Weighing Reconciliation Techniques for Military …...2014/05/14 · Weighing an aircraft is not...

INTERNATIONAL

SOCIETY OF ALLIED

WEIGHTS ENGINEERS,

INC.

R E C O M M E N D E D P R A C T I C E N U M B E R PD RP A-11

Serving the Aerospace - Shipbuilding - Land

Vehicle and Allied Industries

Executive Director

P.O. Box 60024, Terminal Annex

Los Angeles, CA 90060

Date Issued 14 May 2014

Weighing Reconciliation Techniques for

Military Aircraft

Revision Letter (Original)

Prepared by

Standards and Practices Committee – Military Aircraft

Society of Allied Weight Engineers, Inc.

All SAWE technical reports, including standards applied and practices recommended, are advisory only. Their use by

anyone engaged in industry or trade is entirely voluntary. There is no agreement to adhere to any SAWE standard or

recommended practice, and no commitment to conform to or be guided by any technical report. In formulating and

approving technical reports, the SAWE will not investigate or consider patents that apply to the subject matter.

Prospective users of the report are responsible for protecting themselves against liability for infringement of patents.

Notwithstanding the above, if this recommended practice is incorporated into a contract, it shall be binding to the extent

specified in the contract.

This Draft Copy of this SAWE Recommended Practice is made available for the sole purpose of public

review. This document is not an official version of this RP and should not be used for any purpose

other than to provide feedback to the author.

SAWE Recommended Practice No. PD RP M-xx

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Change Record

Issue No. Date Description Entered by

-

-

04/25/12

05/21/13

05/14/14

Committee Draft

Revised Committee Draft: included

contributions by Gregor Lehnertz from

SAWE Paper 3542, May 2012

Public Draft

L. Linner

L. Linner

L. Linner

Recommended Practice

RP-Mxx


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Weighing Reconciliation Techniques for Military Aircraft

1. This SAWE Recommended Practice provides guidelines for reconciling weight

and balance measurements against predicted values for military aircraft.

2. Beneficial comments (recommendations, additions, and deletions) and any

pertinent data that may be of use in improving this document should be forwarded

to the Executive Director at the above address.


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TABLE OF CONTENTS

Page

1 Scope 7

2 Introduction 7

3 Reference Documents 8

4 Definitions 8

5 Weight Reconciliation Process 9

5.1 Cause and Effect Diagram (Fish Bone Diagram) 9

5.2 Methods 11

5.2.1 Failure to Exercise the scales 11

5.2.2 Failure to rotate the scales 11

5.2.3 Attempting to shim the scales 12

5.2.4 Not leveling prior to jacking 12

5.2.5 Leveling laterally while on platform scales 12

5.2.6 Tires not centered on the platform scales 12

5.2.7 Improper defuel 12

5.2.8 Improper aircraft configuration 13

5.2.9 Failure to account for all Column I items 13

5.2.10 Tire resting against wheel chock 13

5.2.11 Rapidly towing the aircraft onto platform scales 13

5.2.12 Non level weighing 14

5.3 Equipment 14

5.3.1 Bad scale or load cell calibration 14

5.3.2 Old/weak batteries 14

5.3.3 Damaged scale/load cell 14

5.3.4 Damaged jacks 15

5.3.5 Incorrect load cell/platform size 15

5.3.6 Inclinometer out of calibration 15

5.3.7 Leveling bar not installed correctly 15

5.3.8 Leveling lugs incorrectly installed 15

5.3.9 Aircraft fuel scavenging system operation 16

5.3.10 Aircraft fuel probe operation 16

5.4 Measurement 17

5.4.1 Incorrect elevation, latitude settings 17

5.4.2 Error recording the displayed values 17

5.4.3 Error reading the inclinometer 17

5.4.4 Error in measuring the strut chrome 17

5.4.5 Incorrect geometry measurements 18

5.5 Environment 18

5.5.1 Non level / uneven floors 18

5.5.2 Temperature variation 19


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5.5.3 Electrical interference 19

5.5.4 Wind / air flow 19

5.6 Personnel 19

5.6.1 Incomplete / Improper training 19

5.6.2 Incorrect, incomplete TOD 20

5.6.3 Not following procedures 20

5.6.4 Records not maintained 20

5.6.5 Incorrect "Hit the Scales" prediction 21

5.7 Prediction 21

5.7.1 Weight Empty 22

5.7.1.1 Propulsion System 22

5.7.1.2 Airframe 23

5.7.2 Basic Weight 24

5.7.3 Column I / Column II Items 25

5.8 Configuration 26

5.8.1 Manufacturing Variation 26

5.8.1.1 Manufacturing Induced trapped / undrainable fuel 26

5.8.1.2 Manufacturing Variation in Stuctures and Systems 27

5.8.1.3 Alignment and Symmetry 27

5.8.1.4 Leveling Lug Installation 28

6 Weighing Event Checklists 29

7 Failure / Root Cause investigation 30

7.1 Scatter related weighing performance investigation 34

7.2 Deviation related weighing performance investigation 35

7.3 Aircraft inventory or build investigation 36

7.4 Scatter related weighing equipment investigation 36

7.5 Deviation related weighing equipment investigation 37

7.6 Unresolved weighing error investigation 26

8 References 29

Figure 1 Cause and effect diagram for sources of error during aircraft

weighings

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Figure 2 Top level failure / root cause investigation flow chart 30

Figure 3 Failure investigation / root cause table 33

Figure 4 Scatter-related weighing performance investigation 34

Figure 5 Deviation-related weighing performance failure investigation 35

Figure 6 Deviation-related aircraft inventory or build failure investigation 36

Figure 7 Scatter-related weighing equipment investigation 36

Figure 8 Deviation-related weighing equipment investigation 37


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1. SCOPE

The objective of this Recommended Practice is to document methods and practices that

have proven effective in the reconciliation of significant differences between measured

weight and balance values obtained from actual weighing measurements and predicted

values. These proven methods are applicable to most military aircraft, including

transport, fighter, and unmanned aircraft, and are applicable to weighings accomplished

using a variety of measurement equipment including portable weighing platforms,

permanently installed weighing platforms, and top-of-jack load cells. Procedures and

calculations relating to applying measured data towards the end product of calculating

Basic Weight and Center of Gravity are well established in other industry standards and

are beyond the scope of this document. For example, it is not the intent of this RP to

address the details of any aircraft’s weighing procedure, but rather to provide guidance on

what to do if weighing results are significantly different from predicted.

2. INTRODUCTION

As aero-performance becomes increasingly important, modern aircraft have incorporated

active load alleviation and weight distribution/center of gravity control to minimize drag,

increase maneuverability, and increase aircraft structural life by minimizing loads. Load

factor control requires that an accurate current Gross Weight be known at all times during

flight to be able to limit control surface deflection in accordance with load factor times

weight (NzW) limitations. To control weight distribution and center of gravity, an

accurate knowledge of the current weight and center of gravity of fixed Operating Weight

is required as well as an accurate assessment of non-fixed Operating Weight and

expendable weight items. This need requires storing aircraft specific mass properties

data within the aircraft flight control system, data that must be updated regularly to

maintain accuracy with current aircraft configuration.

Operationally, accurately knowing the weight and center of gravity is largely dependent

on the accuracy of the periodic weight and center of gravity measurement of the aircraft.

If the aircraft Basic Weight and/or center of gravity are not accurately determined during

the aircraft weight and balance measurement process, all calculations using these

weighing results are flawed. Significant error in weight or center of gravity may degrade

aircraft performance, reduce aircraft life, or contribute to unsafe aircraft operation.

A significant weight and balance difference is defined in the Joint Service Technical

Manual for Aircraft Weight and Balance, Tech Order 1-1B-50 dated 30 September 2011,

as greater than 0.4% of Basic Weight or 0.2% Mean Aerodynamic Chord for aircraft with

a Basic Weight less than 75,000 pounds, or as greater than 0.5% of Basic Weight or 0.5%

Mean Aerodynamic Chord for aircraft with a Basic Weight greater than 75,000 pounds.

Weighing results with differences from predicted in excess of this tolerance require a

complete reweighing, usually including completely refueling and defueling the aircraft, a

time consuming and costly effort in terms of use of resources and lost availability time

for the aircraft. A recent survey of measured to predicted weight and center of gravity


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agreement for an operational fighter aircraft has shown that a high proportion of

operational aircraft weighings have been accepted without reweighing, even though

outside the allowable range, likely due to the difficulty and expense involved in

reweighings and the perceived difficulty in avoiding or reconciling weighing error. This

RP provides guidance by way of a “fishbone” diagram on possible sources of error to aid

in reconciling, correcting, and potentially reducing occurrences of weighing measurement

error and a failure / root cause investigation process for aiding in the identification and

elimination of error sources.

3. REFERENCE DOCUMENTS

The following specifications and recommended practices of the latest revision are herein

referenced for this Recommended Practice.

3.1 MILITARY SPECIFICATION

This recommended practice provides supplemental techniques for meeting the

requirements of the Joint Service Technical Manual, Tech Order 01-1B-50, dated 30

September 2011.

3.2 RECOMMENDED PRACTICE

This recommended practice provides supplemental techniques for meeting the

requirements of SAWE Recommended Practice 7, “Mass Properties Management and

Control for Military Aircraft.”

4. DEFINITIONS Term Definition

TBD TBD


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5. WEIGHT RECONCILIATION PROCESS

Weighing an aircraft is not easy. An aircraft weighing takes a long time to set up,

involves coordination with a great number of people from multiple organizations whose

sense of priorities are varied. The process has many opportunities for pitfalls, and there

is usually a great deal of pressure from the supporting cast and supervision to take short

cuts and complete the weighing faster. By the time the scales are being read, it has

already been a long day (or two). If results don’t match predictions and no errors in

predictions are evident, usually the only option is a reweigh.

There are many potential sources of weighing error. Understanding where these errors

can be introduced is key to reconciling what could have gone wrong with a weighing that

caused large actual to predicted discrepancies. Knowing these sources of error ahead of

time and taking steps to avoid them during the weighing process is best.

5.1 CAUSE AND EFFECT DIAGRAM (FISH BONE DIAGRAM)

Figure 1 is provided as an aid in capturing and describing sources of weighing error.

Sources of differences between measured and predicted weight and center of gravity are

subdivided into categories: Prediction, Measurement, Methods, Configuration,

Personnel, Environment, and Equipment. These categories are represented by the

“bones” on the chart. All of these potential sources of error contribute to the overall

difference in weighing results from predicted.

Potential sources of weighing error during the aircraft operational phase are shown in

black font; additional sources of error during the aircraft manufacturing phase are shown

in green font. Operational weighing predicted weight and center of gravity are normally

based off of incremental changes added to a previous actual aircraft weighing. During

aircraft manufacture, however, sources of differences between actual and predicted are

greater than in operational weighings because predicted weight and center of gravity are

based on calculations only, without the benefit of the aircraft having ever been previously

weighed.

Operational and Manufacturing Phase categories are Measurement, Methods, Personnel,

Environment, and Equipment. The category Method captures sources of error introduced

by execution errors or application of improperly techniques. Equipment sources of error

are related to weighing equipment malfunctions or failures. The Measurement category

captures physical measurement errors, such as misreading the tape measure or scale

displays. Environment error sources are related to the physical environment in which the

weighing is being conducted. Personnel describes errors introduced by individuals

participating in the weighing.

Figure 1. Cause and effect diagram for sources of error during aircraft weighings

Prediction

Weight EmptyPropulsion configuration

Missing or incorrect records

Incorrect effectivity of records

Engine Fluids weight incorrect

Failure to account for all Column I items

Column I ItemsTooling/weighing equip weight incorrect

Trapped Fuel incorrect

Basic Weight

Government Furnished Equipment

Aircraft Fluids weight incorrect

Supplier weights incorrect

Weighing

Error

Measurement

Incorrect elevation, latitude settings

Error recording the displayed values

Error reading inclinometer

Error in measuring strut chrome*

Incorrect geometry measurements*

Equipment

Old/weak batteries

Bad scale or load cell calibration

Damaged scale/load cell

Damaged jacks

Incorrect load cell/platform size

Inclinometer out of calibration*

Leveling bar not installed correctly*

Leveling lug incorrectly installed*

Environment

Non level floor

Uneven floor

Temperature Variation

Electrical interference

Wind/air flow

Incomplete/Improper Training

Records not maintained

Not following TOD

Incorrect, incomplete TOD

Personnel

* Primarily affects CG

MethodsFailure to Exercise scales

Failure to Rotate scales

Attempting to “shim” scales

Not leveling aircraft prior to jacking

Leveling laterally while on platform scales

Improper defuel

Improper aircraft configuration


Tire resting against wheel chock

Moving the aircraft onto platform too quickly

Non level weighing*

Tires not centered on platform scales

Configuration

Manufacturing variation

Structure

Supplier components/parts

Sealant

Finishes

Manufacturing induced trapped fuel

Part substitution

Aircraft alignment and symmetry*

Leveling lug incorrectly installed*

Manufacturing Weighings Only

Aircraft fuel scavenging system

Aircraft fuel probe operation

“Hit the Scales” prediction

Previous weighing was in error


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Manufacturing Phase categories are Prediction and Configuration. Prediction errors are

associated with errors in the weight records or calculations involved in developing of

original predicted weight and center of gravity of the aircraft. Configuration error

sources are due to the aircraft not being assembled as expected or configured as expected

during the weight and balance measurement step in the aircraft production line.

The remained of Section 5 of this RP discusses, for each category, each potential source

of error.

5.2 METHODS 5.2.1 Failure to exercise the scales Prior to taking the first reading on a set of scales placed in a new position during a

weighing, first load the scales by either pulling the aircraft onto the scales, if platforms,

or by lifting the aircraft by the jacks, if on top of jack load cells. Check the readings to

verify that the scales are behaving normally and are displaying a result that is close to

what you are expecting. Then unload the scales, zero any tares, and reapply the load and

proceed with the weighing. This allows verification that each scale is operating

normally, and allows the scales to seat into place. It is recommended that this be done

each time a platform or load cell is placed in a new position, including when scales are

rotated (see paragraph 5.2.2).

5.2.2 Failure to rotate the scales

Scales should be rotated among the reaction points for each set of scale readings. For

example, if three weight measurements are to be taken during a weighing using three

reaction points, the scales are placed in their first position, such as scale 1 under the

forward point, scale 2 under the right reaction point, and scale 3 under the left reaction

point. Exercise the scales (see 5.2.1), then load the scales and take the first readings.

When complete, unload the scales and move scale 1 to the right reaction point, scale 2 to

the left reaction point, and scale 3 to the forward reaction point. Exercise the scales

again, then load the scales and record the second readings. Compare the scales readings

at each reaction point with the previous reading. For example, compare the right reaction

point read using scale 2 during the first measurement against the reading of scale 1 under

the right reaction point in the second measurement. Do the same for all reaction points

watching for evidence of a scale reading erroneously high or low as compared to the

others. For the third measurement, move scale 1 to the left reaction point, scale 2 to the

forward reaction point, and scale 3 to the right reaction point, exercise the scales, and

take the third set of measurements. If aircraft pitch and roll attitudes are unchanged from

measurement to measurement, the weight at each reaction point should remain relatively

unchanged (within the scale measurement tolerance). Comparing what each scale reads

to what the other scales read at each reaction point can immediately identify a scale that

is not behaving correctly. If a scale is suspect, replace it with a spare.


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5.2.3 Attempting to “shim” the scales

Do not attempt to weigh aircraft on uneven, “lumpy”, or non-level floors. While

Technical Orders cite limits for non-level floors, they don’t discuss “lumpy” floors where

bumps in the floor can cause platform scales to rock or jacks to lean. Do not attempt to

use shims under platform scales or under the foot of a jack to try to limit rocking. Flat,

level floors must be selected for use in aircraft weighings; shimming can damage the

scales, introduce false readings, and can cause a potentially hazardous condition.

5.2.4 Not leveling prior to jacking

Aircraft must be level prior to jacking the aircraft while accomplishing a weighing using

top-of-jack load cells. Failure to having the aircraft level before beginning to lift the

aircraft with top-of-jack load cells cause side loads in the load cells. Excessive side loads

will cause load cell error, could damage the load cell, and could cause the aircraft to slide

off of the jack. Level the aircraft prior to jacking by using the struts. Once level, place

the jacks with the top-of-jack load cells in place and begin the lifts, maintaining the

aircraft level attitude. (Remember to rotate load cells for each set of measurements per

5.2.2).

5.2.5 Leveling laterally while on platform scales

If your aircraft has outwardly canted main landing gear, significant changes in strut

deflection will introduce side loads into the platform scales under main gear.

Compression of the struts effectively reduces the distance between the tires which cause

the tires to slide toward each other. Extension of the struts increases the distance between

the tires which cause the tires to slide away from each other. Attempting to laterally level

the aircraft by inflating or deflating the main gear struts while the aircraft is on top of the

scale platforms introduces these side loads which can cause the platforms to read

incorrectly.

5.2.6 Tires not centered on the platform scales

Tires should be placed as close as possible to the center of the platform scale load plate.

Although many scale manufacturers build in some flexibility into where the tires can be,

platform scales will perform best if the tire is at the center of the load plate. As an aid,

mark an “X” at the center of your platform scale load plate, and strive for placing the

center of the tire on the mark.

5.2.7 Improper defuel

Care must be taken to perform the aircraft defuel exactly as stated in Technical Order

instructions. Trapped/Undrainable fuel is usually determined experimentally during the

aircraft model’s fuel calibration test. The manufacturer determines the optimal attitude at

which to defuel the aircraft to achieve a repeatable trapped/undrainable fuel condition,


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and then verifies this amount by weighing one or more previously unfueled aircraft

before and after fueling and defueling to determine the value to be deducted in Column I

of the Form B. Any deviation from defuel instructions may result in differences from the

manufacturer’s value and result in more or less fuel (and at a different center of gravity)

than TO values which will cause errors in weighing results.

5.2.8 Improper aircraft configuration

Remove all non-Chart A items such as external fuel tanks, bombs, ammunition, cargo,

crew members, and equipment not having a fixed weight and location in the aircraft.

These types of items are not part of the Basic Weight on the Chart A and, therefore,

should not be in the aircraft when weighed. Check all reservoirs and tanks for liquids

such as drinking and washing water, engine oil, hydraulic fluid, anti-icing fluid, cooling

fluids, and liquid oxygen. Reservoirs and tanks should be empty or filled to normal

capacity prior to weighing. Never weigh aircraft with partially filled reservoirs or tanks.

All waste tanks shall be empty. Ensure that all doors and panels are closed except as

allowed by TOD. Verify that all moveable control surfaces are in the neutral position.

Fully close the canopy, and raise the arresting hook to the stowed position. If doors must

remain open, ensure that doors are symmetric so as to not cause a lateral asymmetry.

(Reference Joint Service Technical manual – Aircraft Weight and balance, September

2011.)

5.2.9 Failure to account for all Column I items

Check to ensure chocks, flight gear, survival kits, fly-away gear, blade ropes, engine

covers, and other non-Basic Weight items were removed or have been correctly

accounted for in Column I. Check that aircraft doors and panels were in proper

configuration.

5.2.10 Tire resting against wheel chock

In platform scale weighings, ensure that tires do not rest against scale mounted wheel

chocks which, in turn, may transfer load onto the platform scale frame. If load is

transferred off of the load plate, the scale will report inaccurate values.

5.2.11 Rapidly towing the aircraft onto platform scales

As an aircraft is moved onto platform scales, the tire transitions from the floor to a ramp

and onto the platform scale. The platform can move as it accepts the weight of the

aircraft moving onto it. Carefully and slowly moving the aircraft onto the scales

minimizes the energy involved in these movements, lessening the potential for damage

and mis-readings. Be sure the aircraft is towed at a slow and steady rate on to the scales

to prevent the scales from tipping, sliding or being pushed out of position.


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5.2.12 Non level weighing

An aircraft’s center of gravity is located at a significant distance above the scales.

Because of this vertical center of gravity component, a non-level attitude will cause the

center of gravity as projected onto the ground, and thus reacted to by the scales, to be

different from the center of gravity of the aircraft when level. Unless the aircraft

manufacturer has provided equations, tables, or graphs to enable adjustment for non-level

weighing attitudes, any deviation from level will cause center of gravity calculations to

be incorrect. If non-level weighing adjustments are available, be careful to apply them

correctly (pay attention to nose up or down sign conventions, and left side up or down

sign conventions).

5.3 EQUIPMENT 5.3.1 Bad scale or load cell calibration The best way to quickly identify a scale or load cell that is not in calibration or has been

calibrated incorrectly is to compare it with others scales or load cells with different

calibration dates by rotating scales as described in 5.2.2. If load cells or scales do not

compare well, work with the calibration lab to determine which are correct. Variation in

readings should not exceed what could be expected from scale calibration ranges.

5.3.2 Old/weak batteries Most portable scales use rechargeable batteries to enable them to operate while

disconnected from power. As the batteries age, the reliability of the voltage supplied

deteriorates resulting in reduced reliability of the weights reported. Replace rechargeable

batteries annually for best results. Non-rechargeable batteries should be replaced more

frequently.

5.3.3 Damaged scale/load cell Examine the platform scales or load cells for damage prior to use. Platform scales are

subject to damage from deformed load plates, bent platform frames (typically from

lumpy floors or use of shims), and damaged displays from tires over running the load

plate. Inspect the battery compartment for signs of corrosion and inspect plugs for

damage. Where possible, inspect platform load cells for signs of binding against the

platform, loose connections, or hydraulic leaks. Load cells are susceptible to bent posts,

damaged cups, and damaged plugs. If damage is found, have the scale evaluated and

recalibrated. Again, the best way to quickly identify a scale or load cell that is not

behaving correctly or has fallen out of calibration is to compare it with others scales or

load cells (again, preferably with different calibration dates) by rotating scales as

described in 5.2.2. In all instances, be sure to follow the scale manufacturers

recommended maintenance procedures. Do not attempt to disassemble the scale in the

field as this should only be performed by a trained technician and/or during normal


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maintance/calibration intervals. If you suspect internal damage as observed or described

herein, return the scale to a calibration lab for further evaluation.

5.3.4 Damaged jacks A top of jack load cell weighing can be compromised by a damaged jack. If the feet of

the jack are bent such that the jack is not vertical, the load cell on top of the jack will not

be vertical, resulting in the introduction of side loads to the load cell. If jack cannot hold

pressure or is difficult to raise, keeping the aircraft level will be difficult and may result

in side loads or may be potentially hazardous to operate. Inspect jacks for damage and

ability to hold pressure.

5.3.5 Incorrect load cell/platform size Ensure that the platform or load cell being used is the correct size for the application.

The applied load should be in the middle of the range of the platform or load cell’s

capacity; for example, don’t try to use a 60,000 lb capacity platform scale to measure a

1000 lb load, and certainly don’t try to measure a 60,000 lb load with a 50,000 lb

capacity scale. In addition to capacity, be sure that platform scales are adequately sized

for the size of the tire. The size of the tire should not overwhelm the size of the platform.

5.3.6 Inclinometer out of calibration The inclinometer calibration can significantly affect weighing results two ways: 1) The

defueling procedure used for preparing the aircraft for weighing likely calls for a specific

pitch and roll attitude. An inaccurate inclinometer can cause trapped/undrainable fuel to

be different than expected, resulting in weight and center of gravity error. 2) An incorrect

inclinometer would also result in an error in the leveling of the aircraft for weight and

balance measurement, resulting in center of gravity measurement error (see 5.2.12).

5.3.7 Leveling bar not installed correctly On some aircraft the inclinometer is placed on a flat bar mounted on jig-located lugs in a

wheel well. Ensure that the leveling bar is seated solidly on the jig-located lugs, and is

not being influenced by nearby wiring or equipment. Verify that the leveling bar has

been built to print. If the leveling bar is not located correctly, the inclinometer cannot

report the correct pitch and roll attitudes, which may result in weight and center of

gravity measurement error.

5.3.8 Leveling lug incorrectly installed If aircraft leveling is dependent on an inclinometer placed on a leveling bar mounted on

jig-located lugs in a wheel well, ensure that the jig-located lugs are correct, that they have

been installed correctly, are the intended diameter, and have not been damaged or

replaced with an incorrect part. If the leveling bar is not located correctly, the


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inclinometer cannot report the correct pitch and roll attitudes, which may result in weight

and center of gravity measurement error.

5.3.9 Aircraft fuel scavenging system operation Trapped/undrainable fuel values used in the derivation of Basic Weight from measured

as-weighed conditions were determined by the aircraft manufacturer using fuel

calibration testing during aircraft development. Changes in how the aircraft defuels in

the operational environment can affect the amount of fuel remaining on the aircraft after a

defuel and drain for weighing, and thus cause errors in the weighing results. Sources of

variation in trapped/undrainable fuel associated with the defueling/scavenging, and

draining process include the following.

Aircraft or truck/building pumps not scavenging fuel correctly

Aircraft pump or fuel system failure

Blocked scavenge lines

Clogged fuel pumps

Clogged drain ports

Inadequate pressure applied to tanks during the drain process

Truck or Building pump failure

Fuel migrating back onto aircraft after defuel (defuel valve not closed)

Truck was full of gas so simply stopped defueling the aircraft

Aircraft defueling software sequence disrupted causing an out of sequence defuel

Aircraft with foam in their tanks pose special problems. Follow manufacturer’s

instructions carefully regarding defueling.

Watch for evidence of these sources of error during the aircraft weighing process. Verify

that the fuel truck called to accomplish the aircraft defuel has sufficient volume to accept

a full load of fuel taken from the aircraft. A partial defuel interrupted by a search for a

different fuel truck usually results in a bad weighing.

If it is suspected that the defuel process was not accomplished correctly, start over.

Better to spend an extra hour or two at this step than to go through the rest of the

procedure, get a bad weighing result, and have to repeat everything over again.

5.3.10 Aircraft fuel probe operation Due to design constraints precluding optimal drain locations some aircraft rely on fuel

probe readings to normalize trapped/undrainable fuel during the weighing procedure.

The intent of these probe readings is to correct for extra residual fuel over what would

normally be drained during the weighing defuel process. The consequence, however, is

that any failure or error in fuel probe indicated values will adversely affect the accuracy

of the weighing. Critically assessing the probe by probe indicated values will usually

identify when a probe is not behaving normally, allowing an opportunity to replace a

faulty probe or identifying when a probe reading is false and should not be used. Probe


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reading checks prior to use in the weighing procedure could avoid unnecessary repeat

weighings.

5.4 MEASUREMENT 5.4.1 Incorrect elevation, latitude settings The force applied to a scale for a given mass is dependent on gravitational acceleration.

Gravitational acceleration varies on the planet with elevation and latitude. Many scales

adjust for elevation and latitude to account for this variation. However, when scales are

calibrated, a hydraulic press is used to simulate the weight of the aircraft on the scale.

Because the hydraulic press is not gravity dependent, gravity adjustments for elevation

and latitude are not required. Therefore, if the scale has setting for elevation and latitude,

part of the calibration process is to set these settings to sea level and 45 degrees latitude.

As part of setting up the scales for use in each weighing event, the elevation and latitude

setting must be verified to ensure they are properly entered for the local elevation and

latitude of the weighing location.

5.4.2 Error recording the displayed values Errors in reading or recording displayed values are easy errors to make, but easy errors to

spot if multiple readings are recorded (see 5.2.2). Beware of translating values, and

beware of mixing up left hand and right hand measurements, and sign conventions used

in dimensions. Watch for inconsistent weighing results at individual scale locations for

each weighing reading. These can indicate scale error or can be the result of an

incorrectly recorded weight reading. Do not accept wide variation in readings at a

reaction point. The variation is indicating a problem that needs to be addressed.

5.4.3 Error reading the inclinometer Although inclinometers display readings to 1/100th of a degree, they are sometimes

confusing to interpret. When looking at the probe indicator, it can be hard to be sure

whether a positive reading indicates nose up or down, or which wing is high.

Inclinometers usually have a symbol to assist the user, but getting the direction right can

be made sure visually by slightly lifting the one end of the inclinometer and noting

whether the displayed value increases from zero or decreases toward zero. By visually

identifying which end of the inclinometer has to rise to approach zero, it is easier to know

how the aircraft attitude needs to be adjusted to reach level.

5.4.4 Error in measuring the strut chrome* Some aircraft weight procedures require measuring the strut chrome to determine tire

location. Ensure that measurements are made with the tape measure or other device

aligned directly along the centerline of the strut, not canted at an angle relative to the


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strut. Try swinging the tape measure through an arc and use the minimum measured

distance. If a table in TOD is used to convert the chrome measurement to axle location,

recheck the location read from the table.

5.4.5 Incorrect geometry measurements* Top of jack load cell measurements do not use projection of points to the floor, so this

category of weighing error source does not apply, except to note that dimension values

used on the Form B should be verified to ensure they are correct in accordance with the

weighing procedure. Do not simply accept the values entered on the Form B by the

technican accomplishing the last weighing; he or she could have been incorrect, or the

previous weighing may have been on platforms.

Platform weighings require that geometry be determined by projecting points to the floor.

Refer to Joint Services Technical Manual 01-1B-50, Naval Sea Systems Command for

detailed instructions and the aircraft -5-2 TO for aircraft specific instructions. The

instructions provided in these sources are complete and will not be duplicated here, but

ensure that these instruction were followed completely.

Note that dimensions are often determined by dropping a plumb bob to mark the points to

be measured onto the hangar floor, allowing reaction points to be accurately located on

the hangar floor by measuring from the marked points. Ensure that these points are

marked prior to moving the aircraft from the scales. Do not mark the points on the

platform scales. The scales are moveable and will shift when the aircraft is moved

making the marks invalid, and the top of the platform is usually offset vertically from the

floor which can cause measurement error due to tape measure “drooping”.

Do not use Dimensions B and D out of the a Sample Form B out of the Chart E nor from

previous Form B weighing results. These dimensions must be determined by

measurement for each weighing. If measurements were taken by placing the 10" mark of

the tape, be sure to subtract the 10 inches.

5.5 ENVIRONMENT 5.5.1 Non level/ uneven floors Ensure that the slope of the floor does not exceed 1/4 inch in one foot (1.2 degrees). Do

not place scales on or over a crack, or drain in the floor. Jacks may straddle engineered

expansion joints providing elevation does not change. Ensure that the floor is not an

uneven, “lumpy” floor. “Lumpy” floors are where bumps in the floor can cause platform

scales to rock or jacks to lean. Do not attempt to use shims under platform scales or

under the foot of a jack to try to limit rocking. Flat, level floors must be selected for use

in aircraft weighings; shimming can damage the scales, introduce false readings, and can

cause a potentially hazardous condition.


19

5.5.2 Temperature variation Allow time for scales to match the temperature of the weighing area and floor or jack and

aircraft. Significant difference in temperature among the scale or load cell and may

affect weighing results. Opening hangar doors during the weighing event may cause

significant temperature change. Allow sufficient time for temperature stabilization.

5.5.3 Electrical interference Some weighing equipment is sensitive to electrical interference such as use of walkie

talkie radios or cell phones, and sometimes local transmission towers. Restrict use of

transmitting equipment at least three minutes before and during weight readings. If using

cords running to the scales or load cells, try picking up the cords and checking to see if

there is movement of weight readings. If so, this may indicate that the cords are picking

up interference that is affecting weighing results. An alternate weighing location away

from this interference may be required.

5.5.4 Wind/air flow Verify that the aircraft is being weighed in an enclosed, draft-free hangar, with fans and

air circulating equipment turned off. Many aircraft react to very slight air movement, and

scale readings will reflect this reaction.

5.6 PERSONNEL 5.6.1 Incomplete/improper training Personnel weighing an aircraft have many opportunities for mistakes as many seemingly

innocuous decisions are made during the course of accomplishing the weighing

procedure. Simple step by step decisions such as deciding how precise to set the aircraft

attitude during defueling, whether two or three repeat weight readings are sufficient,

whether the platform scales are centered under the tires well enough, judging if the top of

jack load cells are indeed vertical, and making the unpopular decision to reweigh after a

bad weighing. Predefined weighing checklists aid in ensuring weighings are conducted

in a complete and repeatable manner (see Section 6.0).

Weight technician efficacy, self confidence in knowing what has to be done and in

knowing that he or she is performing the task correctly is essential to commanding the

team of mechanics and line crew assembled to weigh the aircraft. Self doubt can lead to

accepting the improper deviations from procedure that are inevitably suggested by the

supporting cast. Knowing what has to be done, and why is has to be done that way

empowers the weight technician, and helps minimize process deviations.


20

5.6.2 Incorrect, incomplete TOD Flight Manual Managers work with contractors to ensure that every effort is made to keep

the Technical Order Data procedures and information correct and current. Regardless,

situations in the operational environment may arise that were not foreseen by the TOD

authors. If something doesn’t make sense, ask for help and guidance. Comments,

corrections, and questions regarding TOD shall be submitted in accordance with TO 00-

5-1 and the appropriate Technical Order Data Change Request (TODCR). These should

be forwarded to the appropriate Flight Manual Manager as specified in applicable TOD.

5.6.3 Not following procedures Review the procedures and ensure that each step was accomplished in accordance with

Technical Order Data. Weighing should be conducted with TOD readily available, and

each step must be accomplished as specified. If the TOD is believed to be incorrect

follow up after the weighing (5.6.2),

5.6.4 Records not maintained Ensure that all changes made to the aircraft since the last weighing have been properly

accounted. Review the Chart C for errors or omissions, and review previous weighing

results to determine if the difference is correcting a previous erroneous weighing. If an

aircraft modification has been accomplished since the last weighing, ensure that all

changes have been properly accounted for in the Chart C and that the Chart A inventory

has been correctly accomplished. Ensure that negative and positive sign conventions

have been entered correctly in all calculations, and verify that center of gravity values

have not been left blank (including lateral, if applicable).


21

5.6.5 Incorrect “Hit the Scales” Prediction

Calculation of the predicted aircraft weight and CG in the as-weighed condition is an

essential step in the preparation for weighing. A prediction of the as-weighed weight &

CG will help to quickly identify a bad (or good!) weighing result before the aircraft is

moved from the weighing site and before the aircraft configuration or fuel state changes.

Failure to have an expected weight and CG value during the weighing means that

troubleshooting can at best be done in hindsight, perhaps not at all. Having a complete

derivation of the weight and CG of the aircraft in the as-weighed condition available

during the weighing provides a means to verify that the actual aircraft configuration

matches the configuration described in the derivation. Reviewing previous results also

provides an opportunity to catch if a previous weighing had accepted although unusual

results; identifying that the previous weighing was erroneous may reconcile current

weighing discrepancies.

5.7 PREDICTION Section 5.7 and 5.8 are directed toward the manufacturing environment where each

aircraft is being weighed for the first time and whose predicted weight and center of

gravity is derived analytically by calculating the weight of tens of thousands of parts,

accumulated supplier weight reports, measured subassemblies and detail parts, and

calculated and measured fluid weights. These weights are accumulated by means of a

mass properties database which is used as the basis for the predicted weight and center of

gravity.

What is thought of as aircraft manufacturing variation is manifested in weight and

balance in two ways: as a shift, where average weight or average arm is consistently

biased in one direction from expected, such as heavy or light, CG forward or aft, left or

right. Manufacturing variation can also affect weight and balance as variation (or scatter)

about the shifted or unshifted average, such as a plus or minus variation from aircraft to

aircraft about predicted, or about some average shifted above or below predicted.

Ideally, over a production series of aircraft the average measured weight and center of

gravity is approximately equal to the predicted weight and center of gravity and the

variation about that average is small. This condition implies that the mass properties

database is representing the as-built aircraft well, that the manufacturing process is

consistent and under control, and that the measurement process is sufficiently accurate

and precise.

Aircraft to aircraft variation in measured weight and center of gravity from predicted

from aircraft to aircraft, swings back and forth above and below (or forward and aft) of

the average measured values is an indication of a highly variable manufacturing process

and/or that the measurement process is not precise. Poor measurement precision is the

subject of many of the fish bones discussed in the preceding sections and is as applicable

to the manufacturing environment as the operational environment.


22

In some cases, however, the average measured weight and center of gravity of a series of

manufactured aircraft is consistently different from predicted and biased in one direction,

such as always heavier than predicted, or always aft of predicted. This condition

indicates that the mass properties database does not adequately represent the as-built

aircraft, or that the measurement process is not accurate.

The mass properties database does not correctly represent the as-designed aircraft when

there are errors or omissions in calculations or bad records. The mass properties database

does not correctly represent the as-built aircraft when there are differences in the

manufactured aircraft from what is described in the nominal product design.

The Prediction fishbone, (Section 5.7), focuses on sources of potential error due to

differences between the as-designed aircraft and the as-designed aircraft, error that

usually manifests itself in a biased shift. The Configuration fishbone addresses potential

error due to differences between the as-designed aircraft and the as-built aircraft.

5.7.1 Weight Empty The following are potential sources of difference between predicted and measured weight

and center of gravity attributable to the Weight Empty of the aircraft. For the purposes of

this RP, the aircraft is divided into two parts; propulsion system and aircraft. Airframe

category consists all components making up the aircraft Weight Empty except the

engines.

5.7.1.1 Propulsion System For a prime airframe contractor, engine mass properties are usually provided by an

engine manufacturer. The engine manufacturer provides weight and center of gravity of

the engine, and these values are used in the derivation of aircraft predicted weight. Often

engine weight is a significant portion of the total aircraft weight, and has a large influence

on aircraft center of gravity. Potential contributions to aircraft weighing differences from

predicted to consider are:

Inaccurate engine calculated weights

o Changes not calculated correctly (math error)

o Components/drawings not reviewed & calculated

Inaccurate actual weights

o Actual weights were of incomplete assemblies or dry assemblies

o Misunderstanding of weighing configuration between engine manufacturer

and prime contractor.

o Scale error or error recording data by supplier.


23

5.7.1.2 Airframe The bulk of the mass properties database is dedicated to building up the weight and

center of gravity calculation details for the airframe. Every effort is spent on identifying

each detail part and assembly, each supplier component, the runs of every harness and

plumbing branch, the thickness and weight of each layer or finish, computing the volume

and weight of every drop of fluid in the aircraft, and calculating the weight of gases in

compressed gas bottles and tires. In these calculations weight and center of gravity data

are usually computed using nominal thicknesses, but may also include allowances for

dimension tolerances (often “half the permitted upper tolerance” thickness). In weight

critical aircraft, nominal as-designed dimensions are used in predictions so that over

nominal dimensions can be challenged for weight reduction if found during the

manufacturing phase.

The following are potential sources of difference between predicted and measured weight

and center of gravity that are usually attributable to the airframe. In the case or a weight

shift, a biased difference between predicted and measured weight, each line item should

be investigated as part of a root cause corrective action plan and to provide objective

evidence for or against whether the line item is a root cause.

Engineering changes unaccounted for or underestimated in mass properties database.

Material specification change made; weight impact not expected or not assessed.

o Engineering spec change made without coordination with or informing Mass

Properties.

o Aircraft build is out of tolerance for paint or sealant

o Material specification change made; weight impact not expected or not

assessed.

o Material densities different than advertised.

o Manufacturing processes are drifting to the high side of tolerances.

Manufacturing Review Board (MRB) engineering changes (repairs, work around)

changes not being captured in weight database

o Mass properties lacks visibility into MRB actions

o Weight effect of MRB action known but not assessed or mis-estimated.

o MRB Action weight impact not entered in mass properties database.

Design change not captured in mass properties database.

o Engineering Change Request (CR) not assessed.

o CR assessed, but effect of CR not understood; more impact/less impact than

expected.

o Inaccurate calculation of released drawings.

Changes miscalculated (math error)

Components/drawing change presumed to have no weight impact, so

not worked.

Interface components (miscellaneous standard parts, etc.) not

accounted for.

Wiring change (gage, wrapping, installation clamping, routing change.


24

Plumbing routing change; routing length change not assessed

correctly.

Plumbing routing change; change in fluids in lines not assessed

correctly

Drawings in Mass Properties backlog awaiting assessment; estimates

carried in database pending final assessment.

Database of standard part weights used for drawing calcs has incorrect

weight data.

Supplier design change

o Suppliers exceeding max spec weights

o Supplier weights increased from previously delivered weights to max spec

weight (i.e., Class II producibility improvements).

o Supplier not reporting correct "wet" weight.

Weight records in error in weight database

o Weight database not reconciled against aircraft build databases. Potential for

missing drawings or assemblies.

o Database software programming error allowing duplication or omission of

individual weight records.

o Error in serialization/version usage of weight impact of a design change.

Government Furnished Equipment (GFE) not weighing as promised

o GFE exceeding max spec weights

o GFE weights increased from previously delivered weights to max spec weight

(e.g., design change driven by another aircraft platform needs)

o GFE not reporting correct weight for current configuration, reporting an

alternate configuration,

5.7.2 Basic Weight Similar to Weight Empty error sources, there are sources of error introduced by fluids

and gases configurations, equipment added that are part of the Basic Weight

configuration. Basic Weight specific hardware items tend to be not as “fixed” in the

aircraft as Weight Empty items, and are thus more likely to be missing from the aircraft

calling for the need of a careful inventory (Chart A items). Fluids and gases that require

servicing require a known volume or pressure so that the weight and center of gravity

match a predicted level.

The following are potential sources of difference between predicted and measured weight

and center of gravity attributable to the Basic Weight of the aircraft. Although these

sources can manifest themselves as a biased shift from predicted, often these sources of

error cause variation from aircraft to aircraft if they are not in control or poorly

inventoried or understood. Each line item should be investigated as part of a root cause

corrective action plan and to provide objective evidence for or against whether the line

item is a root cause.


25

Engine not serviced as expected

o Additional trapped fluids

o Servicing status unknown or inconsistent

o Servicing records not correct

Unexpected aircraft fuel system change

o Fuel system modified by MRB action

o Mass Properties marked as not affected by CR (software change?)

o Fuel system not operating as expected by Fuel IPT (system design deficiency)

Alternate Mission Equipment (AME) weight and/or center of gravity incorrect

o AME weight not understood, miscalculated, and not verified by individual

weighing

o AME installed when should have been removed for weighing

o AME center of gravity not correctly determined (coordinate system

transformation error or simple miscalculation)

Non-Chart A items such as external fuel tanks, bombs, ammunition, cargo, crew

members, and equipment left in aircraft.

Fluids in reservoirs and tanks for liquids such as drinking and washing water, engine

oil, hydraulic fluid, anti-icing fluid, cooling fluids, and liquid oxygen not in a known

condition (either empty or filled to normal capacity prior to weighing.

Failure to empty waste tanks.

5.7.3 Column I/Column II Items As in operational weighings, understanding and correctly accounting for Column I and

Column II items on the Form B is critical in deriving an accurate Basic Weight from

measured readings. In the manufacturing environment additional challenges are present

over the operational environment because of manufacturing sequencing, variability in the

availability of parts, and constraints in assembly line steps. In addition, the manufacture

is developing the source data used for Column I and II items, and error in these weights

are potential sources of error.

The weight and/or CG of panels removed for weighing are not known, making Form

B corrections are in error.

Weighing equipment / tooling weight / CG not known

o Jack adaptors

o Canopy safety lock /

o Landing Gear safety pins

o Arresting hook support

Non-Chart A items such as bombs, ammunition, cargo, crew members, and

equipment left in aircraft.

Trapped fuel value incorrect

o Fuel calibration test not conducted correctly. While the details are outside the

scope of this RP, fuel calibrations tests should be conducted using similar

equipment and under conditions that will be typical for future operational

aircraft weighings.


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o Fuel calibration test not valid for aircraft variant.

o Defuel/drain pitch angle not adequate for achieving consistency in trapped

fuel values.

o Defuel not accomplished per procedure

o Personnel not sufficiently trained in defuel and draining procedures

o Defuel and draining procedures incomplete, inadequate, or insufficiently

documented

Incorrect aircraft inventory or configuration

o Servicing of fluids has changed from previous weighing configurations

o Fluids now in areas not filled during fuel calibration.

o Fluids are different in aircraft variant.

Coatings resequenced

o Build Plan resequenced without engineering input/notification

GFE or AME installed but not correctly accounted for

Additional equipment not identified during physical inventory

Additional equipment not identified during electronic/records inventory

Weight of GFE/AME different than expected.

Aircraft configuration unknown

Aircraft inventory not known (non-Chart A equipment missing)

Aircraft weighing occurring at new point in build sequence

Aircraft weighing occurring at different point in build sequence for one variant versus

another.

5.8 CONFIGURATION The Configuration fishbone addresses potential error due to differences between the as-

designed aircraft and the as-built aircraft. Major sources of configuration-related sources

of error associated with inventory are addressed in other sections (5.1 and 5.7), so are not

repeated here. The configuration sources of error addressed in this section are associated

with differences between the as-designed aircraft and the as-built aircraft. These

difference can manifest themselves as a weight or center of gravity biased shift (more

often seen in hardware), or can be variable (mostly found in fluids variation or

measurement process precision errors). Measurement process is addressed in previous

sections, so will not be repeated here. This section focuses on hardware or fluids

manufacturing variation.

5.8.1 Manufacturing Variation Each line item should be investigated as part of a root cause corrective action plan and to

provide objective evidence for or against whether the line item is a root cause.

5.8.1.1 Manufacturing Induced trapped/undrainable fuel o Fuel drain/weep holes not allowing fuel to drain as planned.

o Excess fuel tank sealant clogging drain/weep holes


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o Drain/weep holes not drilled per engineering drawing requirements.

o Drain/weep holes inadvertently blocked by design changes made subsequent

to fuel calibration tests.

o Fuel scavenge lines not performing as designed.

o Scavenge lines pinched during manufacturing.

o Scavenge lines clogged with sealant/debris.

o Drain ports clogged with sealant/debris.

5.8.1.2 Manufacturing Variation in Structures and Systems o Additional fuel sealant, primer, filler, paint, finishes

o Over or upper side of tolerance thicknesses being applied for sealants (fuel,

environmental).

o Over or upper side of tolerance thicknesses being applied for primer.

o Over or upper side of tolerance thicknesses being applied for final finish.

o Material densities different than advertised.

o Excess fill and fair material (aerofill) being applied to improve smoothness,

reduce steps, or simply for better appearance.

o Change in part tolerances/as-built thicknesses

o Machining tolerances increased to reduce cost in machining operations.

o Casting tolerances increased to reduce scrap.

o Machining leaving additional material due to changes in milling/machining

speeds.

o Wear-out of machining bits leaving additional material

o

o Alternate supplier providing high side of tolerance forgings, castings, machinings.

o Suppliers have incorporated Class II design changes that have affected the weight of

their components.

o Change in supplier or sub-tier supplier resulting in component or part weight

differences

o Repair to structure or systems routing made during construction

o Allowable part substitution

o Substitution of steel fasteners as a replacement for titanium due to repair or

part availability.

o Alternate supplier part used as replacement

5.8.1.3 Alignment and Symmetry* A source of error that is related to the construction of the aircraft that can cause weight or

center of gravity error is aircraft alignment and symmetry. Platform scale weighing

center of gravity calculation from weighing measurements is very dependent on an

accurate knowledge of the geometry of the reaction loads calling for accurate use of truly

jig-point controlled dimensions. Top of jack load cell weighings rely on understanding

the geometry of the jack points. While jack points are normally well known and fixed, a

manufacturing twist or bend in the fuselage, while still within manufacturing tolerances,


28

may influence center of gravity measurement accuracy. Part of a center of gravity root

cause corrective action plan should be to verify the accuracy of points used to control the

geometry and dimensional measurement used in the aircraft weighing procedure.

Leveling the aircraft requires a known / controlled location on the aircraft, usually in a

wheel well, where a leveling bar and inclinometer or bubble balance is placed for use in

determining and controlling aircraft pitch and roll attitude. Defueling attitude and

leveling for weighing require that the geometry of this location be accurately known. If

the pitch/roll attitude is incorrect, aircraft defueling may produce unexpected amounts of

trapped/undrainable fuel and non-level weight readings will result in center of gravity

miscalculation error. Part of a weight and/or center of gravity root cause corrective

action plan should be to verify the accuracy of points used to control the geometry and

dimensional measurement used in the aircraft weighing procedure.

5.8.1.4 Leveling Lug Installation Accurate, jig-controlled installation of the leveling lugs is required to establish a known /

controlled location where a leveling bar and inclinometer or bubble balance can be placed

for use in determining and controlling aircraft pitch and roll attitude. As stated in 5.8.1.3,

defueling attitude and leveling for weighing require that the geometry of this location be

accurately known in order to avoid unexpected amounts of trapped/undrainable fuel and

non-level weight readings resulting in center of gravity miscalculation error. Incorrect

installation of these leveling lugs such as use of incorrectly dimensioned fasteners or

improper location on the aircraft should be investigated as part of a weight and/or center

of gravity root cause corrective action plan.


29

6. Weighing Event Checklists

Weighing event checklists are a means of ensuring a repeatable process under

comparable conditions. Checklists should reference documented Technical Order Data

(TOD), providing additional details and delineated steps as supplemental information, but

are never a means for replacing TOD.

A weighing event checklist should begin with aircraft preparation, describing each step

sequentially, assigning responsibility for each step, and providing graphical location

guidance as needed. Critical steps and tasks should be highlighted. Special attention

should be addressed to variable components such as consumable and expendable items,

especially defueling and fuel draining, and should include careful attention to stowage

items.

The weighing event checklist should provide guidance regarding weighing

environment/site preparation, including guidance for controlling environmental

conditions if needed. The weighing event checklist should direct the weighing

technicians to scale manufacturer requirements regarding weighing equipment

preparation, following the instructions provided in weighing equipment manufacturers’

manuals, but providing additional site-specific guidance as required.

Finally, the checklist should lead through the aircraft weighing procedure, providing

special handling advice for critical aspects of steps needed for controlling the aircraft

weighing attitude as well as warning and cautions for safety handling.


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7. Failure Investigation

It is strongly recommended that expected weighing results be prepared in advance of the

actual weighing and that the expected values be available at the weighing site at the time

of the weighing. Having the expected weighing results available at the weighing site at

the time of the weighing greatly facilitates trouble shooting weighing error. In addition

to having the expected Basic Weight and center of gravity available, it is important to

have an expected “hit the scales” weight and center of gravity for direct comparison to

reading that will be reported by the scales.

Criteria defining maximum acceptable weighing deviation from expected must be

established prior to the weighing event. When weighing results do not meet the given

acceptance criteria, a failure investigation is required. Failure investigations can be

facilitated through use of a flow chart to assist in narrowing down potential error sources

to likely probable causes.

The following flow chart presents an example of a top-level failure investigation:

Figure 2. Top level failure / root cause investigation flow chart

Weighing

Scatter OK?Acceptable Deviation?

Weighing OK

Performance investigation (scatter-related)

Performance investigation (deviation-related)

Failure Resolved?

Failure Resolved?

Weighing Equipment Investigation

Aircraft Inventory or Build Investigation

Repeat weighing (aircraft in prior weigh condition)

avoiding detected weighing performance

errors.


replacing suspect equipment components

(in case of no investigation result replace complete

weighing kit)


replacing suspect equipment components

(in case of no investigation result replace complete

weighing kit AND repeat aircraft preparation

including refuel/defuel and drain)

Failure Resolved?

Weighing Equipment Investigation

Repeat weighing avoiding detected weighing

performance deficiencies (repeat aircraft

preparation including refuel/defuel and drain)

Calculated adjustment acceptable

?Correct A/C configuration and reweigh (aircraft may

be in prior weigh condition if unaffected by

configuration change, otherwise repeat aircraft

preparation including refuel/defuel and drain)

Correct predicted weight data.

YES

NO

YES

NO

YES NO YES

NO YES

NO YES

NO


31

The flow chart of Figure 2 illustrates two main branches:

Scatter-related investigation branch

Deviation-related investigation branch

A scatter-related failure investigation only applies in the case where multiple weighing

measurements are conducted in the course of a single weighing event. This is the typical

practice for most government and aircraft manufacturer’s weighing instructions as

multiple weighing measurements provide the means of detecting unusual scatter in

weighing results. Because the aircraft configuration is unchanged during the course of

these multiple measurements, unacceptable scatter cannot be attributable to defuel

variation or other aircraft weighing configuration issues. Unacceptable scatter narrows

the probable causes to weighing equipment and/or weighing performance error.

A deviation-related failure investigation is initiated when weighing results are

unacceptably different than the expected results (i.e., a consistently repeated high or low

weight, forward or aft center of gravity, or left or right center of gravity). Such a

deviation can be caused either by incorrect weighing measurements or by incorrect

predictions of expected weights. Because expected weighing results are derived from an

expected aircraft configuration, the aircraft configuration must be considered as a

potential error source in addition to the potential error sources of the scatter-related

investigation. Also, note that often a basis of expected weight is a prior weighing

measurement; therefore, an erroneous prior weighing is also a potential source of error.

The top-level flow chart calls for the specific failure investigations identified by the

yellow, green, and orange colored boxes. These specific investigations are described in

Figures 3 through 8.

The following is the general flow of the Failure Investigation process. Based on

weighing results, determine if the results indicate a difference from predicted that is

scattered about the expected value or if the results differ from expected in a tight cluster,

but shifted away from the predicted weight and/or center of gravity (high or low, forward

or aft, or left or right). In the case where results are widely scattered AND have a shift

(deviation) from predicted there are likely multiple simultaneous error sources and all

branches must be investigated.

If the error can be categorized as scatter related, follow the left major branch of the flow

diagram in Figure 2. First conduct the performance investigation that is tailored toward

scatter related probable causes as described in Section 7.1. If the root cause is identified

and resolved by the scatter related performance investigation, the root cause is not aircraft

configuration related, and the aircraft can be reweighed in the current condition without

the need for repeating the inventory, fuel, defuel, and drain aircraft preparation steps.

If the root cause cannot be identified and resolved by the scatter-related performance

investigation, the next step is to conduct a scatter related weighing equipment


32

investigation as described in Section 7.4. If the Scatter Related Weighing Equipment

Investigation identifies the root cause of the scatter error, the root cause is not aircraft

configuration related, and again, the aircraft can be reweighed in the current condition

without the need for repeating the inventory, fuel, defuel, and drain weighing steps.

If the error is categorized as deviation related, follow the right major branch of the flow

diagram in Figure 2. First perform the performance investigation that is tailored toward

deviation related probable causes as described in Section 7.2. If the root cause is

identified and resolved by the deviation related performance investigation, the root cause

was aircraft configuration related, and the aircraft must be reweighed completing all steps

of the aircraft preparation process including inventory, fuel, defuel, and drain procedures.

If the root cause cannot be identified and resolved by the deviation related performance

investigation, the next step is to conduct an aircraft build investigation as described in

Section 7.3. If the aircraft build investigation identifies the root cause, perform the

mathematical correction as needed. If corrected results are acceptable, the aircraft does

not have to be reweighed and the corrected data may be used to close out the weighing

process. If the results are still not acceptable, make the mathematical corrections as

needed to the predictions, correct any identified configuration issues, and reweigh the

aircraft. If the aircraft configuration corrections were not associated with elements

affected by aircraft preparation steps (such as defueling or fluid levels) the aircraft may

be reweighed without repeating those steps.

If the root cause cannot be identified and resolved by the Aircraft Inventory or Build

Investigation, the next step is to conduct the deviation related weighing equipment

investigation as described in Section 7.5. If the Deviation Related Weighing Equipment

Investigation identifies the root cause of the deviation error, the root cause is not aircraft

configuration related, and the aircraft can be reweighed in the current condition without

the need for repeating the inventory, fuel, defuel, and drain weighing steps.

If the above process steps have not identified the root cause and resolved the weighing

differences from predicted, the aircraft must be reweighed, including all steps in the

aircraft inventory and aircraft preparation steps including refueling, defueling, and

draining the aircraft until TOD criteria have been satisfied.

The mapping of the Failure investigation Flow Chart of Figure 2 to the Fishbone Diagram

of Figure 1 is summarized as Figure 3. To aid in the investigation, Figure 3 provides

reference to the paragraph and page number in Section 5 of this document where each

error source is discussed. Figure 3 also provides insight into the primary and secondary

effects of the root cause, an impact weight and/or center of gravity, whether the affect

would likely appear as in a deviation or a scatter of results, and a color code pointing to

the investigation box in Figure 2 that pursues each root cause.

Once a root cause has been identified and corrected, the post failure investigation

recommendation of the reweighing requirement is also provided in the most right column

for Figure 3.


33

Figure 3. Failure investigation / root cause table.

Fishbone Page

Scatter or

Deviation

Branch

WeightCenter of

GravityDeviation Scatter Deviation Scatter

5.2

5.2.1 Methods 11 D, S ● ○ ● ● ● ● Reweigh in current configuration

5.2.2 Methods 11 D ● ○ ● ● Reweigh in current configuration



5.2.5 Methods 12 S ● ○ ● ● Reweigh in current configuration

5.2.6 Methods 12 S, D ● ○ ○ ● ○ ● Reweigh in current configuration

5.2.7 Methods 12 D ● ○ ● ● Reweigh including aircraft preps

5.2.8 Methods 13 D ● ○ ● ● Reweigh including aircraft preps (See Note 1)

5.2.9 Methods 13 D ● ○ ● ● Correct calculation error, no reweigh

5.2.10 Methods 13 D, S ● ○ ● ○ ● ○ Reweigh in current configuration


5.2.12 Methods 14 D ● ● Reweigh in current configuration

5.3

5.3.1 Equipment 14 D ● ○ ● ● Reweigh in current configuration

5.3.2 Equipment 14 D, S ● ○ ● ● ● Reweigh in current configuration


5.3.4 Equipment 15 S, D ● ○ ○ ● ○ ● Reweigh in current configuration

5.3.5 Equipment 15 D, S ● ○ ● ● ● ● Reweigh in current configuration

5.3.6 Equipment 15 D ○ ● ● Reweigh in current configuration (see Note 2)


5.3.8 Equipment 15 D ○ ● ● Reweigh including aircraft preps

5.3.9 Equipment 16 D ● ○ ● ● Reweigh including aircraft preps


5.4

5.4.1 Measurement 17 D ● ○ ● ● Reweigh in current configuration

5.4.2 Measurement 17 S ● ○ ● ● Reweigh in current configuration

5.4.3 Measurement 17 D ○ ● ● Reweigh in current configuration

5.4.4 Measurement 17 D ● ● Correct calculation error, no reweigh


5.5

5.5.1 Environment 18 D, S ● ○ ● ● ● ● Reweigh in current configuration


5.5.3 Environment 19 S, D ● ○ ○ ● ○ ● Reweigh in current configuration


5.6

5.6.1 Personnel 19 D ● ○ ● ○ ● ○ Reweigh including aircraft preps



5.6.4 Personnel 20 D ● ○ ● ● Correct calculation error, no reweigh


● Primary effect of root cause D

○ Secondary effect of root cause S

Note 1 If incorrect aircraft configuration can be corrected without affecting fuel, fluids, or gases, the aicraft may be rewieghed without repreparations.

Note 2 If aircraft attitude during defuel may have been affected by root casue, reweigh aircraft including all preparation steps.

Deviation from Expected Weight and/or Center of gravity

Scatter in weighing results during multiple weighing during a single weighing event.

Section No.

Incorrect "Hit the Scales" prediction

Wind / air flow

Personnel

Incomplete / Improper training


Not following procedures


Error in measuring the strut chrome

Incorrect geometry measurements

Environment

Non level / uneven floors

Temperature variation


Aircraft fuel scavenging system operation


Measurement



Error reading the inclinometer


Damaged jacks


Inclinometer out of calibration

Leveling bar not installed correctly

Leveling lugs incorrectly installed


Moving the aircraft onto platforms too quickly

Non level weighing

Equipment


Old/weak batteries

Not leveling prior to jacking


Tires not centered on the platform scales

Improper defuel



Methods

Failure to Exercise the scales

Failure to rotate the scales

Attempting to shim the scales

Discrepancy Weight Center of Gravity

Investigation

Box Color

Code

Post Failure Investigation - Reweigh Action

required after Error is Corrected

Root Cause / Error Source


34

7.1 SCATTER-RELATED WEIGHING PERFORMANCE INVESTIGATION

A scatter-related weighing performance investigation should be performed immediately

at the weighing location together with the involved personnel prior to removing weighing

equipment and prior to allowing any aircraft movement or reconfiguration. Any delay in

the investigation may lead to unsolvable trouble shooting and unnecessary multiple

reweighings.

Figure 3 can be reduced to focus on possible scatter related weighing performance root

causes as shown in Figure 4.

Figure 4. Scatter related weighing performance investigation.

Figure 4 demonstrates the process by which the list of root causes can be filtered to focus

on weighing performance errors that would most often result in a scattering of weight or

center of gravity values. Each of these potential root causes can be investigated

individually to determine if the weighing event may have compromised by any of these

errors.

Fishbone Page

Scatter or

Deviation

Branch

WeightCenter of


5.2




5.2.5 Methods 12 S ● ○ ● ● Reweigh in current configuration




5.3

5.4

5.4.2 Measurement 17 S ● ○ ● ● Reweigh in current configuration

5.5





5.6



Section No.


Scatter in results during multiple weighings during a single weighing event.

Wind / air flow

Personnel

Environment




Measurement




Equipment




Methods




Investigation

Box Color

Code





35

7.2 DEVIATION-RELATED WEIGHING PERFORMANCE INVESTIGATION

Figure 5 demonstrates the process by which the list of root causes can be filtered to focus

on weighing performance errors that would most often result in a deviation in weight or

center of gravity values. Again, each of these potential root causes can be investigated

individually to determine if the weighing event may have compromised by any of these

errors. Note that some of the root causes can manifest is deviation or scatter, as noted by

listing a D and an S designation in the Scatter or Deviation Branch column of Figure 5.

Figure 5. Deviation-related weighing performance failure investigation.

Many of these potential root causes for error can be corrected and, once the error source

has been eliminated, the aircraft can be reweighed without aircraft re-preparation. At

minimum these potential sources should be investigated prior to moving the aircraft from

the weighing site and prior to any aircraft configuration changes.

Fishbone Page

Scatter or

Deviation

Branch

WeightCenter of


5.2


5.2.2 Methods 11 D ● ○ ● ● Reweigh in current configuration




5.2.7 Methods 12 D ● ○ ● ● Reweigh including aircraft preps

5.2.8 Methods 13 D ● ○ ● ● Reweigh including aircraft preps (See Note 1)



5.2.12 Methods 14 D ● ● Reweigh in current configuration

5.3

5.3.8 Equipment 15 D ○ ● ● Reweigh including aircraft preps



5.4

5.4.1 Measurement 17 D ● ○ ● ● Reweigh in current configuration

5.4.3 Measurement 17 D ○ ● ● Reweigh in current configuration

5.5





5.6






Note 1 If incorrect aircraft configuration can be corrected without affecting fuel, fluids, or gases, the aicraft may be rewieghed without repreparations.

Section No.



Wind / air flow

Personnel

Incomplete / Improper training


Not following procedures

Environment




Aircraft fuel scavenging system operation


Measurement


Error reading the inclinometer

Leveling lugs incorrectly installed



Non level weighing

Equipment



Improper defuel


Methods


Failure to rotate the scales



Investigation

Box Color

Code





36

7.3 AIRCRAFT INVENTORY OR BUILD INVESTIGATION

The table in Figure 6 describes the general investigating process for identifying errors

associated with the aircraft having a different configuration than expected, or conversely,

an error in the development in the expected weight and/or center of gravity.

Figure 6. Deviation-related aircraft inventory or build failure investigation.

The aircraft build investigation can be more detailed and time consuming, so except for a

visual review of the aircraft configuration for obvious missed aircraft mounted

equipment, checking scale calibration and a review of scale performance during scale

rotation, errors in recording scale readings or scale position, the majority of this effort is

conducted after the weighing.

7.4 SCATTER-RELATED WEIGHING EQUIPMENT INVESTIGATION

Figure 7 describes the general investigating process for identifying errors associated with

the weighing equipment that would result in high weighing result scatter.

A scatter-related weighing equipment investigation should be performed immediately at

the weighing location together with the involved personnel prior to removing weighing

equipment and prior to allowing any aircraft movement or reconfiguration.

Figure 7. Scatter-related weighing equipment investigation.

Fishbone Page

Scatter or

Deviation

Branch

WeightCenter of


5.2

5.2.9 Methods 13 D ● ○ ● ● Correct calculation error, no reweigh

5.3

5.4



5.5

5.6





Section No.



Incorrect "Hit the Scales" prediction

Personnel


Error in measuring the strut chrome

Incorrect geometry measurements

Environment

Measurement

Equipment


Methods


Investigation

Box Color

Code




Fishbone Page

Scatter or

Deviation

Branch

WeightCenter of


5.2

5.3





5.4

5.5

5.6



Section No.



Personnel

Environment

Measurement


Damaged jacks


Equipment

Old/weak batteries

Methods


Investigation

Box Color

Code





37

7.5 DEVIATION-RELATED WEIGHING EQUIPMENT INVESTIGATION

The table in Figure 8 describes the general investigating process for identifying errors

associated with the weighing equipment that would result in a significant difference in

weight and/or center of gravity from expected values.

Figure 8. Deviation related weighing equipment investigation.

Most failures associated with errors within the weighing equipment would likely be due

to equipment malfunction, damage, or improper calibration. As such, other than a review

of the equipment at the weighing site for loose connections, improper placement (such as

on non-level floors or scale placed spanning cracks in the floor), this kind of failure is

indirectly detected by repeated aircraft weighing using a different set of weighing

equipment and comparing results.

7.6 UNRESOLVED WEIGHING ERROR INVESTIGATION

Finally it should be realized that a failure investigation could also end up without

identifying a root cause. When a failure investigation fails to uncover root cause, the

weighing results must be confirmed through a repeat weighing before acceptance. This

must be achieved by way of a repeat aircraft weighing, including complete aircraft

weighing preparation in accordance with TOD instructions (usually including refueling,

defueling, and draining the aircraft). It is recommended that the repeat weighing be

accomplished using both the original weighing equipment as well as an additional set of

backup equipment to aid in identifying equipment.

Fishbone Page

Scatter or

Deviation

Branch

WeightCenter of


5.2

5.3

5.3.1 Equipment 14 D ● ○ ● ● Reweigh in current configuration







5.4

5.5

5.6



Note 2 If aircraft attitude during defuel may have been affected by root casue, reweigh aircraft including all preparation steps.

Section No.



Personnel

Environment

Measurement


Damaged jacks


Inclinometer out of calibration

Leveling bar not installed correctly

Equipment


Old/weak batteries

Methods


Investigation

Box Color

Code





38

8. REFERENCES Lehnertz, Gregor, 2012, Optimization of Aircraft Weighing Results and Failure Investigation,

SAWE Paper No. 3542.

NAVAIR. 2011. Aircraft Weight and Balance, Joint Services Technical Manual 01-1B-50, Naval

Sea Systems Command.

SAWE RP 7D. 2004. Mass Properties Control of Military Aircraft. SAWE Recommended

Practice.

Ship Name ____________________________

Compartment Survey Data Sheet

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