibration are more likely to produce mechanical ratherthan electrical failures. Although not all mechanical failures immediately preventthe transformer from functioning properly, they eventually result in destruction ofthe transformer and damage to surrounding components. Of 80 transformerssubjected to shock and vibration, 17 had the mounting stubs break at the weld, fourhad the bottom frame fail, and two suffered broken internal leads due to motion ofthe coil in the case.

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TABLE 10-9. EFFECT OF ACCELERATION ON MILITARY EQUIPMENT (Ref. 6)

Item Effect

Mechanical: moving parts, Pins may bend or shear; pins and reeds deflect; shock mounts may break awaystructures, fasteners from mounting base; mating surfaces and finishes may be scored.

Electronic and electrical Filament windings may break; items may break away if mounted only by theirleads; normally closed pressure contacts may open; normally open pressurecontacts may close; closely spaced parts may short.

Electromagnetic Rotating or sliding devices may be displaced; hinged part may temporarily engageor disengage; windings and cores may be displaced.

Thermally active Heater wires may break; bimetallic strips can bend; calibration may change.

Finishes Cracks and blisters may occur.

Materials Under load, materials may bend, shear, or splinter; glue lines can separate; weldscan break.

tively larger amounts of deposited energy, however, pro-duce permanent and accumulative damage.

The design of material that will be exposed to a nuclearradiation environment can be extremely complex. Ofprime consideration is the radiation level for which theitem is being designed. For example, in the design ofsemiconductor devices for nuclear radiation environ-ments, the use of gold–-which has a large absorptioncross section—is sometimes avoided. For items that aresensitive to radiation, the use of lead shielding is em-ployed. In electronic circuits, for example, knowledge ofthe radiation sensitivity of particular circuit elements isemployed in the design to provide for continued opera-tion of the circuit even when the properties of the sensitiveelements may vary within wide limits.

In addition to blast. fire, and radiation resulting from anuclear explosion, a large electrical charge is transportedin a short period of time. This produces a large transientpulse of electromagnetic energy known as electromag-netic pulse (EMP). The EMP can be considered to be verysimilar to the common phenomenon of a lightning stroke.The magnitude and extent of EMP far exceeds electro-magnetic fields created by any other means; its duration isless than 1 ms. The electromagnetic signal from the EMPconsists of a continuous spectrum with most of the energycentered about a median frequency of 10 to 15 kHz (Ref.6).

10-3 .2.1.8 Electromagnetic Radiation Effects(Ref. 6)

The following is excerpted from Ref. 6:“For electronic equipment operating within the com-

munications and microwave bands, environmental elec-tromagnetic fields can be harmful in three basic ways: (1)interference, (2) overheating, and (3) electric breakdown.First, the presence of extraneous electromagnetic fieldscan produce interference, particularly in communicationchannels, but also in other electronic equipment such asnavigation, radar, and command and control units. Thisinterference to a system can be caused by (1) other systems

operating in frequency ranges that interfere with theoperation of the desired equipment, and (2) undesiredsignals generated by the system itself. Good design prac-tice and proper siting of equipment are usually sufficientto eliminate problems encountered in the second category.

“Electromagnetic interference is classified in a numberof ways but, for measurement purposes, it is usually clas-sified according to its spectral characteristics. The twogeneral classifications are broadband interference, inwhich a wide range of frequencies are involved, and nar-rowband interference, which is centered about a discretefrequency. In addition, the interference is classified withrespect to its duration. That which is constant withoutinterruption is called continuous wave or CW interfer-ence. Interference that is periodic and occurs in burstswith a regular period is called pulse interference. Pulseinterference can be either narrowband or broadbanddepending upon the pulse duration. In addition, nonre-petitive short duration bursts of broadband noise arecalled transient interference. Lightning, for example, is atypical example of transient interference. Electromag-netic interference can be coupled into equipment either bydirect radiation or by conduction on power lines or struc-tures. [For a thorough discussion of electromagneticinterference and compatibility, see Ref. 21.]

“Through proper frequency management, many inter-ference problems can be reduced. Unfortunately, theproblem is complicated because the electromagneticenvironment contains not only the desired electromag-netic radiation, but also spurious and undesired interfer-ence from both natural and man-made sources. As thenumber, complexity, and output power of electronic sys-tems in use grow, the problem of the electromagneticenvironment and equipment compatibility becomes moreserious. For example, within the military, the density ofelectronic equipment in the field has grown to the pointthat hundreds of equipments now occupy the same opera-tional environment as did a few equipments in World War11. It is noted that, in discussing electromagnetic interfer-ence, the fields usually spoken of are not high enough to

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cause permanent damage to the system or equipmentunder consideration.

“When electromagnetic fields become very large, per-manent damage can occur to operating equipment. Forexample, if the electrical field becomes sufficiently high,electronic breakdown can occur, destroying the equip-ment. On the other hand, at some intermediate values offield strength, overheating can occur in which the RF fieldinduces currents that contribute to the heat load alreadypresent as a consequence of operation of the equipment.This overheating can lead to failure of components andmalfunction of the system.

“In addition to the effects of electromagnetic radiationon equipment, another consideration involves the effectsof the electromagnetic environment on man and theextent that this must be considered in the design of elec-

tronic materiel. Basically, the effects produced by elec-tromagnetic fields on man are classified into thermal andnonthermal. Some portions of a man’s physiology areparticularly susceptible to certain frequencies of electro-magnetic energy. One of the prime areas of environmentalconcern involves the effect of microwaves on humanbeings. ”

10-3.2.2 Summary of Environmental EffectsThe environmental conditions under which unshel-

tered equipment should be designed are given in Table10-10. A summary of the major environmental effects isgiven in Table 10-11 (Ref. 6), and the failure modes ofelectronic components due to some of these environmen-tal factors are presented in Table 10-12 (Ref. 22).

TABLE 10-10. ENVIRONMENTAL REQUIREMENTS FOR UNSHELTERED EQUIPMENT

Environment

Temperature

Standard Area

OperatingNonoperating

Cold Weather Area

OperatingOperatingNonoperating

Desert and Tropical Areas

OperatingNonoperating

Humidity

OperatingNonoperating

Solar Radiation

Wind

Barometer Pressure

Operating

Nonoperating

Environmental Limits

-29 to 52°C (-20 to-54 to 54° C (-65 to

25°F)30°F)

-40°C (-40°F) if operator is unsheltered-54°C (-65°F) if operator is sheltered-62°C (-80°F) for 3 days and achieve rated capacity after 30

min preheating and warm-up

52°C (125°F)71°C (160°F) for 4 h per day indefinitely

Up to 100% at 38°C (100°F) including condensationUp to 100% including condensation

Endure a solar intensity of 4X106 J/m 2 (360 Btu/ft2 for aperiod of 4 h at 52°C (125°F)

Withstand wind pressures up to 1435 Pa (30 lb/ft2) ofprojected surface, either empty or under load

From 101 to 57 kPa (30 to 16.8 in. of mercury)4572 m (0-15,000 ft)

From 101 to 19 kPa (30 to 5.54 in. of mercury)12,190 m (0-40,000 ft)

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TABLE 10-11. SUMMARY OF MAJOR ENVIRONMENTAL EFFECTS (Ref. 6)

(cont’d on next page)

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TABLE 10-11 (cont’d)

EnvironmentalFactor

Wind

Rain

Water immersion

Insects and bacteria

Fungi

Temperature shock

High-speed particles(nuclear irradiation)

Ozone

Explosive decompression

Dissociated gases

Acceleration

10-20

Principal Effects

Force application

Deposition of materials

Heat loss (low velocity)Heat gain (high velocity)

Physical stressWater absorption and immersion

Erosion

Corrosion

Corrosion of metals

Chemical deteriorationHigh pressure

(13 psi at 30-ft depth)

Penetration into equipmentNibbling by termites

Growth of molds, hyphae

Mechanical stress

Heating

Transmutation and ionization

Chemical reactions:Crazing, cracking

EmbrittlementGranulation

Reduced dielectric strength of air

Severe mechanical stress

Chemical reactions:

ContaminationReduced dielectric strength

Mechanical stress

Typical Failures Induced

Structural collapseInterference with functionLoss of mechanical strengthMechanical interference and cloggingAbrasion acceleratedAccelerated low-temperature effectsAccelerated high-temperature effects

Structural collapseIncrease in weightStructural weakeningAccelerates coolingElectrical failureRemoves protective coatingsStructural weakeningSurface deterioriationEnhances chemical reactions

Structural weakness, seizure of parts,contamination of products

Dissolving out and changing of materialsMechanical damage

Blockage of small parts, meters, etc.Damage to plastic cables or other organic

insulating materials, causing shorts

Damage to optical equipment; leakage pathsin high impedance circuits; blockage ofsmall parts, meters, etc.; breakdown ofmechanical strength of all organic materials

Structural collapse or weakeningSeal damage

Thermal agingOxidationAlteration of chemical, physical, and

electrical propertiesProduction of gases and secondary particles

Rapid oxidationAlteration of chemical, physical, and

electrical propertiesLoss o f mechan ica l s t r eng thInterference with functionInsulation breakdown and arc-over

Rupture and crackingStructural collapse

Alteration of physical and electricalproperties

Insulation breakdown and arc-over

Structural collapse

(cont’d on next page)

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TABLE 10-11 (cont’d)

EnvironmentalFactor Principal Effects

Vibration Mechanical stress

Magnetic fields Induced magnetization

Typical Failures Induced

Loss of mechanical strengthInterference with functionIncreased wearStructural collapse

Interference with functionAlteration of electrical propertiesInduced heating

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TABLE 10-12.

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1.

2.

3.

4.

5.

6.

7.

8.

9.10.

11.

REFERENCESMIL-STD-1165, Glossary of Environmental Terms,1968.AMCP 706-118, Engineering Design Handbook,Environmental Series, Part Four, Life Cycle Eniron-ments, 1975.J. Sheldon, “Nondevelopment Item Acquisition”,Army Research, Development and AcquisitionMagazine, p. 9, March-April 1985.AMCP 706-115, Engineering Design Handbook,Environmental Series, Part One, Basic Environmen-tal Concepts, 1974.AMCP 706-116, Engineering Design Handbook,Environmental Series, Part Two, Natural Environ-mental Factors, 1975.AMCP 706-117, Engineering Design Handbook,Environmental Series, Part Three, Induced Environ-mental Factors, 1976.MIL-HDBK-759, Human Factors EngineeringDesign for Army Materiel, 1981.HEL STD S-1-63C, Materiel Design Standard forNoise Levels of Army Materiel Command Equip-ment, Human Engineering Laboratory, AberdeenProving Ground, MD, 1972.TB MED 501, Hearing Conservation, March 1980.TM 11, “Report on Preliminary observations ofHuman Engineering Problems Under Desert Condi-tions”, Human Engineering Laboratory, AberdeenProving Ground, MD.TB MED 507, Occupational and EnvironmentalHealth, Prevention, Treatment, and Control of HeatInjury, 25 July 1980.

10-26

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

IS0 DIS 2631, Guide to the Evaluation of HumanExposure to Whole- Body Vibration.MIL-HDB-721, Corrosion and Corrosion Protec-tion of Metals, 1 1965.MIL-E-5400, Electronic Equipment, Airborne,General Specificatios for, 1980.MIL-E-16400, Electronic, Interior Communicationand Navigation Equipment, Naval Ship and Shore,General Specifications for, 1976.TM 20, “Visual Efficiency Under Desert Condi-tions”, Human Engineering Laboratory, AberdeenProving Ground. MD.IDA Record Document D-21, F-16 APG-66 FireControl Radar Case Study Report, p. 111-30, August1983.MIL-HDBK-772, Military Packaging Engineering,30 March 1981.DARCOM-P 706-298, Engineering Design Hand-book, Rocket and Missile Container EngineeringGuide, January 1982.TM 38-230-1, Packaging of Materiel: Preservation(Vol. 1), 1 August 1982.DARCOM-P 706-410. Engineering Design Hand-book, Electromagnetic Compatibility (EMC), March1977.Space, Aeronautics. 36, No. 6, December 1961.

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GLOSSARY

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A

Accessibility. A design feature that affects the ease ofadmission to an area for the performance of visual andmanipulative maintenance.

Active maintenance time. The time during which preven-tive and/ or corrective maintenance work is being doneon the item.

Active repair time. The time during which one or moretechnicians are working on the item to effect a repair.

Active technician time. That time (expressed in man-hours) expended by the technician(s) in active perfor-mance of a maintenance task.

Adjustment and calibration time. That element of activemaintenance time required to make the adjustmentsand/or calibrations necessary to place the item in aspecified condition.

Administrative time. The downtime due to nonavailabil-ity of test equipment or maintenance facilities and thetime due to nonavailability of maintenance technicianscaused by administrative functions. It is that portion ofnonactive maintenance time that is not included inlogistic time.

Alignment. Performing the adjustments that are neces-sary to return an item to a specified level of operation.

Artificial intelligence (AI). A field aimed at pursuing thepossibility that a computer can be made to behave in amanner that humans recognize as intelligent behaviorin each other.

Automatic test equipment (ATE). Equipment designed toconduct automatically the analysis of functional orstatic parameters and to evaluate the degree of theperformance degradation of the unit under test. Thetest equipment is not an integral part of the unit undertest.

Automatic testing. The process by which the localizationof faults, possible prediction of failure, or validationthat the equipment is operating satisfactorily is deter-mined by a device that is programmed to perform aseries of self-sequencing test measurements without thenecessity of human direction after its operations havebeen initiated.

Availabifity. A measure of the degree to which an item isin operable and committable state at the start of themission, when the mission is called for at an unknown(random) point in time.

AvailabiIity (achieved). The percentage of time the systemis operating when considering only operating time andtotal maintenance (scheduled and unscheduled) time.The equation is

whereO T C

TCM =

TPM =

A a =

operating time during a given calen-dar time periodtotal corrective (unscheduled) main-tenance downtime during a givencalendar time periodtotal preventive (scheduled) mainte-nance downtime during a given calen-dar time periodachieved availability.

Availability (inherent). The percentage of time the systemis operating when considering only operational timeand unscheduled (corrective) maintenance time. Theequation is

whereOT = operating time during a given calen-

dar time periodTCM = total corrective (unscheduled) main-

tenance downtime during a givencalendar time period

A, = inherent availability.

Availability (operational). A measure of the degree towhich an item is either operating or is capable of operat-ing at any random point in time when used in a typicalmaintenance and supply environment. The equation is

whereOT = operating time during a given calen-

dar time period

G-1

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ST =

TCM =

TPM =

TALDT =

A o =

standby time (not operating, butassumed operable) during a givencalendar time periodtotal corrective (unscheduled) main-tenance downtime during a givencalendar time periodtotal preventive (scheduled) mainte-nance downtime during a given calen-dar time periodtotal administrative and logistic down-time spent waiting for parts, mainte-nance personnel, or transportationduring a given calendar time periodoperational availability.

B

Best operating capability (BOC). The upper level main-tainability value estimated to be technically feasiblewithin the stated time frame and within reasonable costconstraints.

Built-in test (BIT). An integral capability of the missionequipment that provides an on-board, automated testcapability to detect, diagnose, or isolate system failures.The fault detection and, possibly, isolation capabilityare used for periodic or continuous monitoring of theoperating condition of a system, and for observationand diagnosis as a prelude to maintenance. BITE maybe automatically or manually triggered.

Built-in test equipment (BITE). Any device which is partof an equipment or system and is used for the expresspurpose of testing the equipment or system. It is anidentifiable unit of the equipment or system. BITE maybe automatically or manually triggered.

C

Calendar time. The total number of calendar days orhours in a designated period of observation.

Calibration. Those measurement services provided bydesignated depot and/or laboratory facility teams, whoby the comparison of two instruments one Of which isa certified standard of known accuracy detect andadjust any discrepancy in the accuracy of the instru-ment being compared with the certified standard.

Cannibalization. The removal of serviceable items from apiece of equipment to repair another.

Capability. A measure of the ability of an item to achievemission objectives given that the item performs as spec-ified throughout the mission.

Catastrophic failure. A sudden change in the operating

characteristics of some part or parameter resulting in acomplete failure of the item, e.g., circuit opens orshorts, structural failure. etc.

Chargeable. Within the responsibility of a given organiza-tional entity (such as Failures, Maintenance time. etc.)

Checkout. A man machine task to determine that theequipment is operating satisfactorily and is ready forreturn to service.

Checkout time. The time required to check out theequipment after a maintenance action or otherwise toverify that a system or equipment is in satisfactoryoperating condition.

Circuit malfunction analysis. The logical, systematicexamination of circuits and their diagrams to identifyand analyze the probability and consequence of poten-tial malfunctions for determining related maintenanceor maintainability design requirements.

Conceptual phase. The first phase in the material lifecycle. The phase in which the technical, military, andeconomic basis for the program. and concept feasibilityare established through pertinent studies.

Configuration control. The systematic evaluation. coor-dination, approval or disapproval, and implementationof all approved changes in the configuration of a con-figuration item after formal establishment of its config-uration identification.

Configuration item. An aggregation of hardware soft-ware. or any of its discrete portions, which satisfies anend use function and is designated by the Governmentfor configuration management. Configuration itemsmay vary widely in complexity, size, and type, from anaircraft, an electronic or ship system to a test meter orround of ammunition. During development and initialproduction. configuration items are only those specifi-cation items that are referenced directly in a contract(or an equivalent in-house agreement). During thedeployment phase, any repairable item designated forseparate procurement is a configuration item.

Contract data requirements list (CDRL). A listing of alltechnical data and information required to be deliveredto the Government by the contractor.

Corrective maintenance. That maintenance performed torestore an item to a satisfactory condition by providingcorrection of a malfunction that has caused degrada-tion of the item below the specified performance.

Corrective maintenance time. The time that begins withthe observance of a malfunction of an item and endswhen the item is restored to a satisfactory operatingcondition. It may be subdivided into active mainte-nance time and nonactive maintenance time. It does not

G-2

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necessarily contribute to equipment or system down-time in cases of alternate modes of operation orredundancy.

Criticality. A measure of the impact of the failure mode ofthe item that includes the severity of the effect com-bined with the frequency or probability of occurrence.

D

Delay time. The component of downtime during whichno maintenance is being accomplished on the itembecause of technician alert and response time, supplydelay, or administrative reasons.

Demonstrated. That which has been measured, withinspecified confidence limits, by the use of objective evi-dence gathered under specified conditions.

Dependability. A measure of the degree to which an itemis operable and capable of performing its required func-tion at any (random) time during a specified missionprofile, given item Availability at the start of the mis-sion. (Item state during a mission includes the com-bined effects of the mission-related system R & Mparameters but excludes nonmission time: see AVAIL-ABILITY.)

Depot maintenance. A category of maintenance organ-ized to support the supply system. It will be production-line oriented and will be performed by special repairactivities, US Army Materiel Command (AMC) depots,and contractor personnel.

Design adequacy. The probability that a system orequipment will successfully accomplish its mission,given that the system is operating within design specifi-cations.

Design review. A multipurpose design verification proce-dure and project management tool used to evaluate thecumulative results of all constituent design verificationcycles at each of several designated major milestones inthe acquisition process in order to provide adequateengineering basis for timely iteration in the total systemengineering cycle.

Development model. A model designed to meet perfor-mance requirements of the specification or to establishtechnical requirements for production equipment. Thismodel need not have the required final form or neces-sarily contain parts of final design. It may be used todemonstrate the reproducibility of the equipment.

Development testing. A series of materiel tests conducted

DOD-HDBK-791(AM)

by the Army developer—with or without contractorassistance—to assess program technical risks, demon-strate that engineering design is complete and accept-able, determine the extent of the design risks, determinespecification compliance, and assess production require-ments.

Diagnostics. The functions performed and the techniquesused in determining and isolating the cause of malfunc-tions in an operating system or determining its opera-tional status.

Direct maintenance man-hours per maintenance action(DMMH/MA). A measure of the maintainabilityparameter related to item demand for maintenancemanpower. The sum of direct maintenance man-hoursat all levels or repair, divided by the total number ofmaintenance actions (preventive and corrective) duringa stated period of time.

Direct maintenance resources. The time in man-hoursand material in dollars expended directly on the itembeing maintained during the period of active mainte-nance.

Disassemble. Opening an item and removing a number ofparts of subassemblies to make the item that is to bereplaced accessible for removal. This does not includethe actual removal of the item to be replaced.

Discard-at-failure maintenance. Maintenance accom-plished by replacing and discarding a failed assembly,subassembly, module, or piece part. The term normallyis associated with modules.

Downtime. That portion of calendar time when the itemcannot perform its intended function.

E

Ease of maintenance. The degree of facility with whichequipment can be retained in, or restored to, operation.It is a function of the rapidity with which maintenanceoperations can be performed to avert malfunctions orcorrect them if they occur. Ease of maintenance isenhanced by any consideration that will reduce the timeand effort necessary to maintain equipment at peakoperating efficiency.

Engineering change proposal (ECP). A proposal tochange the design or engineering features of materielundergoing development or production.

Environment. The aggregate of all external and internalconditions—such as temperature, humidity, radiation,magnetic and electric fields, shock, and vibration—either natural or man-made or self-induced that influ-

G-3

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ences the form, performance, reliability or survival ofan item.

Equipment repair time. The 50th percentile, i.e., themedian, of the distribution of repair time.

Expert system. An intelligent computer system that usesknowledge and inference procedures to solve problemsthat are difficult enough to require significant humanexpertise in their solution.

F

Failure. A detected cessation of ability to perform a speci-fied function or functions, within previously estab-lished limits, in the area of interest. It is a malfunctionthat is beyond adjustment by the operator by means ofcontrols normally accessible to him during the routineoperation of the device.

Failure analysis. The logical, systematic examination ofan item to identify and diagnose the cause of observedfailures.

Failure effect. The condition(s) introduced by a failurethat reduce or modify the ability of an item to performits required function.

Failure mode, effects, and criticality analysis (FMECA).A general evaluation procedure that

1. Documents all possible potential failures in an itemdesign

2. Determines by analysis the effect of each potentialfailure on item operation and on each reliability andmaintainability parameter applicable to the type ofitem

3. Ranks potential failures according to their impact(see CRITICALITY) on each applicable systemreliability and maintainability parameter.

Failure rate. The number of failures of an item per unitmeasure of use—cycles, time, miles, events, etc., asapplicable. It is also called renewal rate.

Fault correction time. That element of active repair timerequired under a specified maintenance philosophy tocorrect the malfunction. It may consist of correcting themalfunction with the faulty item in place, removing andreplacing the item with a like serviceable item, or re-moving the item for corrective maintenance and rein-stalling the same item.

Fault detection time. Time between the occurrence of afailure and the point at which it is recognized that thesystem or equipment does not respond to operationaldemand during the mission sequence.

required for testing and analyzing an item to isolate amalfunction.

Field maintenance. Maintenance authorized and per-formed by designated maintenance activities in supportof using organizations.

Final test time. That element of active repair timerequired after completion of maintenance, adjustments,and calibration to verify by measurement of perfor-mance that the item is in a condition to perform itsfunction satisfactorily.

Free time. Time during which operational use of a systemor equipment is not required; this time may or may notbe downtime, depending on whether or not the systemis in operable condition.

Frequency-of-use-principle (equipment design). Theprinciple of positioning the most frequently maintaineditems in preferred locations to facilitate maintenance.

Full-scale development phase. The third phase in themateriel acquisition process. During this phase, asystem –including all items necessary for its support—is fully developed and engineered, fabricated, tested,and initially type classified. Concurrently, nonmaterialaspects required to field an integrated system arerefined and finalized.

Function analysis for maintainability. The analyticalbasis for allocating tasks to personnel and equipmentso as to achieve optimum system maintainability.

Functional interchangeability. A condition in which apart or unit, regardless of its physical specifications.can perform the specific functions of another part orunit.

Functional principle (equipment design). The principle ofarrangement that provides for the grouping of hard-ware items according to their functions.

G

General-purpose test equipment. A category of testequipment, normally available in the supply system orfrom commercial stock, that can be used to test morethan one system or equipment type.

Geometric mean-time-to-repair (MTTRG). A measure ofcentral tendency for repair time. Generally used withthe lognormal distribution.

Go/no-go display. A display that indicates the operableor nonoperable condition of equipment.

Fault location time. That element of active repair time

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H

Human engineering. The area of human factors that ap-plies scientific knowledge to the design of items toachieve effective operation, maintenance, and man/machine integration.

Human factors. Human characteristics relative to com-plex systems and the development and application ofprinciples and procedures for accomplishing optimumman/machine integration and utilization. The term isused in a broad sense to cover all biomedical and psy-chosocial considerations pertaining to man in thesystem.

I

Inactive time. The period of time when the item is avail-able, but it is neither needed nor operated for itsintended use.

Indirect maintenance resources. That time in man-hoursand material in dollars which. although not directlyexpended in active maintenance tasks, contributes tothe overall maintenance mission through the support ofoverhead operations, administration, accumulation offacility records and statistics, supervision, and facilitiesupkeep.

Inherent value. A measure of maintainability whichincludes only the effects of item design and applicationand assumes an ideal operation and support environ-ment.

In-process review (IPR) (nonmajor hardware systems). Areview conducted at critical points in the acquisitionprocess to evaluate military utility, accomplish effectivecoordination, and facilitate proper and timely decisions.

Interchangeability. A condition when two or more partsare physically and functionally interchangeable in allpossible applications, i.e., when both parts are capableof full, mutual substitution in all directions.

Intermediate maintenance. A category of maintenanceorganized as forward and rear. Forward maintenance ischaracterized by high mobility and repair by replace-ment, Division maintenance units will support ma-neuver elements, and nondivisional units will providearea and backup support to the division. Rear interme-diate maintenance is characterized by semifixed facili-ties. Its fundamental purpose is to support the theatersupply system through repair of components and directexchange of items.

Item. A nonspecific term used to denote any product,including systems, materials, parts, subassemblies, sets,accessories. etc.

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Item obtainment time. The time required for the techni-cian to obtain replacement parts, assemblies, or units,depending on the maintenance concept and the loca-tion and method of storing the supply items.

L

Life cycle costs. The sum of the funds expended duringthe life cycle of materiel for development, test, pro-curement, operation, support, and disposal.

Line-replaceable unit (LRU). An item whose removal andreplacement with a like serviceable item is consideredthe optimum corrective method for a specific higherindenture level item.

Logistic resources. The support personnel and materielrequired by an item to assure its mission performance.It includes such things as tools, test equipment, repairparts, facilities, technical manuals, and administrativeand supply procedures necessary to assure the availabil-ity of these resources when needed.

Logistic support. Maintenance and supply support to beprovided at unit and intermediate and depot levels.Logistic support is influenced by the degree of unitiza-tion or modularization, ruggedness, cost, test points,test equipment, tactical employment, and transporta-tion requirements.

Logistic time. The portion of downtime attributable todelay in the acquisition of replacement parts.

Logistics. Those aspects of military operations which dealwith (a) design and development, acquisition, storage,movement, distribution, maintenance, evacuation, anddisposal of materiel, (b) movement, evacuation, andhospitalization of personnel, (c) acquisition or con-struction, maintenance. operation and disposition offacilities, and (d) acquisition or furnishing of services.

M

Maintainability. A measure of the ease and rapidity withwhich a system or equipment can be restored to opera-tional status following a failure or retained in a speci-fied condition. It is characteristic of equipment designand installation, personnel availability in the requiredskill levels, adequacy of maintenance procedures andtest equipment, and the physical environment underwhich maintenance is performed. One expression ofmaintainability is the probability that an item will beretained in or restored to a specified condition within agiven period of time when the maintenance is per-formed in accordance with prescribed procedures andresources.

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Maintainability analysis. The definition of maintainabil-ity design procedures and evaluation of achievement ofmaintainability design goals through use of prediction,verification, demonstration and assessment techniquesto insure that system or equipment design characteris-tics meet operational objectives with a minimum ex-penditure of maintenance and support effort.

Maintainability data. Data (other than administrativedata) resulting from performance of maintainabilitytasks in direct support of an equipment or systemacquisition program.

Maintainability demonstration tests. Tests, usually at theequipment or subsystem level, for the major items thatwill comprise the integrated system to demonstrateconformance with specified quantitative maintainabil-ity requirements.

Maintainability constraints. A group of factors—environ-mental, human, hardware—which establishes limits tothe performance of maintenance on an item.

Maintainability program. The planning, development,and implementation of those organized sets of tasksdirectly related to the specification, prediction, verifica-tion and assessment of the design characteristics of anitem with the goal of meeting operational objectiveswith a minimum expenditure of maintenance and sup-port effort.

Maintainability requirement. A comprehensive statementof required maintenance characteristics, expressed inqualitative and quantitative terms, to be satisfied by thedesign of an item.

Maintenance. All actions necessary for retaining an itemin, or restoring it to, a serviceable condition. Mainte-nance includes servicing, repair, modification, over-haul, inspection, and condition determination.

Maintenance ability. A figure of merit for a crew of ausing organization defined as the ratio of maintenanceman-hours established on a specific item by a trainedand expert maintenance crew to the maintenance man-hour figure established by the crew of the using organi-zation on the same item and under similar maintenanceconditions.

Maintenance analysis. The process of identifying requiredmaintenance functions through analysis of a fixed orassumed design and determining the most effectivemeans of accomplishing these functions.

Maintenance category. Division of maintenance of mate-riel, based on difficulty and requisite technical skill, inwhich jobs are allocated to organizations in accordancewith the availability of personnel, tools, supplies, andtime within the organization.

Maintenance concept. A description of the general scheme

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for maintenance and support of an item in the opera-tional environment.

Maintenance element. A discrete portion of a mainte-nance task which can be described or measured.

Maintenance engineering. The application of techniques,engineering skills. and effort during the life cycle ofmateriel to insure the planning and implementation ofan effective maintenance program.

Maintenance error. An error on the part of maintenancepersonnel in performing maintenance on an item whichresults in subsequent failure or malfunction, or an errorin published maintenance procedures which results insubsequent failure or malfunction. (Note: Items re-moved because of maintenance errors are consideredUnscheduled Removals. )

Maintenance functions. Actions that must be accom-plished for a system or system element to return a failedsystem element to readiness (corrective maintenancefunctions) or to insure continuing normal system read-iness (preventive maintenance functions).

Maintenance level. One of several organizational entitiesto which materiel maintenance functions may beassigned. The maintenance levels are unit, interme-diate, and depot.

Maintenance man-hours. The number of active mainte-nance man-hours—total number of maintenance per-sonnel required multiplied by number of hours workedto perform a maintenance operation.

Maintenance plan. A part of the plan for logistic support.The maintenance plan contains conditions of materieluse, reliability and maintainability requirements, themaintenance concept, a definition of the using andsupport organizations, maintenance test and physicalteardown information, and a maintenance allocationchart.

Maintenance procedures. Established methods for peri-odic checking and servicing items to prevent failure orto effect a repair.

Maintenance proficiency. The ability of maintenance per-sonnel to apply job skills in the maintenance of an item.

Maintenance resources. Facilities; ground support equip-ment; test, measurement, and diagnostic equipment(TMDE); manpower; repair parts; consumables; andfunds available to maintain and support an item in itsoperational environment.

Manpower and personnel integration (MANPRINT). Aprogram whose purpose is to impose human factors,manpower, personnel, training, system safety, andhealth hazard considerations across the entire materielacquisition process.

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DOD-HDBK-791(AM)

Maintenance task. Any action or actions required to pre-clude the occurrence of a malfunction or to restore anequipment to satisfactory operating condition.

Malfuction. A general term used to denote the failure ofa product to give satisfactory performance. It need notconstitute a failure if readjustment of operator controlscan restore an acceptable operating condition.

Maximum time to repair (Mmaxct). The maximum timerequired to complete a specified percentage of all main-tenance actions.

Mean. A quantity representing the average of two ormore other quantities arrived at by adding the quanti-ties together and dividing by their number. Also called“arithmetic mean”. The “geometric mean” of two quan-tities is the square root of the product of the quantities.

Mean maintenance time. The mean time required tocomplete a maintenance action, i.e., total maintenancedowntime divided by the total maintenance actions.

Mean time between downtime events (MTBDE). A mea-sure of the system reliability parameter related to avail-ability and readiness. The total number of system lifeunits, divided by the total number of events in which thesystem becomes unavailable to initiate its mission(s),during a stated period of time.

Mean time between failures (MTBF). The totat function-ing life of a population of an item divided by the totalnumber of failures within the population during themeasurement interval. The definition holds for time,cycles, miles, events, or other measures of life units.

Mean-time-to-correct-failure. The expected value of thetime required to restore an equipment or system to acondition of satisfactory operation, measured from themoment it is judged unsatisfactory for normal use.

Mean task time. A representative task time equal to thesummation of task times required to perform a specifictask a number of different times divided by the numberof times performed.

Mean-time-to-repair (MTTR). The statistical mean of thedistribution of times-to-repair. The summation ofactive repair times during a given period of time dividedby the total number of malfunctions during the sametime interval.

Mean-time-to-restore-system (MTTRS). A measure ofthe system maintainability parameter related to avail-ability and readiness. The total corrective maintenancetime, divided by the total number of downing events,during a stated period of time. (Excludes time for off-system maintenance and repair of detached com-ponents.)

Median corrective maintenance time The down-time within which 50% of all corrective maintenanceactions can be completed under the specified mainte-nance conditions. (Also called Equiment Repair Time(ERT).)

Median preventive maintenance time The equip-ment downtime required to perform 50% of all sched-uled preventive maintenance actions on the equipmentunder the specified conditions.

Military standard. A document that establishes engineer-ing and technical requirements for items, equipments,processes, procedures, practices, and methods thathave been adopted as standard.

Military specification. A document intended primarilyfor use in procurement, which clearly and accuratelydescribes the essential technical requirements for items,materials, or services, including the procedures bywhich it will be determined that the requirements havebeen met. Specifications for items and materials mayalso contain preservation-packaging, packing, andmarking requirements.

Minimum acceptable value (MA V). A minimum valueconsistent with system operation and support concepts.

Mission reliability. The probability that, under statedconditions, a system or equipment will operate in themode for which it was designed, i.e., with no malfunc-tions, for the duration of a mission, given that it wasoperating in this mode at the beginning of the mission.

Mission time. The period of time in which an item mustperform a specified mission.

Model. Any device, technique, or process with which thespecific relationships of a set of quantifiable systemparameters may be investigated.

Modification. A major or minor change in the design ofan item of materiel, performed to correct a deficiency,to facilitate production, or to improve operationaleffectiveness.

Modularization. The design of equipment such that itsfunctional grouping, arrangement and size and partsand/or assemblies improve both the ability to test andease of maintenance.

Module. A part, subassembly, assembly, or componentdesigned to be handled as a single unit to facilitatesupply and installation, operations, and/ or mainte-nance.

N

National inventory control point (NICP). An organiza-tional segment, within the overall supply system of acommodity command, to which has been assigned

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DOD-HDBK-791(AM)

responsibility for integrated material inventory man-agement of a group of items.

O

Operating and support phase. The period in the systemlife cycle which starts with the delivery of the first itemof equipment to the using unit and terminates withdisposition of the system from the inventory.

Operational readiness. The probability that, at any pointin time, a system or equipment is either operating satis-factorily or ready to be placed in operation on demandwhen used under stated conditions, including statedallowable warning time. Thus, total calendar time is thebasis for computation of operational readiness.

Operational testing. A series of tests conducted by thedesignated user to determine operational effectiveness,suitability, and military desirability of materiel and theadequacy of the organization, doctrine, and tacticsproposed for use.

Operational value. A measure of maintainability whichincludes the combined effects of item design, quality,installation, environment, operation, maintenance, andand repair.

Operating time. The time during which a system orequipment is operating in a manner acceptable to theoperator, although unsatisfactory operation (or fail-ure) is sometimes the result of the judgment of themaintenance technician.

Overhaul To restore an item to a completely serviceablecondition as prescribed by maintenance serviceabilitystandards.

P

Periodic maintenance. Maintenance performed on equip-ment on the basis of hours of operation or calendartime elapsed since last inspection.

Physical interchangeability. A condition in which anytwo or more parts or units made to the same specifica-tion can be mounted, connected, and used effectively inthe same position in an assembly or system.

Plan for logistic support. A major section of the materielacquisition plan that deals with all aspects of materielsupport planning.

Preparation time. That element of active repair timerequired to obtain necessary test equipment and main-tenance manuals and to setup the necessary equipmentin preparation for fault location.

Preventive maintenance. That maintenance performed toretain an item in satisfactory operational condition byproviding systematic inspection. detection, and preven-tion of incipient failures.

Preventive maintenance time. That portion of calendartime used in accomplishing preventive maintenance. Itcomprises time spent in performance measurement;care of mechanical wear-out items; front panel adjust-ment, calibration, and alignment; cleaning; and sched-uled replacement of items.

Probability of fault detection. By using authorized dis-plays, manuals, checklists, test points, and test equip-ment, the probability that an existing fault—whichwould render a system or equipment inoperable (ormarginally effective) will be detected.

Production model. An item in its final mechanical andelectrical form—of final production design made byproduction tools, jigs, fixtures, and methods.

Production and deployment phase. The fourth phase inthe materiel life cycle. During this phase, all hardware,software, and trained personnel required to deploy anoperational system are acquired.

Proportion of faults isolatable. Given that a fault hasoccurred which renders a system or equipment inoper-able, the percentage of the faults that can be traced toan isolatable unit by using authorized displays. manu-als, checklists, test points, and test equipment.

Prototype model. A model suitable for complete evalua-tion of mechanical and electrical form, design, andperformance. It is in final mechanical and electricalform, uses approved parts. and is completely representa-tive of final equipment.

Q

Qualitative maintainability requirement. A maintainabil-ity requirement expressed in qualitative terms—e.g.,niinimize complexity, design for a minimum number oftools and items of test equipment, and design for opti-mum accessibility.

Quantitative maintainability requirement. A maintain-ability requirement expressed in quantitative termsi.e., a figure of merit in measurable units of time orresources required to accomplish a specific mainte-nance task or group of tasks in relation to the applica-ble performance requirements (reaction time. availabil-ities, downtime, repair time, turnaround time, etc.)

R

Random failure. Any Failure whose exact time of occur-rence cannot be predicted.

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Reaction time. The time required to initiate a mission;measured from the time the command is received.

Ready time. The period of time during a mission that theitem is available for operation but is not required.

Reassembly. A technician task for replacement of itemsremoved to gain access to facilitate repair and for clos-ing the equipment for return to service.

Rebuild. To restore to a condition comparable to new bydisassembling the item to determine the condition ofeach of its component parts, and reassembling it usingserviceable, rebuilt, or new assemblies, subassemblies,and parts.

Redundancy. The existence of more than one means foraccomplishing a given task, where some number ofmeans must fail before there is an overall failure to thesystem. Parallel redundancy applies to systems whereboth means are working at the same time to accomplishthe task and either of the systems is capable of handlingthe job itself in case of failure of the other system. Seriesor standby redundancy applies to a system where thereis an alternate means of accomplishing the task, i.e., thestandby redundancy is switched in by a malfunctionsensing device when the primary system fails.

Reliability. The probability that an item will perform itsintended function for a specified interval under statedconditions.

Reliability centered maintenance (RCM). A concept thatuses decision logic to evaluate and construct mainte-nance tasks which are based on the equipment func-tions and failure modes.

Repair. The process of returning an item to a specifiedcondition including preparation, fault location, itemprocurement, fault correction, adjustment and calibra-tion, and final test.

Repairability. The capability of an item to be repaired.

Repairable item. An item which can be restored to per-form all of its required functions by corrective mainte-nance.

Repair rate. A measure of repair capability, i.e., thenumber of repair actions completed per unit of time.

Repair time. See active repair time.

Replacement schedule. The specified periods when itemsof operating equipment are to be replaced. Replace-ment means removal of items approaching the end oftheir maximum useful life, or the time interval specifiedfor item overhaul or rework, and installation of a ser-viceable item in its place.

Replacing. Substituting one unit for another unit. Usu-ally done to substitute a properly functioning unit for amalfunctioning unit.

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S

Self test. A test or series of tests, performed by a deviceupon itself, that shows whether or not the device isoperating within designed limits. This includes testprograms on computers and automatic test equipmentthat check out their performance status and readiness.

Serviceability. The design, configuration, and installationfeatures that will minimize periodic or preventive main-tenance requirements, including the use of special tools,support equipment, skills, and manpower, and enhancethe ease of performance of such maintenance, includinginspection and servicing.

Service life. The period of time during which an item canremain in the operational inventory under specifiedconditions of use and maintenance.

Servicing. The performance of any act–-other than pre-ventive or corrective maintenance—required to keep anitem of equipment in operating condition, such as lubri-cating, fueling, oiling, cleaning, etc. This does notinclude periodic replacement of parts or any correctivemaintenance tasks.

Skill level. Level of proficiency required for performanceof a specific job, and the level of proficiency at which anindividual qualifies in that occupational specialty.

Sneak circuit. An unexpected path or logic flow within asystem which, under certain conditions, can initiate anundesired function or can inhibit a desired function.

Software. That portion of the support subsystem requiredin addition to personnel and hardware. Softwareincludes technical data, computer programs and tapes,training documents, etc.

Special tools. Tools peculiar to a specific end product.

Standardization. The use of common items, parts, mate-rials, and practices throughout the life cycle of systemsand equipment.

Storage time. Time during which a system or equipment ispresumed to be in operable condition but is being heldfor subsequent use.

Support cost. The total cost of ownership, excludingoperating crews and using personnel, of an item duringits operational life including the total impact of require-ments for skill levels; technical data; test, measurement,and diagnostic equipment (TMDE); spares; repairparts; special tools; maintenance equipment; facilities;levels and location of maintenance facilites; manpower;and training and training equipment.

Supportability. A measure of the capability of materiel tobe supported easily and economically.

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Support equipment. Items necessary for the maintenanceor operation of the system which are not physically partof the system.

Support parameter. Any of the several categories of sup-port resources—such as personnel, repair parts, andfacilities—required to support materiel.

System effectiveness. The probability that a system canmeet successfully an operational demand within a giventime when operated under specified conditions.

System engineering. The application of scientific andengineering knowledge to the planning, design, con-struction, and evaluation of man/machine systems andcomponents. It includes the overall consideration ofpossible methods for accomplishing a desired result,determination of technical specification, identificationand solution of interfaces among parts of the system,development of coordinated test programs, assessmentof data, integrated logistic support planning, andsupervision of design work.

T

Testability. A design characteristic that allows the statusof a unit or system to be determined in a timely andcost-effective manner.

Testability analysis. An element in equipment designanalysis effort related to developing a diagnostic ap-proach and then implementing that approach.

Test and checkout level. The functional level at whichequipment status is verified.

Test, measurement, and diagnostic equipment (TMDE).Any system or device used to evaluate the operationalcondition of materiel to identify and/or isolate anyactual or potential malfunction.

Test point. A convenient and safe access to functionalportions of materiel that is to be used so that a signifi-cant quantity can be measured or an access introducedto facilitate maintenance, repair, calibration, align-ment, or monitoring.

Total downtime. That portion of calendar time duringwhich a system is not in condition to perform itsintended function. It includes active maintenance (pre-ventive and corrective), supply downtime due to unavail-ability of needed items, and waiting and administrativetime.

Total technician time. The total man-hour expenditurerequired to complete a maintenance task. It includesactive technician time and delay technician time.

Trade-off. A comparison of two or more ways of doingsomething in order to make a decision. Decision crite-ria normally are quantitative.

Troubleshooting. Locating and diagnosing malfunctionsor breakdowns in equipment by means of systematicchecking or analysis:

u

Unitization. The process of providing a series of plug-inunits or similar subassemblies. each of which containsall parts necessary to make up a complete functioningcircuit or stage. Each circuit or stage can be indepen-dently removed and replaced with a like unit or subas-sembly. See also module.

Unit maintenance. A category of maintenance which iscategorized by quick turnaround based on repair byreplacement and minor repair -e. g., adjust, clean, lubri-cate, tighten. The maintenance is performed by theoperator, crew, and company or battalion maintenancepersonnel.

Unscheduled maintenance. All maintenance work notspecifically planned to occur at a prearranged time.

Useful life. The total operating time in which an itemremains operationally effective and economically use-ful before wear-out.

v

Validation phase. The second phase in the materiel acqui-sition process. This phase consists of those steps neces-sary to resolve or minimize special logistic problemsidentified during the conceptual phase. verify prelimi-nary design and engineering, accomplish necessaryplanning, fully analyze trade-off proposals. and pre-pare contracts as required for full-scale development.

Value engineering (VE). An organized effort directed atanalyzing the function of an item with the purpose ofachieving the function at the lowest overall cost.

w

Wear-out. The point at which further operation isuneconomical.

Wear-out failure. A failure that occurs as a result ofdeterioration or mechanical wear and whose probabil-ity of occurrence increases with time. Wear-out failuresgenerally occur near the end of the life of an item andare usually characterized by chemical or mechanicalchanges. These failures frequently can be prevented byadopting an appropriate replacement policy based onthe known wear-out characteristics of the item.

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INDEX

A

Abbreviations, use in labeling, 6-8Absorber, shock, 4-27Access, 4-1, 4-2, 4-7. See also Accessibility

aircraft, 4-24, 4-25criteria for, 4-2covers, 4-3doors, 4-3electronic equipment, 4-10fire control equipment, 4-20for lubrication, 8-10for maintenance actions, 4-2,4-40for reciprocating engine accessories, 4-25,4-26,4-27location of, 4-6

component arrangement and mounting, 4-6, 4-7importance of, 4-6to avoid dangerous situations, 4-6

marine equipment, 4-24missiles, 4-20modules, use of, 5-1openings, size and shape, 4-5, 4-10, 4-12, 4-13, 4-15,4-26

for gloved hand, 4-15,4-25,4-26one handed, 4-12

physical access for visual inspection, test, and serviceequipment, 4-4

tank and automotive, 4-22trailers, 4-20, 4-21types, 4-3visual inspection, light intensity required, 4-3windows, 4-3

materials, 4-5Accessibility, 1-7, 4-1, 4-2. See also Access

checklist, 4-28, 4-29factors affecting, 4-2planning for, 4-2safety considerations, 4-1

Accident, aviation, due to human error, 9-14Active maintenance time, 1-2Adaption kits, 4-2, 5-9Advanced Attack Helicopter (AAH), 5-6

artificial intelligence application, 7-10Aircraft materiel, 4-24

access to equipment, 4-25Advanced Attack Helicopter (AAH), 5-6Helicopter UH-60A, BLACK HAWK, 2-4landing gear, 4-27propulsion systems, 4-25

Aligning, 4-20, 8-16Aluminum, attack by alkaline solutions, 8-15Ammunition supply, 3-6Analysis, module postmortem, 5-3Anchors, 4-22

Anthropometric data, 4-1,4- 10,9-1,9-2,9-3sources, 9-2

MIL-HDBK-759, 9-2MIL-STD-1472, 9-2

Antifouling paint, 4-24AR 702-3, Army Materiel, System Reliability, Availabil-

ity, and Maintainability (RAM), 1-7Armament, modular design, 5-9Army Maintenance Management System (TAMMS),

9-15Army Master Data File, 3-6, 3-8Army Test Program Sets (TPS), 3-10ATE. See Automatic test equipmentAuditory system, 9-11Automatic test equipment (ATE), 5-4, 7-2, 7-8

artificial intelligence, use of, 7-9block diagrams, use of, 7-8cost, 7-8definition, 7-8EQUATE, 7-9GENRAD, 7-9logic flow technique, use of, 7-8output format, 7-16sneak circuits in, 7-11stimuli, 7-10, 7-16

transducers (sensors) selection characteristics, 7-16test features for consideration, 7-8test point compatibility, 7-12versus built-in test equipment (BITE) 7-11

Automotive replacement parts, frequency of occurrence,4-23,4-24

Availability, 1-1,4-1, 8-3

B

Bacteria, 10-2, 10-20Batteries, 10-14, 10-15Bearings, 4-28

lubrication, 4-28, 8-7split, 4-28

BITE. See Built-in test equipmentBLACK HAWK Helicopter, UH-60A, 2-4, 7-7

example of decrease in mean time to repair by use ofstandard/ interchangeable parts, 3-5

Blind mountings, 4-5,4-6,4-10, 8-8Bolts, 4-8

ball and socket, 4-25clearance, 4-18self-aligning, 4-25

Borescope, 7-7Brakes, for mounting stands, 4-22Built-in test equipment (BITE), 1-2,5-4,7-1,7-2,7-3, 7-6,

7-9artificial intelligence, use of, 7-9

I-1

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DOD-HDBK-791(AM)

INDEX (cont’d)

block diagram, use of, 7-17definition, 7-9example, maintenance planning effort, 7-3maturation process, 7-9output format, 7-16sneak circuits in, 7-11stimuli selection, 7-10test point compatibility, 7-12types, 7-16versus automatic test equipment (ATE), factors to be

considered, 7-11

C

Cables, 4-10,4-18connectors, 4-19, 4-20routing, 4-19, 4-22

Calibration, sensors, 7-16Capacitors, 4-8,5-5, 10-14, 10-22Caps

color coded, 8-13fuel, 8-10,8-13spring loaded, 4-4

Career Management Fields (CM F), 9-13Categories, sneak circuits, 7-16CHAIN GUN, 5-13Checklists

accessibility, 4-29diagnostics, 7-26identification, 6-17interchangeability, 3-12modularization, 5-16preventive maintenance and servicing, 8-17simplification, 2-7standardization, 3-12testability, 7-25

Chemical environment, 10-3Circuit, 5-5

breakers, 10-22diagrams, 6-1, 6-4grouping, 5-5simplicity, 3-1sneak, 7-11

Cleaning, 4-26,5-5,8-7,8-15solvents, 8-15

Clearance, nuts and bolts, 4-18Clothing, 4-2, 6-4Clutches, magnetic, 10-22Collimation, 4-20Color code

fuel caps, 8-10, 8-13labels and signs, 6-6parts, 6-5perception, 9-6piping, 6-4, 6-14safety, 6-11, 6-14, 6-16, 6-17

Color, contrast and background, 6-11,6-14,6-15

Components. See also Throwaway and Modulesarrangement, 4-6complexity, 1-7dispersal, 4-8grouping, 5-5high failure rate, 4-8location, 4-5,4-6,4-10, 8-8off-the-shelf, 3-1stacking, 4-8

Computer-aided designs for testability, 7-5Conditioning monitoring, mechanical systems, 7-6Connectors, electrical, 4-10,4-18,4-19,4-20, 4-22Controls, 4-6Corrective maintenance, 1-1,4-23Corrosion

biological, 10-13galvanic, 6-6, 8-14,8-15, 10-13marine, 4-24protection against, 4-24, 8-15, 8-16stress. 6-6

Costs, 1-1, 1-5, 2-2, 5-2, 5-3, 5-7, 8-6manufacturing minimization, 5-2ownership, 8-3, 8-6versus preventive maintenance, decision on

effectiveness, 8-6Covers. See also Doors

method of securing, 4-6types, 4-3

Cowlings, 4-25,4-26fasteners, 4-25

Crystals, 10-23

D

DA Pam 700-12-1, DA TMDE Preferred ltems List(PIL), 3-10

Damping, 10-16Decalcomania, 6-6Decision Coordinating Paper (DCP), 1-1Defense Logistics Service Center (DLSC), 3-3, 3-8, 3-9Delphi, technique for error estimation, 9-15DELTA program, application of artificial intelligence,

7-10Department of Defense Index of Specifications andStandards (DODISS), 3-6, 3-8

Desert environment, 10-14Design trade-offs, 8-6Detuning, 10-16Diagnostics, 7-1. See also Testability

analysis for T700 gas turbine, 7-20, 7-21, 7-22, 7-24automatic inspection, diagnostic, and prognosticsystem (AIDAPS), 7-12

capability, 7-5block diagrams, use of, 7- I 7checklist, 7-20, 7-26condition monitoring, 7-6definition, 7-5design considerations

I-2

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INDEX (cont’d)

DOD-HDBK-791(AM)

built-in test equipment (BITE) versus automatictest equipment (ATE), 7-10

software, 7-10, 7-12stimuli, 7-10, 7-16test point identification, 7-10, 7-12test point selection, 7-10, 7-11test sequence, 7-10, 7-12transducer (sensor), selection characteristics, 7-16

design needs, 7-5equipment, 7-2, 7-6, 7-11

design needs for reliability, 7-5function test performed

fault localization, isolation, and prediction, 7-10hidden faults, 7-10laboratory versus field performance, 7-5, 7-6,

7-10verification by test and demonstration, 7-5verification testing, 7-10

false alarms, 5-3, 7-2, 7-4, 7-5, 7-6, 7-8, 7-9faults, 4-2, 7-1, 7-2, 7-5, 7-6, 7-9, 7-10, 7-12likelihood analysis computer programs, 7-17

application to Advanced Attack Helicopter(AAH), 7-17,7-18

maturation process, 7-6mechanical systems, for, 7-6output format, 7-16reliability centered maintenance (RCM), relationshipto, 8-3

requirement, 7-2techniques

guidelines to improve, 7-5importance of, 7-9types, 7-7

turbine, gas, T700, analysis of, 7-20, 7-21, 7-22, 7-24transducers (sensors) selection, 7-16versus testability, 7-1

Dipsticks, 8-8, 8-10Discard at failure, 5-2, 5-4Documentation reduction, 5-2

due to standardization interchangeability, 3-2, 3-4DoD Instruction 4120.19, Department of Defense

Parts Control Program, 3-6, 3-7Doors, access, 4-3

bottom support, 4-5, 4-6captive bolts, quick opening, 4-3hinged, 4-3, 4-4,4-5screwed down, 4-3types, 4-3

Downtime, 5-1, 8-3, 8-7Drainage, 8-16

cocks, 8-13holes, 8-14, 10-2, IO- I 3location, 8-14plugs, 8-10, 8-13requirements for, 8-13valves, 8-13

Drawers, 4-4,4-5,4-10.4-19, 4-28Dust, 10-14

EElectromagnetic pulse (EM P), 10-3, 10-17Electromagnetic radiation, 10-3, 10-17Electronic equipment, 7-1

environmental effect ofacceleration, 10-15, 10-17arctic climate, 10-14desert, 10-14electromagnetism, 10-3, 10-17nuclear radiation, 10-3, 10-4, 10-15, 10-17

failure modes, 10-22foldout construction, 4-10,4-16hinged assembly, 4-10,4-16lugs, use of, 4-10,4-18modular design, 5-6repair of, 4-10

use of logic flow diagram technique, 7-8use of integral stands, 4-10, 4-16

service loops, 4-19solid-state, 10-16terminals, 4-10, 4-18

Encapsulation of modules, 5-2, 5-4Engines, reciprocating, access to components, 4-25,4-26,4-27

Engraving, 6-6Environment

defined, 10-1effects on materiel, 10-3, 10-12

acceleration and protection against, 10-16, 10-17arctic and protection against, 10-2, 10-14desert and protection against, 10-2, 10-14electromagnetic and protection against, 10-3, 10-17moisture and protection against, 10-2, 10-13nuclear and protection against, 10-15shock and protection against, 10-2, 10-15tropic and protection against, 8-15vibration and protection against, 10-2, 10-15

effect on personnel, 10-2, 10-5noise, 105temperature, 10-2, 10-5vibration and motion, 10-2, 10-10, 10-11, 10-12

factors, 10-1conductive or synergistic, 10-2, 10-3, 10-4induced, natural, combined, 10-2, 10-3, 10-4limitation for unsheltered materiel, effect of factors

and equipment failure made, 10-19wartime, 10-2, 10-3, 10-4, 10-5

EQUATE, 7-9Equipment accesses. See Access doors and OpeningsError, human, 9-13

causes, 9-13contributing factors, 9-13due to mislabeling, 6-1quantification models, 9-15rate estimation, 9-15, 9-16reduction, 3-4Siegel-Wolf model, for determining, 9-15

Expert system, 7-9

I-3

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DOD-HDBK-791(AM)

INDEX (cont’d)

F

Failure category severity, 8-3Failure modes

detectable, 7-10isolatable, 7-17

Failure modes and effects analysis (FMEA), 7-2Failure modes, effects, and criticality analysis(FMECA), 7-3

False alarms, 5-3, 7-2, 7-4, 7-5, 7-6, 7-8, 7-9Fasteners, 4-3

captive, 4-3cowl, 4-25effects of acceleration, on, 10-17examples of, 4-21nuts and bolts, 4-8quick opening, 4-20,4-25, 5-6self-aligning, 4-25standardization, 4-25

Faultshidden, 7-10insertion tests, 7-5, 7-6isolation, 7-1, 7-9, 7-10, 7-12localization, 7-10prediction, 7-10simulation, 7-2tolerance, 7-9

Filters, 4-22oil, 4-23, 4-24, 4-28

Fire control equipment, 4-20Fluids

access. 8-13drains. See Drainagefilling, 8-10

fuel tanks, vehicle, caps, rate, size, 8-10injection systems, 5-9

Fonts, size and style for labels, 6-12,6-13,6-14Frequency grouping, 5-5Fuels

access to, 8-13cells, 4-25drains, See Drainagefilling, 8-10

tanks, vehicle, size, rate, caps, 8-10filters, 4-22,4-23,4-24,4-28injection systems, 5-9

Function consolidation, 2-2Fungous, 8-15,8-16, 10-2, 10-20

inert materials, 8-15protection, 8-15

Fuses, 2-2,4-8, 5-5

G

Galvanic action, 6-6,8-14,8-15, 10-13Gas turbine T700, diagnostic evaluation, 7-20,7-21,7-22.7-24

Gaskets, 8-16seals, 8-16

Gears, 7-17, 8-7, 8-10Generators, 8-7GENRAD, 7-9Gloved hand, size openings for, 4-15Grouping for modularization, 5-4, 5-5

evaluation, 5-5Gun, M230, 2-4Gyroscopes, 10-23

H

Handgrips, 4-10,4-18Handles, 4-8,4-10,4-18,4-20, 5-6,6-10

design, 4-10,4-20dimensionless, 4-17

Hand-tool usefrequency, 4-11space requirements, 4-10

Hazards. See SafetyHearing, 9-10Helicopters

Advanced Attack (AAH), 5-6Army Oil Analysis Program (AOAP), 8-10artificial intelligence application, 7-10availability, 8-6built-in test equipment (BITE) application, 7-9BLACK HAWK, UH-60A, 2-4component dispersal, 4-8, 4-9diagnostics, 7-7engine, modular example, 5-13fuel

drains, 8-14lines, 8-14

modular design, 5-13likelihood analysis program, 7-17

application to Advance Attack Helicopter (AAH),7-17, 7-18

shock absorbers, 4-27HELLFIRE missile system, 5-9High failure rate components

delicate, 4-8fuses, 4-8location of, 4-8stacking, 4-8

Hingedassemblies, electronic equipment, 4-16doors, 4-13,4-19,4-25

Hoists, 4-23Housings, 4-28Howitzer, 155-mm, 2-4Human body measurements. See Anthropometric dataHuman error. See Error, humanHuman factors, 2-1, 4-1,4-10,9-1

design applicabilityMIL-STD-759, 9-1

I-4

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DOD-HDBK-791(AM)

INDEX (cont’d)

MIL-STD-1472, 9-1failures attributable to, 6-1hearing, 9-10

audio signals, 9-10versus vision, 9-10

noise, speaker-to-listener distance, 9-11sound intensity levels, 9-12

principles, MIL-H-46855, 9-1relative to maintenance, 1-2, 4-10sensory capacity, 9-4sight, 9-4, 9-7

versus audio signal, 9-10signals, when to use audio or visual, 9-11strength, 9-4, 9-5, 9-6touch, 9-11

recognizable knob shapes, 9-11Humidity, 10-19. See also MoistureHydraulic equipment, 4-8,4-27, 5-5

I

Identification of equipment and parts. See Labels andLabeling

Identification List, 3-8Illumination requirements, 4-3, 9-1,9-6,9-7Indicator levels, 8-10Information sources for standard parts, 3-6

AR 700-60, Department of Defense Parts ControlProgram, 3-6, 3-7

AR 700-43, Test, Measurement, and DiagnosticEquipment (TMDE), 3-10

Infrared suppressors, 2-4Inspection, 4-3Insulators, 4-10, 10-23Integrated logistical support, 8-2Intensity levels of sound, 9-12, 10-15Interchangeability, 1-7, 3-1, 3-2

advantages, 3-3, 3-4application to design, 3-3checklist, 3-12classes

local, 3-2universal, 3-2

design principles, 3-3examples of, 3-4functional, 3-3physical, 3-3

Jacks, 4-23Job design, 2-2Joints, solder, 10-23Junction boxes, 4-24

K

Kolmogorov-Smirnov Test, 1-3Knobs, 6-10

shapes, 9-11

L

Labeling, 6-1. See also Labelsaspects of, 6-1contributions to human error, 6-1lubrication fittingsmethods, advantages and disadvantages of

adhesive-backed labels, 6-7decalcomania transfers, 6-6engraving, 6-6ink stamping, 6-6metal plates, 6-7molding-in, 6-6photocontact, 6-7photoetching, 6-7screen printing, 6-7steel stamping, 6-6stenciling, 6-6tags, 6-7

purpose, 6-1Labels, 10-2. See also Labeling

abbreviations, use of, 6-8arrows, directional, 6-8, 6-9basic characteristics, 6-2color

recognition, 6-15selection, 6-14

composition, 6-8design, 6-11

character height, spacing, width, 6-12, 6-13, 6-14color contrast, 6-11font and size, 6-12, 6-15

display of information, 6-8examples of, 6-3, 6-8, 6-9, 6-10, 6-11font sizes, 6-2functional information, 6-4, 6-8hazards, 6-4information to be contained in, 6-2, 6-7instructional information, 6-4, 6-8location/ position, 6-1, 6-2, 6-5, 6-9, 6-10NATO force information, 6-1purpose, 6-2types, 6-2warranty clauses, 6-2, 6-3

Languages, software, 2-4, 5-1, 5-3, 7-8, 7-10, 7-12Lasers, 4-20,6-4, 10-4Level indicators, fluid, 4-2, 8-10Lever handles 5-6Lids, spring loaded, 4-4Life cycle costs, 2-2, 5-3Lifting equipment, 4-23Lift points, 6-4Likelihood analysis computer program, 7-17

application to Advanced Attack helicopter, 7-17, 7-18Line-replaceable unit, 5-1Logic flow diagram technique, 7-7, 7-8Logistic support, example of improvement bystandardization/ interchangeability, 3-4

Logistic support plan, 2-1, 2-3,5-1, 5-3, 7-1

I-5

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DOD-HDBK-791(AM)

INDEX (cont’d)

Logistics function of built-in test equipment (BITE) and design consideration. 8-7automatic test equipment (ATE), 7-11

LOGMOD, 7-5Lubrication, 4-6,6-4, 8-7, 8-8, 8-9, 8-10

aircraft, 4-26, 4-27bearings, 4-28, 8-8charts, 8-11designs, MIL-STD-838, 8-7fittings, 8-7, 8-8, 8-9

MIL-STD-454, 8-8MIL-F-3541 ,8-8

order, 8-10, 8-11standard products, use of, 8-7, 8-8

Lugs, O- and U-types, use of, 4-10,4-18

M

Maintainabilitycharacteristics, 1-2cooperation with industry, 1-7costs, 1-1, 1-2, 1-5, 1-7criteria, 1-1, 2-1, 2-3features to facilitate, 1-7goals, 2-3importance of, 1-1inherent value, 1-4level of effort, 1-5measures, 1-2

quantitative, 1-1objectives, 1-5operational readiness, 1-1physical factors

relevancy of, 9-15built-in test equipment (BITE) and automatic testequipment (ATE) related, 7-11

plan, 1-6, 7-11application matrix, 1-6MIL-STD-470, 1-6tailoring, 1-6

program, 1-5, 1-7requirement, verification, demonstration, andevaluation of 1-7

AR 702-3, 1-7MIL-STD-471 , 1-7

risk area, 1-1versus maintenance, 1-1

Maintenance, 2-1actions, 4-2availability, related to, 8-3

equations related to quantitative maintenanceparameters, 8-3

built-in test equipment (BITE) related, 7-3climate extremes, performance in, 10-6corrective, 4-1, 4-23, 8-1

design considerations, 8-7relationship to preventive maintenance, 8-2

costs, 2-1, 8-3, 8-6depot, 2-4

1-6

diagnostics, importance of, 7-7distributions, 1-2, 3-5

Gaussian (normal), 1-3lognormal, 1-3time, 1-4

float, 5-2human Factors influence, 2-1, 4-1, 9-1management, 9-15manuals, 3-1, 3-4, 3-8, 5-2, 5-4, 6-1,.8-3, 9-13personnel, 9-13

Manpower and Personnel Integration Program(MANPRINT), 9-1

plan, 8-2preventive, 1-1, 4-23, 8-1

checklist, 8-17cost of ownership, effect on, 8-3, 8-6relationship to corrective maintenance, 8-6trade-offs, 8-3, 8-6versus repair, decision on cost-effectiveness, 8-6

proceduresscheduling, 2-3, 2-4streamlining, 2-3

redundancy, effect on, 7-2reliability centered (RCM), 8-2

analysis process, 8-3, 8-4concept exploration phase, relation to, 8-2data, disposition of, 8-3, 8-5objectives, 8-2time of application, 8-3

time, active, l-2trade-offs, 8-3, 8-6scheduled, 1-1, 7-5servicing, 8-1

checklist, 8-17simplification, 2-3unit level, 2-4unscheduled, 1-1

Magnetic chip detectors, 8-8MS35844, 8-8

Malfunction isolation, 2-3Manuals, 3-1, 3-4, 3-8, 5-2, 5-4, 6-1, 8-3, 9-13Marine

equipment, 4-24fouling, 4-24

antifouling agents, 4-24Marking. See also Labels and Labeling

center of gravity (CG), 6-4doors, access, 4-24,4-25functional information, 6-4hazards, 6-4instructional information, 6-4safety, 6-4, 6-7, 6-11, 6-14, 6-16, 6-17, 10-14security, 5-4

Materialsacceleration, effect of, 10-17corrosion resistance, 8-16flexible, 10-15

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DOD-HDBK-791(AM)

INDEX (cont’d)

fungous inert, 10-13metals, precious, 5-4, 10-17paints, lacquers, varnishes, 10-14rubber, 10-14

Metalsdissimilar, 6-6, 8-14, 8-15, 10-13precious, 5-4, 10-17

Meters. 10-16readout, 7-16

Microbiological organisms, 10-2Microwave radiation, 10-4, 10-5Military environment, 10-2, 10-3, 10-4, 10-5Military Occupational Specialty (MOS), 9-13Military Occupational Specialty Code (MOSC), 9-13Missiles, 4-20, 9-1Mobile equipment, design, 4-20Modification, 5-2Modular design, 5-9Modularization, 2-2, 5-1. See also Modules andThrowaway items

advantages, 5-1, 5-2checklist, 5-16design guidelines, 5-6examples of technique

armament, 5-9electronics and electrical equipment, 5-6helicopter engine, 5-13missiles and rockets, 5-9tank-automotive, 5-9

functional grouping forcircuits, 5-5, 5-7components, 5-5, 5-7evaluation of techniques, 5-5frequency of replacement, 5-5logical flow, 5-4standard construction, 5-5

tank engine assembly, 5-9, 5-10testability and diagnostics, related to, 7-2vehicle design, 5-6

Modules. See a/so Modularization and Throwawayitems

automatic test equipment (ATE) related, 7-8cost considerations. 5-1criteria for use, 5-4definition, 5-1disposal, 5-4encapsulation, 5-4independent check capability, 5-6interchangeability, 5-1Navy Standard Electronic Module Program (SEM),

5-7plug in, 4-2replacement, 5-1self-isolating, 7-13software, 5-3throwaway, 5-3, 5-4trade-off with piece part, 5-4weight limitations, 5-5, 5-6

Moisture, 4-4, 10-13. See also Humidityprotection against, 8-15, 10-13

Molded-in identification label, 6-6Monitoring equipment, on-line, 7-7Motion sickness, 10-10, 10-12Motivation, 9-13Motors, 8-7, 10-23Mounting, blind. See Blind mountings

N

Nacelles, engines, 4-28Nameplates. See Labels and LabelingNational Codification Bureau Code (NCBC), 3-3, 3-9National Item Identification Number (NIIN), 3-9National Stock Number, 3-6, 3-8, 3-9, 6-5Navy Standard Electronic Module Program (SEM), 5-Noise, 9-11, 9-12Nondestructive testing, 7-7Nondevelopment Item (NDI), 10-1Nuclear radiation

environment, 10-15hazard warning symbol, 6-17

Numeral and letter sizes, 6-12, 6-13, 6-14Nuts, 4-8

O

Oilfilters, 4-23,4-24,4-28levels, 8-10pressure, 8-8standard products, use of, 8-7, 8-8

Openings. See DoorsOzone, 8-15, 10-20

Packaging for access, 4-1Paint, 10-14

antifouling, 4-24Parts Control Board, 3-7Parts

access to, 2-3configuration, 2-3

P

identification, 1-7, 6-1information sources on control of

Military Parts Control Advisory Group (MPAG),3 7

MIL-STD-143, 3-6, 3-7, 3-8MIL-STD-965, 1-7Program Parts Selection List (PPSL), 4-23SB 700-20, 3-8

marking, 6-5. See also Labels and Labelingnumbering system, 3-9

Photocontact, 6-7Photoetching, 6-7Plastic covers, 4-3

I-7

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DOD-HDBK-791(AM)

INDEX (cont’d)

Platforms, 4-22Plug-in items

isolatable, 7-2orientation of, 4-8

Plugsdrain. See Drainagethreaded, 8-10, 8-13

Pressure, 10-19Preventive maintenance. See Maintenance, preventivePreventive Maintenance Checks and Service (PMSC),

4-23Program Parts Selection List (PPSL), 3-7Propulsion systems

aircraft, 4-25parts to be readily accessed, 4-25,4-26related to diagnostic, 7-6

Publications. See also Technical manualsreduction of by standardization, 3-2

Psychological factors, 4-1,9-1,9-13MIL-STD-1472, 9-13motivation, 9-13noise, 9-11training, 9-13

Q

Qualified Products List (QPL), 3-6,3-8Quick-release fasteners, 4-20,4-25,5-6

R

Racks, 44,4-5Radar, 7-14, 7-15Radio frequency warning sign, 6-16Rails, use of, 4-20,4-25Rationalization, Standardization, and Interoperability(RSI) program, 3-3

objectives, 3-3Receptacles, marking, 6-4Reciprocating engine components, 4-25,4-26,4-27Redundancy, 7-2,7-20Relays, 5-5, 10-14, 10-16, 10-24Reliability, 1-1, 5-2, 5-3, 7-2, 7-5

related to built-in test equipment (BITE) andautomatic test equipment (ATE), 7-11

Reliability centered maintenance (RCM), 8-2Repair time, attributes affecting, 1-2Resins, synthetic, 8-15Resistors, 4-8,5-5, 10-24Rubber, 8-15

eyepiece, 4-20

S

Safety, 4-1,4-2,4-4,4-6,4-8, 6-4,6-7,6-11, 8-3, 8-10,8-15, 10-5

color, 6-11markings, 6-4, 6-11, 6-14, 6-16, 6-17

Salt spray, 10-19

1-8

Sand and dust, 10-2, 10-19Scheduled maintenance, 1-1. See also MaintenanceScreen printing, 6-7Screwed-down covers, 4-3Seals, 10-14, 10-20

compounds, 10-15hermetic, 7-1, 10-13oil, 8-8

Security classification, throwaway modulus, 5-4Sensors, selection characteristics. 7-16. See alsoTransducers

Servomechanisms, 10-24Shelves, 4-4, 4-5Shock, 10-15

absorber, 4-27mounts, 10-2

Siegel-Wolf model for human error determination, 9-15Sights, 4-20Simplification, 1-7, 2-1

checklist, 2-5, 2-7design techniques, 2-1examples of

IR suppressor, 2-5M230 Gun, 205muzzle plug, 155-mm howitzer, 2-5UH-60A Helicopter (BLACK HAWK), 2-4

modules, use of, 5-1Skin, aircraft, hinged, 4-25Sneak circuits, 7-11

categories, 7-11Snow, 10-14Software

languages—ADA, C, PASCAL, 2-4maintenance, 2-4modules, 5-1, 5-3

advantages, 5-3design criteria, 5-3

relationship to diagnostic techniques, 7-8, 7-10, 7-12automatic inspection, diagnostic, and prognostic

system (AIDAPS), 7-12debugging, 7-12language selection, 7-12structured programming, 7-12

Solid-state electronic devices, 10-16, 10-25Solvents, 8-15Sound levels

associated with weapons, 10-5limits of intensity, 9-12

Split-line design, 4-25Stacking, components, 4-8STAMP, 7-5Stamping

rubber, 6-6steel, 6-6

Standardization, 3-1. 3-9advantages, 3-2, 3-4applications to design, 3-3checklist, 3-12

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DOD-HDBK-791(AM)

INDEX (cont’d)

definition, 3-1examples of, 3-4goals and principles, 3-2importance of, 3-2

Stands, maintenance, 4-16,4-22design for stability. 4-22

Static electricity, 10-16Stenciling, 6-6Surfaces, walking, 4-22Switches

environmental effects on, 10-14, 10-24location of, 4-8

Symmetrical designs, 3-4System

availability, 1-1, 4-1, 8-3experts, 7-9

System Concept Paper, 1-1

T

Tank automotive equipment, 4-22example of increased availability of redesign of

U-joint, 4-23frequency of maintenance tasks, 4-23, 4-24lubrication order, 8-10, 8-11modular design, 5-9stress cracks, access panel corners, 4-24

Target Acquisition Designation System (TADS), 5-6,5-8

Technical manuals, 3-1, 3-4, 3-8, 5-2.5-4, 6-1, 9-13Technician, typical, 9-13Temperature, 10-5

effective, 10-6, 10-7limits and extremes, 10-6, 10-7, 10-8, 10-9, 10-10wet-bulb global, 10-6windchill index, 10-6, 10-9

Termites, 8-15Testability, 1-7, 7-1

analysis, 7-1guidelines for, 7-2

checklist, 7-20, 7-25computer-aided techniques, 7-5definition, 7-1method of achieving, 7-1relation to reliability centered maintenance (RCM),

8-3test point

example of selection process, 7-12identification, 7-12

test sequence, 7-12.7-13examples of. 7-14.7-15

Test equipment, 3-1flow chart selection, 3-11Test, Measurement, and Diagnostic Equipment(TMDE), 3-6, 3-9, 3-10

AMC’s role as executive director, 3-10US Army TMDE Support Group (USATSG), 3-10

Testing, 4-2for maintainability, 1-7verification, 7-10

Test, Measurement, and Diagnostic Equipment(TMDE), 3-6,3-9,3-10

Test Pointsexample of selection process, 7-12identification of, 7-12

Thermistors, 10-24Throwaway items, 7-13

advantages, 5-2classified items, 5-4concept, 5-2containing precious metals, 5-4criteria for, 5-4feasibility, 5-3hazards, 5-3level of accessibility, 4-1location, 4-2strategy, 5-2

Touch, sense, 4-2, 9-11Track radar transmitter, example of test sequence, 7-14,

7-15Trade-offs

for maintainability, 2-1parameters, 8-3preventive maintenance versus repair costs, 8-6

Trailers, design, 4-20mating surfaces, 4-20, 4-22

Training, 9-13reduction due to standardization, 3-2

Transducers, 7-10, 7-16selection characteristics, 7-16

Transformers, 10-16, 10-25Transistors, 5-5, 10-15, 10-25Transmissions, 7-17Tropical environment, 7-15Troubleshooting, 7-1, 7-5, 7-7, 7-8, 7-9, 7-10. See also

DiagnosticsTurbines, 4-26

blade removal, 4-26,4-27gas, T700, diagnostic evaluation of, 7-20, 7-21, 7-22,

724Type sizes for labels, 6-2,6-12,6-13,6-14

U

Units, International System (SI) of, 3-3Unscheduled maintenance, 1-1. See also MaintenanceUS Army TMDE Support Group (USATSG), 3-10

VValue engineering, 2-2Valves

check, 2-2control marking, 6-11

Van trailers, 4-22Vehicle replacement parts, frequency of occurrence,4-23,4-24

I-9

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DOD-HDBK-791(AM)

INDEX (cont’d)

Verification of maintainability, 1-7Very high-speed integrated circuits (VHSIC). artificialintelligence related, 7-9

Very large-scale integrated circuits (VLSIC). artificialintelligence related, 7-9

Vibration and motion, 10-12, 10-15limits and tolerances, 10-10, 10-11, 10-12whole body, 10-10, 10-11

Visibility of components, 1-7Visual fields, 9-6, 9-7Voltage, marking, 6-4Warranties, placed on labels, 6-2Warsaw Pact Forces, 3-3

Wartime environment. 10-2Wet-bulb global temperature. 10-6Windchill index, 10-6, 10-9, 10-10Wiring, 4-8, 4-10, 10-16

cables. 4-10, 4-18, 4-19connections, 4-20, 4-22

spacing, 4-10, 4-18terminals, 4-10, 4-18use of O- and U-type lugs, 4-10, 4-18

X

X rays, 7-7, 10-4. 10-5

1-10

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MIL-HDBK-791 (AM)

CUSTODIAN:ARMY-AM

PREPARING ACTIVITY:ARMY-AM

(PROJECT MISC-A009)

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INSTRUCTIONS: In a continuing effort to make our standardization documents better, the DoD provides this form for use insubmitting comments and suggestions for improvements. All users of military standardization documents are invited to providesuggestions. This form may be detached, folded along the lines indicated, taped along the loose edge (DO NOT STAPLE), andmailed. In block 5, be as specific as possible about particular problem areas such as wording which required interpretation, wootoo rigid, restrictive, loose, ambiguous, or was incompatible, and give proposed wording changes which would alleviate theproblems. Enter in block 6 any remarks not related to a specific paragraph of the document. If block 7 is filled out, anacknowledgement will be mailed to you within 30 days to let you know that your comments were received and are beingconsidered.

NOTE: This form may not be used to request copies of documents, nor to request waivers, deviation, or clarification ofspecification requirement on current contracts. Comments submitted on this form do not constitute or imply authorizationto waive any portion of the referenced document (s) or to amend contractual requirements.

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