Module 7 (Maintenance Practices) Sub Module 7.5 (Engineering Drawings, Diagrams & Standards)...

For Training Purpose Only ISO 9001:2008 Certified

PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES Category A/B1 Sub Module 7.5 - Engineering Drawings, Diagrams & Standards

PTC/CM/B1.1 Basic/M7/02 Rev.00 7.5 Mar 2014

MODULE 7

Sub Module 7.5

ENGINEERING DRAWINGS, DIAGRAMS & STANDARDS

ISO 9001:2008 Certified For Training Purpose Only


PTC/CM/B1.1 Basic/M7/02 Rev.00 7.5 - i Mar 2014

Contents

ENGINEERING DRAWING, DIAGRAMS AND STANDARDS -- 1

TYPES OF DRAWINGS AND DIAGRAMS -------------------------- 3

SYMBOLS ------------------------------------------------------------------ 13

DIMENSIONS ------------------------------------------------------------- 29

TOLERANCES ------------------------------------------------------------ 36

PROJECTIONS ----------------------------------------------------------- 40

IDENTIFYING TITLE BLOCK INFORMATION -------------------- 50

MICROFILM, MICROFICHE AND COMPUTERISED

PRESENTATIONS ------------------------------------------------------- 53

AERONAUTICAL STANDARDS -------------------------------------- 55

AIR TRANSPORT ASSOCIATION SPECIFICATION NO. 100 55

INTERNATIONAL ORGANISATION FOR STANDARDISATION

(ISO) ------------------------------------------------------------------------- 58

BRITISH STANDARDS (BS) ------------------------------------------- 58

MILITARY STANDARD (MS) ------------------------------------------ 58

AIR FORCE AND NAVY (AN) ----------------------------------------- 58

NATIONAL AEROSPACE STANDARD (NAS) -------------------- 58

WIRING DIAGRAMS AND SCHEMETIC DIAGRAMS ----------- 59

SCHEMATIC DIAGRAMS ---------------------------------------------- 61



PTC/CM/B1.1 Basic/M7/02 Rev.00 7.5 - 1 Mar 2014

ENGINEERING DRAWING, DIAGRAMS AND STANDARDS The purpose of an engineering drawing is to record and convey the designers requirements to other, interested, people. The drawing must, therefore, include sufficient information to enable production planning, manufacture, assembly, testing, inspection and subsequent maintenance of the particular component or assembly to be achieved in the most cost-effective manner. So that there can be no misrepresentation of drawings, it is essential that the person preparing the drawing and those using the drawing should have a knowledge of the methods of presentation symbols, terms, and abbreviations, used in the preparation of an engineering drawing. This section is not intended as a standard for the production of drawings, but should be regarded as a general guide to drawing procedures and interpretation. The reference for drawing practices, in the United Kingdom, is that produced by the British Standards Institution, (BSI), in their publication BS 308. There are other standards available, which supplement BS 308, such as the Society of British Aerospace Companies (SBAC) Technical Specification (TS) 88. Companies, that have design approval from the CAA or the JAA, can modify these standards to suit their own particular drawing requirements. They must, however, publish their preferred standards of drawing, to obtain the approval of their National Aviation Authority (NAA).

There are four main types of drawings recommended by the BSI, although there are many other types and sub-types of drawing used at different times. The main drawing types are the:

Single-part: unique parts or assemblies

Collective: parts or assemblies of similar shape, but of different dimensions

Combined: complete assemblies, including all individual

parts on a single drawing

Constructional: assembly drawing with sufficient dimensional and other information to describe the component parts of a construction.

A complete set of drawings for an aircraft, and any documents or specifications referenced on the drawings, represents a complete record of the information required to manufacture and assemble that aircraft. The manner, in which a set of aircraft drawings is arranged, enables any particular component, material, dimension, procedure or operation to be traced.




Drawings of individual parts contain all the information necessary to enable the parts to be manufactured to design requirements. The material specification, dimensions and tolerances, machining details and surface finish, and any other treatment required, would all be specified on the drawings. Sub-Assembly drawings are issued to convey specific information on the assembly of component parts. When the method of assembly involves welding or a similar process, the drawing will include details of any heat treatment or anti-corrosive treatment that may be necessary. Sub-assembly drawings are sometimes issued in connection with spares provisioning and also in instances where assembly would be difficult without special tools, jigs or techniques. Installation drawings are issued to clarify the details of external dimensions and attitudes of components, locations, adjustments, clearances, settings, connections, adapters and locking methods between components and assemblies. A main General Arrangement (GA) drawing of the aircraft and GA drawings of main assemblies and systems are also provided. These drawings usually contain overall profile particulars only, with locations and references of the associated main assembly and installation drawings. They also provide a guide to the identification of drawing groups used by the particular design organisation. Main Assembly drawings may also contain profile particulars only, but will include the information required for the assembly

of individual parts of sub-assemblies.

The sequence of assembly is given where appropriate but the information contained in single-part or sub-assembly drawings is not repeated. Parts, as such, are referenced but, in the case of sub-assemblies, only the sub-assembly will be referenced and not its individual parts. There are a number of other drawings, which are used to display alternative views of a component, or to show where that component appears in a system, while pictorial diagrams or charts, are used, to show complete or part representations of functional systems such as hydraulic and electrical systems.




TYPES OF DRAWINGS AND DIAGRAMS As an aircraft technician there are several types of drawings and graphic representations you must become familiar with. Each type of drawing is designed to transmit a certain piece of information. The most common type of drawing you will use is the working drawing. There are three classes of working drawings, the detail drawing, the assembly drawing, and the installation drawing. Other types of drawings include sectional drawings, exploded view drawings, block diagrams, logic flowcharts, electrical wiring diagrams, pictorial diagrams, and schematic diagrams. Each type of drawing is designed to transmit a certain type of information.

Detail drawings

When an aircraft is designed, a detail drawing is made for every

part. A detail drawing supplies all the information required to

construct a part, including all dimensions, materials, and type of

finish.

When needed, an enlarged section or a drawing of another view is added to make the drawing easier to understand. When a detail drawing is made, it is carefully and accurately drawn to scale and dimensioned. However, when a print is made, the copy may be made to shrink or stretch. Therefore, a measurement should never be scaled from a print. Instead, all measurements should be derived from the dimensions given. An example on detail drawing is given in fig. (a)




Assembly drawings

After individual parts are fabricated, they are assembled into various subassemblies with the aid of an assembly drawing. An assembly drawing depicts the relationship between two or more parts. These drawings reference individual parts by their part number and specify the type and number of fasteners needed to join them. Because there are detail drawings for each component, no materials are specified and only those dimensions needed to assemble the parts are included. An assembly drawing is shown in figure (a)




Installation drawings

All sub-assemblies are brought together in an installation drawing. This type of drawing shows the general arrangement or position of parts with respect to an aircraft and provides the information needed to install them. Like the assembly drawing, the bill of material on an installation drawing lists the fasteners needed, as well as any instructions required for the installation. In the figure (a) below dimensions are given only for those adjustments necessary for the part to function. Often, portions of an aircraft that are not involved in the installation are shown using phantom lines. This helps you locate where a part is installed. Parts that are used only as a reference are often identified by their part name and the word "Ref" is noted beside it.




Sectional drawings

When it is necessary to show the internal construction or shape of a part a sectional drawing is used. There are four types of sectional drawings, the revolved section, the removed section, the complete section, and the half section. Revolved section

In a revolved section, a portion of an object is turned or revolved to show a different view. In the example shown in figure (a), the I-beam has been broken in two places with long break lines and the cross section is shown between the breaks. When only the shape of a part needs to be shown, it is shown with either a revolved or removed section. The revolved section drawing is often used to illustrate simple items with no interior parts. Basically, a revolved section drawing shows how a part is sectioned and revolved to illustrate it from a different view. Removed section

Like the revolved section drawing, the removed section drawing is also used to illustrate simple objects. However, to do this, the object is cut by a cutting plane line and a section is removed to illustrate another angle. In a removed section drawing, the object illustrated is cut and a section is removed to illustrate another angle. An example of a removed section is given in figure (b)

Complete sections

Complex assemblies like cable connectors are typically shown incomplete sections. With this type of view, it is easy to identify individual parts within a complex assembly. This feature is further enhanced through the proper use of section lines. In the example given in figure (c), the use of a sectional view to illustrate a cable connector makes it easy to identify the units separate parts. Half-sections

When it is helpful to see the outside of a part as well as the inside, half-sections are made. With this type of drawing, typically the upper half of a drawing shows the internal construction of the assembly, while the lower half shows the entire assembly as it appears from the outside. The half-sectional view allows the inside and outside of a part to be seen at the same time. An example for the half sections is given in figure (d)




Exploded- view drawing

Illustrated parts drawings often make use of exploded view drawings to show every part in an assembly. In this type of drawing, all parts are typically in their relative positions and expanded outward. Both its physical appearance and its reference number, which is used on the parts list, identify each part. An exploded view of a cable end assembly is given in figure (a). Block diagrams

With electrical systems and electronic components becoming more complex, procedures and graphical aids have been developed to aid you in locating problems. One such aid is the block diagram. A block diagram consists of individual blocks that represent several components such as a printed circuit board or some other type of replaceable module. Since most of the maintenance needed on complex systems consists of identifying a malfunctioning subassembly and replacing it, block diagrams greatly enhance this process. When using a block diagram you must trace the problem to the module that receives the correct input, but does not produce the required output. Once this is done, the module is removed as a whole and replaced. An example of such Block Diagram is given in figure (b).




Logic flowcharts

Logic flowcharts are another aid used in troubleshooting. A logic flowchart represents the mechanical, electrical, or electronic action of a system without expressing construction or engineering information. When using a logic flowchart, go to the oblong START symbol and follow the arrows through the logical testing sequence. As it can be seen on the example in figure (c) below, on most flow charts rectangular boxes explain a procedure, while diamonds identify questions that require a specific answer. In other words, after using a rectangular box to test something, you must match the existing condition before proceeding to the next course of action. Each diamond has one input and at least two outputs. In order to assure that all discrepancies are addressed, you must follow a flow chart to the oblong END.OF TEST symbol. In addition to identifying the probable cause of a problem, many flow charts specify a fix for each circumstance. By using this information, troubleshooting time is reduced to a minimum.

Figure C: Flow charts




SYMBOLS Purpose

A symbol is a visible sign used instead of a word or words to represent ideas, operations, quantities, qualities, relations, or positions. It may be an emblem, such as a picture of a lion to represent courage or an owl to represent wisdom. Symbols and abbreviations are used extensively in place of long explanatory notes on drawings and prints. A few of these symbols and abbreviations are common to every trade, whereas special trades have special symbols and abbreviations of their own.

Material symbols

The American National Standards Institute (ANSI) has standardized certain symbols used to represent materials in section view. It must be clearly understood that, when used, these symbols are intended only for general information and not to indicate specific types of material. For example, in the upper-left-hand corner of figure (a) below there is a symbol for iron, including cast iron and malleable iron, but it does not tell the specific type of iron to be used. Such information appears elsewhere on each print. The print reader must determine what metal is intended in each case. These material symbols are not generally used on section views unless it is desired to call special attention to section parts; hence their appearance is an invitation to observe the drawing closely. Colors are used more and more in modern illustrations in manuals and other textbooks or reference material, especially to show the flow of fluids or the movement of parts. However, the blueprinting process does not permit the use of color on ordinary drawings.




Finish and surface-roughness symbols

The surface of a metal part is "finished" by performing a machining, coating, or hand-finishing operation on that surface. Scraping, file-fitting, reaming, lathe turning, shaping, and grinding are some finishing operations. On many existing blueprints the symbol for a finished surface is a letter V with its point touching the surface to be finished, drawn with an angle of 60 between the sides of the V. Numbers may be placed within the angle formed by the sides of the V to represent the type of finish to be applied to that particular surface. When a part is to be finished on all surfaces, the abbreviation F.A.O. is sometimes used to represent "finish all over." Many manufacturers in the aerospace industry have adopted the root-mean-square (rms) micro-inch system of surface-roughness designation. This system has been standardized by the National Aerospace Standards Committee in Specification NAS30 and is also set forth in MIL-STD-10A. All new drawings of machined castings, machined forgings, and other machined parts will use this method of specifying surface finished. Surface roughness is a term used to designate recurrent or random irregularities that may be considered as being superimposed upon a plane or wavy surface. On smooth-machined" surfaces, these irregularities generally have a maximum crest-to-crest distance of not greater than 0.010 in. and height that may vary from 0.000001 to 0.00005 in.

Waviness" should not be confused with roughness, as the crest distances are much greater, generally running from 0.04 to 1.00 in and the height as much as several thousandths of an inch. The need for a simple control of the surface quality of a machined part by means of production drawings has long been apparent. Dimension tolerances as well as process notes such as "rough machine, " "smooth machine Finish", "grind", and "polish limit the surface characteristics in a general way but are not sufficiently specific to describe the desired result. By means of the rms system of surface-roughness designation, it becomes possible for the engineering department of any company to specify precisely the degree of finish required and for the shop to produce the specified finish without resorting to judgment. The rms average is a unit of measurement of surface roughness and is expressed in micro-inches. The micro-inch is one-millionth (0.000001) of an inch. The rms average is chiefly affected by the highest and lowest deviations from a mean surface and is a mathematical indication of average surface roughness. Figure (a) below symbolizes roughness numbering for surface-finish. The roughness number must always be on the left side of the long leg close to the horizontal bar as indicated in the figure (a). Figure (b) shows a straightened wavy profile with cuts due to machining.




The NAS Committee has selected a series of preferred roughness numbers that cover the range of aircraft requirements. These numbers are 2, 5, 10, 20, 40, 100, 250, and 500. They indicate maximum allowable or acceptable roughness of the surface on which they are specified in rms micro-inches. Figure (a) below shows some of the roughness numbers. Rms 500. This is a very rough, low-grade machine surface resulting from heavy cuts and coarse feeds in milling, turning, shaping, and boring, as well as from rough filing and rough disk grinding. This is also the natural finish of some forgings and sand castings. The extremely smooth finishes are indicated by rms 10, 5 & 2. Honing, lapping, micro-hone, polishing, or buffing produce these finishes. Root-mean-square 10 and the finer finishes may have either a dull or bright appearance, depending upon the method used to produce them. The surface appearance must not be considered in judging quality, but the degree of smoothness must be determined by "feel" or roughness-measuring instruments. Lay of a machined surface may be defined, for the purpose of this discussion, as the direction of tool marks or the grain of the surface roughness. Waviness and tool lay designations also covered in the National Aerospace standards Committee Specification NAS30.




Process and identification markings

Drawings will often call for identification markings on parts, and will indicate both the position of the markings and the method of application, e.g. rubber stamp. In addition, it is sometimes necessary to mark the component to show that a particular process has been carried out, and this will also be specified on the drawing. Symbols are normally used for this purpose, and some of the more common ones are shown in figure below. Some Design Organizations may use different symbols or code letters, which should be obtained from the Drawing Office Handbook, or similar publication, produced by the organization concerned. Some of the commonly used markings are given below.




Abbreviations

The use of abbreviations is not encouraged in the aerospace industry except where a saving of space is necessary. Use of capital letters is preferred on drawings and generally restricts the use of small (lowercase) letters to reports, manuals and other technical publications, where they are used along with capital letters. The period (.) is used after an abbreviation only when the abbreviation spells an English word. For example ADD. for additional, and AIL. for aileron are used with periods because the words add and ail are common English words. In any case of doubt about the use of an abbreviation, the work or words should be given in full. Each company standardizes its abbreviations in accordance with MIL-STD-12, a military specification for government drawings. An example list of abbreviations & symbols are shown in table (a) below




/Fine




Drawing practices

Types of lines

Before you can properly interpret drawings, you must first become familiar with the types of lines used to illustrate various concepts. Different line widths, arrowheads, and alternating breaks in lines all identify specific things. In order to display information contained in a drawing, lines with different appearances are needed. Lines can be in the form of a solid line, a dashed line, or a combination of the two. Furthermore, several drawings use three line widths or intensities, thin, medium, and thick. Figure (a) shows lines of different types and the following list describes the properties of lines used on aircraft drawings. Visible lines on outlines are used to illustrate a visible part. A visible line consists of a medium-weight solid line and is the most common type of line used on most drawings. Hidden lines indicate invisible edges or contours. Hidden lines consist of a dashed line of medium weight. Centerlines are made up of alternating long and short dashes and are used to show the middle of a symmetrical part. In the case of a hole, the exact center is marked by the intersection of two short dashes.

Extension lines are light lines that extend from the point where a measurement is made. These lines do not actually touch the visible lines of an object, but are approximately 1/16 inch from a part's edge. Dimension lines are light lines that are broken in the center so a dimension can be inserted. Typically, dimension lines have an arrowhead placed at each end and touch an extension line. This shows the exact location from which the dimension is made. All dimensions are placed so that they read from left to right. The dimension of an angle is indicated by placing the degree of the angle in its arc. Circular part dimensions are always given in terms of the circle diameter and are usually marked with the letter "D" or the abbreviation "DIA." The dimension of an arc is given in terms of its radius and is marked with the letter R following the dimension. Cutting-plane lines consist of medium or heavy alternating long dashes and two short dashes with an arrowhead at each end. A cutting-plane line is used to indicate the plane in which a sectional view of an object is taken. The arrowheads show the direction in which the view is seen and have letters to identify the section shown. Phantom lines are light lines made of alternating long dashes and two short dashes. These lines indicate the presence of another part and are included for reference or to indicate a part's alternate position. For example, a movable part is illustrated by solid lines in one position, and by phantom lines for its alternate position.




Short break lines are used across small dimensions to show that a part continues. Break lines are medium weight lines that are often drawn freehand. Long break lines are used across a large part and consist of a light line with a series of irregular breaks or zigzags. Long break lines usually extend beyond the solid lines indicating the edges of the part. Leader lines are light lines with arrowheads that extend from a note, number, or information box to a part. To minimize confusion, leader lines should never cross a dimension line, an extension line, or another leader line. Section lines are used to show differences in types of materials or exposed surfaces. Although different section lines can illustrate various materials, if the materials used are listed in the bill of materials, the symbol for cast iron is frequently used to represent all metals




Lettering

The most important consideration for an aircraft drawing is that it accurately portrays information. Therefore, lettering is often used to help identify some items. For legibility and speed, all lettering is done freehand, using single-stroke Gothic upper-case letters. For ease of reading, single-stroke Gothic letters are used on most aircraft drawings. When it comes to placing letters on a diagram it is common practice to draw very light guidelines and to space letters so there is approximately the same distance between them for uniformity. Appearance is what makes the lettering attractive and easy to read. Words should be separated by the amount of space required for the letter "I" with space on each side of it. Fractions are always made with a horizontal division line and numbers should be two thirds as high as whole numbers. The letters on a drawing are normally in a range of inch to as large as one inch high and may be drawn vertically or on a slant. Slanted letters make an angle of 68 from the horizontal. Figure (a) below shows standard lettering and the directions for making each stroke. Straight portions of lettering are drawn with one stroke from top to bottom or from left to right. Curved portions may be made with a clockwise or counterclockwise stroke, depending upon to which direction will produce best results.

Since all notes, dimensions, material specifications, etc., are read from the bottom of a drawing, all lettering and numbering should be made in horizontal lines with the letters and figures upright as viewed from the bottom of the drawing. This makes it unnecessary for the person using the drawing to turn it at an angle or sideways in order to read it easily. The lettering pencil should be of a hardness that will make a solid, black line on the drawing paper; the point of the pencil should be reasonably sharp but should have a slightly rounded point. This is necessary so that the pencil will not cut through the paper and so that the line will be of the proper width. The spacing of letters and words is important so that each word and figure will be clear and distinct from the others. The letters in each word should be close together with a uniform amount of white paper between each letter Proper and improper character spacing are shown in figure (b)




DIMENSIONS Working drawings must indicate all the necessary dimensions in a way most convenient for the workman. The size of the object or its separate parts is usually indicated in drawings by means of dimension lines, complete with figures showing the actual measurement irrespective of the scale. Dimension lines are made with fine continuous lines, so as to contrast with the heavier outline of the drawing. They are drawn parallel to the sections whose length they indicate and are terminated by carefully made arrowheads at the ends of the dimension line. Dimension figures must be written clearly and neatly to avoid confusion and possible errors. They should be written above and parallel to the dimension line and as close to its center as possible. An example is given in figure (a) The figures may also be inserted in a gap in the dimension line. If a view has a break, however, the dimension line must be drawn without a gap. Each dimension in a drawing must be given only once; duplicate dimensions should be avoided. Dimension lines are preferably (but not obligatory) drawn outside the drawing outlines. Avoid intersection of extension and dimension lines. When a series of parallel dimension lines are in close proximity to one another, the dimensions should be staggered. Dimension lines must not be the continuation of outlines, axial, center or extension lines.

On the other hand, the outlines, axial, center and extension lines must not be used as dimension lines. Arrowheads terminating the dimension lines must just touch the corresponding outlines, or centerlines, or extension lines. The size of arrowheads depends on the thickness of visible outlines and must be one and the same for all dimension lines of a given drawing. Extension lines must extend 2 to 5 mm beyond the ends of the arrowheads. When dimensioning very narrow spaces do as shown in figure (b). As is seen from the figure (b), if there is no room for arrowheads at the ends of dimension lines, arrange in a continuous chain, draw a leader line and place the dimension next. On half-sectioned views with an axis of symmetry it is permissible to dimension as in figure (c). In this case the dimension line must extend somewhat beyond the axis of symmetry.




For a circle the diameter is the only essential dimension. The diameter should always be given for a circle, since this is the dimension the workman will use. The correct method of placing dimension lines and numerals for diameters is shown in figure (a). When dimensioning a number of equi-spaced similar elements of a machine part, say, holes proceed as in figure (a) (usually only one hole is dimensioned and the number of holes indicated). It should be noted that for circular and angular dimensioning, centerlines could be used as reference. Also note that radius can also be used for arcs and circular dimensions. Figure (b) illustrates dimensioning circular arcs, while Figure (c) shows how spherical surfaces are dimensioned.

Fig (C) Dimensioning Spherical Surfaces




Where the center of an arc cannot conveniently be shown in its correct position, and yet needs to be located, one of the methods illustrated in figure (a) may be used. The portion of the dimension line, which touches the arc, should be in line with the true center. Figure (b) and (c) show the recommended practice for placing angular dimensions. Where space limitations do not permit giving a separate line for each dimension, the dimensions may be placed in one line as shown in fig. (c). in this method, called progressive (or consecutive) dimensioning, there is only one arrow for each dimension, thus indicating that each dimension goes back to the original base line.

Fig. (a) Dimensioning Where The Center of Arrow is not visible




A curved line composed of circular arcs should preferably be dimensioned by radii as in figure (a). or by ordinates as in figure (b). Ordinates method should only be used if the radii method is impracticable. Where the ordinates method is used, the ordinates should be close enough to reduce possible deviations of curves to reasonable amounts. The co-ordinates may be rectangular or polar.




TOLERANCES A tolerance dimension defines limits of size of a feature, and also has bearing on the geometrical form of the feature. Where the work piece is defined by limits of size only, accuracy of form may be achieved as a result of the inherent accuracy of the process used. In theory, the maximum material limit of size (i.e. the high limit of size of an external feature or the low limit of size of an internal feature) defines a maximum limit of perfect form for the relevant surfaces. In other words, if an individual feature is everywhere in its material limit of size, it should be perfect in form. If the individual feature is not on its maximum material size, errors of form are permissible provided no part of the finished surfaces crosses maximum material limit of perfect form and the feature is everywhere in accordance with its specified limits of size. Figure (a) shows the drawing specifications and figure (b) shows diagrammatically typical extreme errors of form. Drawings prepared for widespread quantity production at home or abroad, or for subcontracting in workshops of widely varying equipment and experience, may be quoted as particular case in which the most complete and explicit tolerance is necessary. This demands that the information given on the drawing be so complete in dimensional and geometrical requirements that the part may be made, and inspected, to suit the full requirements of the designer.

A tolerance of size, when specified alone, affects a degree of control of form, but in some circumstances dimensions and tolerances of size, however, well applied, and would not impose the desired control. If a different degree of control of form is required form tolerances should be specified in accordance with these standards. Such form tolerances then take precedence over the form control implied by the size tolerance.




Geometrical tolerancing Geometrical Tolerance is the maximum permissible overall variation of form or position of a feature. It defines the size and shape of a tolerance zone within which the surface or median plane or axis of the feature to lie. The zone within which the feature is required to be contained is called Tolerance zone. The diagram composed of the constructional dimensions, which serve to establish the true geometrical relationships between the positional features in one group, is called Geometrical Reference Frame Figure (a) shows the details of the Tolerance Zone and Geometrical Tolerance Frame. Unless otherwise specified, a geometrical tolerance applies to the whole length or surface of the feature. The requirement given in figure (a) is such that the centers of the 4 circles may be at the intersections of the geometric reference frame and each center should lie within a circle of 0.02 diameter tolerance zone. The geometric reference frame is a geometrically perfect square of side 40 mm. The geometrical tolerance is indicated in a rectangular frame, which is divided into compartments as shown in figure (a). The symbol for the characteristic [see table (a)] being tolerance is shown in the left hand compartment. The tolerance value (total value) in the units used for linear dimensions is shown in the second compartment from the left. In cases where datum or datum system is to be identified, a third compartment can be used. The height of this tolerance frame should not more than 7 mm.




PROJECTIONS Several methods are employed in representing three-

dimensional, solid objects on the flat surface of a sheet of paper

(or of other materials, used in producing engineering drawings).

The two common methods, used to depict components, in

drawings, are by:

Pictorial Projections Orthographic Projections.




Pictorial projections

Pictorial Projections provide a three-dimensional, single image of the object, as if it were being viewed, in perspective, by eye (in a similar manner to a painting or a photograph). The main types of pictorial projections (refer to Fig. 1) may be considered as the Perspective Projection, Oblique Projection and Isometric Projection. A Parallel Perspective Projection is when one of the principal faces is parallel to the picture plane. Whilst perspective and oblique projections are not normally, used in aircraft engineering drawings, they may sometimes, be used in Maintenance or Overhaul manuals, to provide initial images of uncomplicated components or to portray a general view of a constructional assembly. Isometric projections are the types mostly used for sketches and for the majority of images in Maintenance and many other manuals, used in aircraft servicing.

Plan

Side Front

Vanishing Point

30 30

Plan

Side Front

Plan

Side Front

45 or 30

Oblique Isometric

Parallel Perspective Projection

Pictorial Projections Fig. 1




Orthographic projections

Orthographic Projections are the types mainly used in the production of aircraft (and most other) engineering drawings of components and structures. They are drawn as if the viewer is infinitely remote from the object and rays (or projectors) lead out from the object so that the projection lines of opposite sides appear to be parallel. This method of projection provides a two-dimensional view of only one surface of the object. This means it must have multiple views (usually three, but there can be as many as six) of the relevant surfaces (drawn on three mutually perpendicular planes) to provide an accurate depiction of the whole object.

There are two conventions, used for orthographic projections (refer to Fig. 2), and they are: The older First Angle Projection The more recent Third Angle Projection. The internationally recognised symbol, of the truncated cone (frustum), is the key as to whether the First or Third Angle projection is being portrayed on a drawing.

Plan View

Front View

Orthographic Projections

Fig. 2

FIRST ANGLE PROJECTION

Side View Plan View

Front View Side View

THIRD ANGLE PROJECTION




The First Angle projection is being used when the truncated end of the cone is viewed and the two concentric circles are drawn at the remote end of the cone. In the same way, the surface of interest (of the object) is drawn remote from that surface in First Angle projections. Third Angle projections show the surface of interest drawn adjacent to that surface, in the same manner that the two concentric circles are drawn adjacent to the truncated end of the cone. Note; It is possible, on some drawings, to find the cone reversed (end for end), but the location of the two concentric circles, relative to the truncated end, will always provide the information as to how the drawing is to be read.




Axonometric projections

Types and methods of axonometric projection

Axonometric projections are widely used in engineering due to their pictorial force and simplicity of construction. Axonometric projections differ from orthographic projections in that in axonometric an object is projected only onto one plane of projection called the axonometric {or picture) plane. The drawing of the object is placed on the picture plane so as to expose three sides. This is shown in figure (a). In figure (a) the 3-plane orthographic projection is drawn in one plane axonometric projection. Exercises in constructing axonometric projections of objects help a great deal in acquiring the skill of reading and understanding the language of engineering drawings, as well as in developing the ability to visualize the shapes of three-dimensional objects and to feel the proportions of machine parts. In mechanical engineering axonometric projections are used as an auxiliary to orthographic projections of a mechanical part when the necessity is felt to give a clearer picture of its shapes, which are difficult to visualize from the orthographic projections. Without the axonometric picture it is very difficult to visualize the shape of the object from the three orthographic projections alone.

Axonometric projections of individual complicated parts or of a whole unit are often of great help in designing and developing new products. Also it is very difficult to read orthographic projections of piping or electrical network diagrams, especially where sections of piping or wire are situated in a vertical or a horizontal plane, since their projections overlap. The axonometric projection of the same piping or wiring makes it possible to easily realize the relative positions of the various sections. Axonometric projections are obtained on the projection plane by the usual method of projecting. One such projection is shown in figure (b). There are two such axonometric projections largely in use. They are

Isometric Drawings (Rectangular Isometric Projections)

Oblique Drawings (Rectangular Dimetric Projections).




Isometric drawings

Shown on figure (a) below is a simple geometrical object in isometric projections. With Isometric Drawings, the object is rotated so three sides are visible. In other words, to make an isometric drawing, an object is rotated so that three views are visible and touching the drawing plane. Also note that the three sides of the object are drawn at an angle of 120. This can be seen figure (b). In isometric drawings, all the edges of the object on the drawing must form the same angle to the drawing plane. And all distances are of same length for equal lengths in the object. This makes an isometric drawing fairly easy since no changes are made to any dimension. Since isometric drawing aids in visualizing a part, most pictorial drawings are illustrated in this way.

Oblique drawings

An oblique drawing is a drawing with one object face parallel to the drawing plane. In other words, two axes are perpendicular to each other, with the front of the object identical to the front view of an orthographic drawing. The depth axis of the oblique drawing is typically 45 degrees. Shown in figure (c) is an example for an oblique drawing and the plane arrangement is shown in figure (d). An oblique drawing is similar to an isometric drawing in that three sides of the object are visible. However, one of the object faces is parallel to the drawing plane. There are two special types of oblique drawings.

The cabinet drawing, and

The cavalier drawing A cabinet drawing gets its name from drawings used for cabinet work. In these drawings, the oblique side is at a 45 degree angle to the front side and is 1/2 the scale. This allows for an accurate and undistorted front view. The remainder of the drawing is present only to illustrate depth. Cavalier drawings use the same scale for the front view as the oblique side lines. However, the oblique sides are still set at a 45 degree angle to the front view. This creates a distorted picture of an object's true proportions. These drawings are primarily used when detailing is required on the oblique side.




Perspective drawings

A perspective drawing is used when you need to see an object similar to the way the human eye sees it. The basic difference between a perspective drawing and an oblique or isometric drawing is that on a perspective drawing the lines, or rays of an object meet at a distant point on the horizon. This point is referred to as the vanishing point. Perspective drawings are not generally used in aircraft drawings. In perspective drawings the rays that project from the drawing intersect at a vanishing point on the horizon.



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IDENTIFYING TITLE BLOCK INFORMATION An aircraft engineering drawing (refer to Fig. 7), must certain data, which is used to prove its validity (and legality). All alterations to drawings must be made in accordance with a drawing amendment system, which will ensure amendment to design records. If an alteration is made, a new issue number and date must be allocated to the drawing. To comply with legislation, procedures must be introduced to progressively amend the total definition of the product in terms of its associated list of drawings at specific issues. Each particular variant of a product and its state of modification must be identifiable in relation to the appropriate list of drawings. Title block

The title block is generally pre-printed and contains the essential information required for the identification, administration and interpretation of the drawing. It is recommended that the title block should be at the bottom of the sheet with the drawing number in the lower right hand corner. Adjacent to this drawing number should be the title and issue (alteration) information. For convenience, the drawing number may appear elsewhere on the drawing, usually inverted so it can be read whichever way it is filed.

Drawing number

No two drawings should bear identical drawing numbers and a design office should maintain a register of all drawings issued. The Drawing Number may refer to elements such as the project identity, the group breakdown, and the individual register number. Except for repair drawings, the Drawing Number is also generally the Part Number. Handed parts

Drawings of handed parts usually have the left-hand, upper, inner or forward part drawn. This item is allocated the odd number, with the opposite hand the consecutive even number. The drawing sheet bears the legend AS DRAWN and OPP HAND in the item quantity column. Where necessary the handed condition is indicated by a local view or annotation. Sheet numbers

Where a complete drawing cannot be contained on a single sheet, successive sheets are used. The first sheet is identified as SHEET 1 of X SHEETS, as applicable and subsequent sheets by the appropriate sheet number. Where a Schedule of Parts (Parts List), applicable to all sheets, is required, it appears on Sheet 1.



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Drawing changes

Change to a design drawing, with the exception of minor clerical corrections, is usually accompanied by a new issue number and date. New parts added to the drawing, or drawn on parts affected by the change, take a new issue number, and parts, which are not affected, retain the original issue number. In all cases where interchangeability is affected, a new Drawing Number and Part Number are allocated. Details of the drawing changes are recorded in the appropriate column on the drawing, or recorded separately on an Alteration Sheet, which is referenced on the drawing. The issue number may, sometimes, be represented by a letter. Some organisations use alphabetical issues for prototype aircraft drawings and numerical issues for production aircraft drawings; thus all drawings of a prototype aircraft become Issue 1 when production commences. An alteration to a single part drawing may also result in changes to associated drawings, and it may be necessary to halt manufacture or assembly of the product. The Drawing Office system usually makes provision for the proper recording of drawing changes, by publishing concurrently with the re-issued drawing, an instruction detailing the effects these will have on other drawings, on work-in-progress and on existing stock. As a further safeguard, some organisations publish Drawing Master Reference Lists, which give details of the current issues of all drawings which are associated with a particular component or assembly.

Part referencing

Every item called up on a drawing is given an item number, which is shown in a balloon on the face of a drawing. No other information is given in, or adjacent to, the balloon, with the exception of information necessary for manufacture or assembly, such as equally spaced or snap head inside. A Schedule of Parts is, normally, also included. Materials such as locking wire and shimming, which are available in rolls and sheets, will be detailed by specification number in the Part No column and the quantity will be entered as As Required or A/R.

Validation of modification/repair drawings

When a modification or a repair is required to be embodied into an aircraft structure or component part, it usually necessitates the use of a working drawing to assist with the work. To ensure the authenticity (and legality) of the drawing, it should bear a Validity stamp (using red ink) which is applied by the issuing department. The stamp consists of the authorisation stamp and signature of the issuing person and the date on which the drawing is obtained from the issuing department. In addition the stamp should bear the words VALID TIL: followed by a second date.



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The additional date will be that of the date of the next scheduled revision (usually Quarterly January, April, July, October or similar) to the relevant manual or document from which the working drawing has been copied. Working drawings must not be used beyond their validation date, but must be returned to the issuing department for checking and re-validation before use.

Summary of recommended drawing information

Table 3 provides a fairly comprehensive summary of the recommended basic and additional information, which is likely to be found on typical aircraft engineering drawings.

Table 3

Recommended Basic and Additional Drawing Information

Recommended Basic Drawing Information Company Identifier (Name, Logo etc.) Drawing number Copyright clause Descriptive title of part/assembly Date of drawing Units of measurement Issue information General tolerances Projection symbol Original scale Sheet number Warning: DO NOT SCALE Number of sheets Grid or zoning system Validation stamp for working drawings Signature(s) Recommended Additional Drawing Information Material and specification Treatment/hardness Surface texture Finish Screw thread forms Tool references Sheet size Gauge references Print-folding marks Reference to drawing standards Supersedes Equivalent part



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MICROFILM, MICROFICHE AND COMPUTERISED PRESENTATIONS Due to the increased complexity of modern aircraft, the amount of information needed, within the Maintenance, Spares and Repair manuals, has grown to an enormous extent. For example, the Maintenance manuals, for one modern aircraft alone, consist of twenty volumes, each of which would be more than 76 mm (3 in) thick. To compress even greater amounts of data, other media are utilised, to make the information more easily available to aircraft servicing technicians. These include: Microfilm Microfiche Computers (CD-ROM). MICROFILM

This method entails one publication being reproduced, on a roll of film and contained in a special cartridge case, approximately three inches (76 mm) square. The pages are sequentially copied onto the film and wound upon a drum, within the cartridge case. A microfilm Reader (a projector) is used, to wind the film through a gate and display a single page of text/drawing upon a screen, which is large enough to enable the text and illustrations to be read and understood.

Because of the condensing of the hard copy books into a small space; a complete set of maintenance manuals can, thus, be contained in a small number of microfilm cartridges which can be stored close to the Reader. A number of these projectors are provided with a printing facility that allows the person, reading the film, to print a copy of any sheets which contain information that is required away from the machine. All copies, removed from the microfilm reading room, must be used once only, and not retained for later work. This practise ensures that amendments and updates are not missed. MICROFICHE

A similar process to microfilm, with the exception that many pages of the manuals are reproduced on one clear sheet of film, measuring approximately 100 mm x 150 mm (4 in x 6 in). Each sheet is capable of storing a large number of pages (over 100) of text/drawings and takes up very little space.



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The Reader is similar to the microfilm Reader except that the film slide is moved about, beneath the viewing lens, until the relevant page appears upon the screen. By simply pressing a button on the machine, a photocopy of the page being viewed can be produced for remote use and, once again, any copies should not be retained for future use. Amendment of both this and the microfilm system is by direct replacement, with local disposal of the unwanted items. Computer cd-rom

The use of computers, with respect to aircraft maintenance manuals, (and other publications), has the primary advantage of the huge amount of information that can be stored on one Compact Disc (CD). A single computer, located within a maintenance facility, could have all the necessary publications (such as the Maintenance Manual, Illustrated Parts Catalogue and Wiring Diagrams), for the relevant aircraft type, held on one CD. As with the other two systems, there should be the facility to print the necessary information required with, of course, the limitation that the information is only valid on-the-day, and must not be used for repetitive jobs. Updating of computer-based systems is by the simple replacement of the relevant CD-ROM, although there may be intermediate amendments.

Supplementary information

It is important that only the current issue, of whichever system is in use, is supplied to servicing technicians. This means that the amendment procedures must be carefully monitored (and especially the disposal of the out-dated material). The new amendments come with a Letter of Transmittal, from the relevant authority, in exactly the same manner as they do with the hard copy technical publications. Because of the need to dispose of large amounts of information, whenever even a minor update or amendment is carried out, it is normal to produce Supplementary Information in hard copy form, as an intermediate source of current information. These issues are in addition to either the film/fiche/CD-ROM systems in use and must be not only carefully monitored, but also well publicised. This ensures that the technicians know that the information, contained in the system they are using, could, possibly, contain small items of out-of-date information.



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AERONAUTICAL STANDARDS A standard is variously defined as: Something, established for use as a rule, or basis of

comparison, in measuring or judging capacity, quantity, content, extent, value or quality, or a level or grade of excellence

Any measure of extent, quality or value, established either by law, or by general use, or by consent.

In the normal performance of their duties, technicians can find a wide array of standards, establishing the characteristics of the materials and components that they encounter in their day-to-day work of maintaining and repairing aircraft. AIR TRANSPORT ASSOCIATION SPECIFICATION NO. 100 Since 1 June 1956, the Air Transport Association of America (ATA), has used a specification, to establish a standard for the presentation of technical data, by aircraft, engine or component manufacturers, that is required for their respective products. This specification is known as ATA Specification No.100 (ATA 100), and its two Chapters clarify the general requirements of the aircraft industry, with reference to the coverage, preparation and organisation of all technical data.

Chapter 2 of the ATA 100 covers policies and standards applicable to specific manuals and it details the names and contents of the various manuals that must be prepared by the manufacturer. Such manuals include the: Aircraft Maintenance Manual Wiring Diagrams Structural Repair Manual Aircraft Illustrated Parts Catalogue Component Maintenance Manual Illustrated Tool and Equipment Manual Service Bulletins Weight and Balance Manual Non-Destructive Testing Manual Power Plant Build-up Manual Aircraft Recovery Manual Fault Reporting and Fault Isolation Manuals Engine Manual Engine Illustrated Parts Catalogue.



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Chapter 1 of the ATA 100 covers policies and standards applicable to all publications and provides a uniform method for arranging technical material, within the relevant publications, in an effort to simplify the technicians problem in locating instructions and parts. In the Arrangement of Material section, in Chapter 1 (1-2) of the ATA 100, the standard details the use of a three-element identifier number. Each element of the identifying number consists of two digits. The first element is designed to provide identification of all topics or systems, within the respective manuals, by reference to specific Chapters. The second element identifies sub-systems (sub-topics) as Sections, while the third element identifies associated sub-sub-systems (sub-sub topics) as Subjects. Table 4 illustrates an example of how the ATA 100 numbering system (in this instance using numbers ranging from 27-00-00 to 27-31-14) is used, to identify the material which is covered at particular locations within a typical Maintenance Manual.

Table 4 Example of ATA 100 Numbering System



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The subject is broken down yet further into Page Blocks to provide maintenance personnel with more detailed information on specific topics (or sub-topics) which relate to the Subject material. Table 5 shows an example of a Page Block system along with the topics and sub-topics, which are allocated to the relevant Page Block numbers. Table 5 Example of ATA 100 Page Block Numbering System Topic or sub-topic Page Block Description and Operation 1 to 100 Trouble-shooting 101 to 200 Maintenance Practices (if brief) 201 to 300 (Otherwise) Servicing 301 to 400 Removal/Installation 401 to 500 Adjustment/Test 501 to 600 Inspection/Check 601 to 700 Cleaning/Painting 701 to 800 Approved Repairs 801 to 900 Note: The word EFFECTIVITY - which may appear on the left hand side of the bottom of a page is used to identify the aircraft serial number, or manufacturers serial number (MSN), or aircraft model to which a particular Subject topic may refer and those numbers will be shown. If the word ALL appears adjacent to the Effectivity then the information concerns all types of aircraft (or components), regardless of any serial numbers.

Chapter 1 of the ATA 100 also details the policies and standards applicable to all publications with reference to the:

Physical Requirements: Format of media (Paper, Film, Page layout/numbering etc.) and Indexing (List of Effective Pages [LEPs], Table of Contents [TOC], Text, Divider Cards, Sequence, etc.)

Issuance and Revision Service

Aircraft and Engine Zoning: Access Door, Port, Panel and Area identification.

Many airlines and similar companies also organise their spare parts in stores departments under the relevant ATA specification numbers and, irrespective of the aircraft type, information on similar components will be found in the same Chapter and Section. A complete table of the ATA numbering system, sub-system and titles, allows the technician to establish, precisely, where the information required can be found in the respective manuals.



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INTERNATIONAL ORGANISATION FOR STANDARDISATION (ISO) This is an international organisation, which has representatives from each

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