Engineering Drawings, Diagrams, and Standards PDF

## Engineering Drawings, Diagrams, and Standards ### Submodule Knowledge Descriptions | Level | | |---|---| | B1 | Engineering Drawings, Diagrams, and Standards | | 2 | Drawing types and diagrams; their symbols, dimensions, tolerances and projections; Identification of title block information; Mi...

## Engineering Drawings, Diagrams, and Standards ### Submodule Knowledge Descriptions | Level | | |---|---| | B1 | Engineering Drawings, Diagrams, and Standards | | 2 | Drawing types and diagrams; their symbols, dimensions, tolerances and projections; Identification of title block information; Microfilm, microfiche, and computerised presentations; Specification 100 of the Air Transport Association (ATA) of America; Aeronautical and other applicable standards including ISO, AN, MS, NAS and MIL; Wiring diagrams and schematic diagrams. | ### 7.5 - Engineering Drawings, Diagrams, and Standards #### Purpose and Function of Aircraft Drawings Drawings and prints are the link between the engineers who design an aircraft and the workers who build, maintain, and repair it. A print may be a copy of a working drawing for an aircraft part or group of parts, or for a design of a system or group of systems. They are made by placing a tracing of the drawing over a sheet of chemically-treated paper and exposing it to a strong light for a short period of time. When the exposed paper is developed, it turns blue where the light has penetrated the transparent tracing. The inked lines of the tracing, having blocked out the light, show as white lines on a blue background. With other types of sensitized paper, prints may have a white background with colored lines or a colored background with white lines. Drawings created using computers may be viewed on the computer monitor or printed out in "hard copy" by use of an ink jet or laser printer. Larger drawings may be printed by use of a plotter or large format printer. Large printers can print drawings up to 42-inches high with widths up to 600-inches by use of continuous roll paper. #### Drawings should be handled carefully Drawings should be handled carefully as they are both expensive and valuable. Open drawings slowly and carefully to prevent tearing of the paper. When the drawing is open, smooth out the fold lines instead of bending them backward. To protect drawings from damage, never spread them on the floor or lay them on a surface covered with tools or other objects that may make holes in the paper. Hands should be free of oil, grease, or other unclean matter that can soil or smudge the print. Never make notes or marks on a print, as they may confuse others and lead to incorrect work. Only authorized individuals are permitted to make notes or changes on prints, and they must sign and date any changes they make. When finished with a drawing, fold and return it to its proper place. Prints are folded originally in an appropriate size for filing. Care should be taken so that the original folds are always used. #### Drawings and Diagrams; Symbols, Dimensions, Tolerances, and Projections ##### Drawing Types Drawings must give information such as size and shape of the object and all its parts, specifications for material to be used, how the material is to be finished, how the parts are to be assembled, and any other information essential to making and assembling the object. Drawings may be divided into three classes: detail, assembly, and installation. ##### Detail Drawings A detail drawing is a description of a single part, describing by lines, notes, and symbols the specifications for size, shape, material, and methods of manufacture to be used in making the part. Detail drawings are usually rather simple. When single parts are small, several detail drawings may be shown on the same sheet or print. ##### Assembly Drawings An assembly drawing is a description of an object made up of two or more parts. It describes the object's size and shape. Its primary purpose is to show the relationship of the various parts. An assembly drawing is usually more complex than a detail drawing and is often accompanied by detail drawings of various parts. ##### Installation Drawings An installation drawing includes all necessary information for a part or an assembly in the final installed position in the aircraft. It shows the dimensions necessary for the location of specific parts with relation to the other parts and reference dimensions that are helpful in later work in the shop. ##### Sectional View Drawings Drawings A section or sectional view is obtained by cutting away part of an object to show the shape and construction at the cutting plane. The part or parts cut away are shown by using section (crosshatching) lines. Types of sections are described in the following paragraphs. * **Full Section** A full section view is used when the interior construction or hidden features of an object cannot be shown clearly by exterior views. For example, is a sectional view of a cable connector and shows the internal construction of the connector. * **Half Section** In a half section, the cutting plane extends only halfway across the object, leaving the other half of the object as an exterior view. Half sections are used with symmetrical objects to show both the interior and exterior. is a half sectional view of a Capstan servo. * **Revolved Section** A revolved section drawn directly on the exterior view shows the shape of the cross section of a part, such as the spoke of a wheel. An example of a revolved section is shown in . * **Removed Section** A removed section illustrates parts of an object. It is drawn like revolved sections, except it is placed at one side and often drawn to a larger scale than the view indicated to bring out pertinent details. is an illustration of removed sections. Section A-A shows the cross-sectional shape of the object at cutting plane line A-A. Section B-B shows the cross-sectional shape at cutting plane line B-B. These sectional views are drawn to the same scale as the principal view. ##### Universal Numbering System The universal numbering system provides a means of identifying standard drawing sizes. In the universal numbering system, each drawing number consists of six or seven digits. The first digit is always 1, 2, 4, or 5 and indicates the size of the drawing. The number 1 indicates a drawing of 81½" x 11"; number 2 indicates an 11" x 17" drawing; number 4 represents a drawing of 17" x 22"; and 5 indicates a width of between 17 and 36-inches but on a continuous roll. Letters are also used (and becoming more prevalent) with the most common letters being A through E. The letter A is 81/2" x 11", B is 11" x 17", C is 17" x 22", D is 22" x 34" and E is 34" x 44". There are additional letters, such as D1 at 24" x 36", E1 at 30" x 42" and additional sizes unique to even larger formats but generally reserved for inter-company operations. The remaining digits identify the drawing. Many firms have modified this basic system to conform to their needs. The letter or number depicting the standard drawing size may be prefixed to the number, separated from it by a dash. Other numbering systems provide a separate box preceding the drawing number for the drawing size identifier. In another modification of this system, the part number of the depicted assembly is assigned as the drawing number. ##### Drawing Standards Drawing standards cover many items such as paper sizes, any notes, numbering systems, geometric dimensions and tolerances, abbreviations, welding symbols, roughness, and electrical symbols. These standards cover metric and inch measurements, as well as computer drafting standards. Different standards for drawings are used in industry and some of the more common ones are published by the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI). ##### Bill of Material A list of the materials and parts necessary for the fabrication or assembly of a component or system is often included on the drawing. The list is usually in ruled columns that provide the part number, name of the part, material the part is to be constructed of, the quantity required, and the source of the part or material. A typical bill of material is shown in Figure 5-3C. On drawings that do not have a bill of material, the data may be indicated directly on the drawing. On assembly drawings, each item is identified by a number in a circle or square. An arrow connecting the number with the item assists in locating it in the bill of material. ##### Revision Block Revisions to a drawing are necessitated by changes in dimensions, design, or materials. The changes are usually listed in ruled columns either adjacent to the title block or at one corner of the drawing. All changes to approved drawings must be carefully noted on all existing prints of the drawing. When drawings contain such corrections, attention is directed to the changes by lettering or numbering them and listing those changes against the symbol in a revision block. The revision block contains the identification symbol, the date, the nature of the revision, the authority for the change, and the name of the draftsman who made the change. To distinguish the corrected drawing from its previous version, a change letter is added to the drawing number. Change letters are usually sequential from "A" to "Z" and indicate the total number of changes made to the drawing. ##### Zone Numbers Zone numbers on drawings are like the numbers and letters printed on the borders of a map. They help locate a point. To find a point, mentally draw horizontal and vertical lines from the letters and numerals specified; the point where these lines intersect is the area sought. ##### Station Numbers and Location Identification on Aircraft A numbering system is used on large assemblies for aircraft to locate stations, such as fuselage frames. Fuselage station 185 indicates a location that is 185-inches from the datum of the aircraft. The measurement is usually taken from the nose or zero station, but in some instances, it may be taken from the firewall or some other point chosen by the manufacturer. Just as forward and aft locations on aircraft are made by reference to the datum, locations left and right of the aircraft's longitudinal axis are made by reference to the buttock line and are called butt stations. Vertical locations on an airplane are made in reference to the waterline. The same station numbering system is used for wing and stabilizer frames. The measurement is taken from the centerline or zero station of the aircraft. shows use of the fuselage stations (FS), waterline locations (WL), and left and right buttock line locations (RBL and LBL). ##### Allowances and Tolerances When a given dimension on a print shows an allowable variation, the plus (+) Figure indicates the maximum, and the minus (-) Figure the minimum allowable variation. The sum of the plus and minus allowance figures is called tolerance. For example, using 0.225 + 0.0025 -0.000 5, the plus and minus figures indicate the part is acceptable if it is not more than 0.002 5 larger than the 0.225 given dimension, or not more than 0.000 5 smaller than the 0.225 dimension. Tolerance in this example is 0.003 0 (0.002 5 max plus 0.000 5 min). If the plus and minus allowances are the same, you will find them presented as 0.225 ± 0.002 5. The tolerance would then be 0.005 0. Allowance can be indicated in either fractional or decimal form. When very accurate dimensions are necessary, decimal allowances are used. Fractional allowances are sufficient when precise tolerances are not required. Standard tolerances of -0.010 or -1/32 may be given in the title block of many drawings, to apply throughout the drawing. ##### Finish Marks Finish marks are used to indicate the surface that must be machine finished. Such finished surfaces have a better appearance and allow a closer fit with adjoining parts. During the finishing process, the required limits and tolerances must be observed. Do not confuse machined finishes with those of paint, enamel, chromium plating, and similar coating. ##### Scale Some drawings are made the same size as the drawn part; reflecting a scale of 1:1. Other scales may be used. However, when drawings are made on a computer, drawing sizes may be easily increased (zoom in) or decreased (zoom out). Some electronic printers have the same capability. Furthermore, when a 1:1 copy of a print is made, the copy size may differ slightly from that of the original. For accurate information, refer to the dimensions shown on the drawing. ##### Application When shown near or in the title block, application may refer to a specific aircraft, assembly, sub-assembly or unique application. For example, in Figure 5-3A the title block indicates the bracket assembly is for a Roll Servo installation for an S-Tec Auto Pilot installation. If this drawing pertained to a B95 Aircraft equipped with an Aero-Tech air conditioning system and the bracket illustrated was unique to that installation, the title block would provide that application information. The title block may indicate Bracket Assy., Roll Servo, with Aero-Tech air conditioner (Model AT103-1) installed. #### Methods of Illustration ##### Applied Geometry Geometry is the branch of mathematics that deals with lines, angles, figures, and certain assumed properties in space. Applied geometry, as used in drawings, makes use of these properties to accurately and correctly represent objects graphically. In the past, draftsmen utilized a variety of instruments with various scales, shapes, and curves to make their drawings. Today, computer software graphics programs show drawings at nearly any scale, shape, and curve imaginable, outdating the need for additional instruments. Several different methods are used to illustrate objects graphically. The most common are orthographic projections, pictorial drawings, diagrams, and flowcharts. ##### Orthographic Projection Drawings to show the exact size and shape of all the parts of complex objects, several views are necessary. This is the system used in orthographic projection. In orthographic projection, there are six possible views of an object, because all objects have six sides-front, top, bottom, rear, right side, and left side. shows an object placed in a transparent box, hinged at the edges. The projections on the sides of the box are the views as seen looking straight at the object through each side. If the outlines of the object are drawn on each surface of the box, and the box is then opened to lay flat , the result is a six-view orthographic projection. It is seldom necessary to show all six views to portray an object clearly; therefore, only those views necessary to illustrate the required characteristics of the object are drawn. One, two, and three-view drawings are the most common. Regardless of the number of views used, the arrangement is generally as shown in . ##### Detail View A detail view shows only a part of the object, but in greater detail and to a larger scale than the principal view. The part that is shown in detail elsewhere on the drawing is usually encircled by a heavy line on the principal view. The principal view shows the complete object, while the detail view is an enlarged drawing of a portion of the object. ##### Pictorial Drawings A pictorial drawing is like a photograph. It shows an object as it appears to the eye, but it is not satisfactory for showing complex forms and shapes. Pictorial drawings are useful in showing the general appearance of an object and are used extensively with orthographic projection drawings. Pictorial drawings are used in Aircraft Maintenance Manuals (AMM), Structural Repair Manuals (SRM), and Illustrated Parts Catalogues (IPC). Three types of pictorial drawings used frequently by aircraft engineers and technicians are: perspective, isometric, oblique, and exploded view. ##### Perspective Drawings A perspective view shows an object as it appears to an observer. It most closely resembles the way an object would look in a photograph. Because of perspective, some of the lines of an object are not parallel and therefore the actual angles and dimensions are not accurate. ##### Isometric Drawings An isometric drawing uses a combination of views from an orthographic projection and tilts the object forward so that portions of all three views can be seen in one view. This provides the observer with a three-dimensional view of the object. Unlike a perspective drawing where lines converge and dimensions are not true, lines in an isometric drawing are parallel and dimensioned as they are in an orthographic projection. ##### Oblique Drawings An oblique view is like an isometric view, except for one distinct difference. In an oblique drawing, two of the three drawing axes are always at right angles to each other. ##### Exploded View Drawings An exploded view drawing is a pictorial drawing of two or more parts that fit together as an assembly. The view shows the individual parts and their relative position to the other parts before they are assembled. ##### Zone Numbers Zone numbers on drawings are like the numbers and letters printed on the borders of a map. They help locate a point. To find a point, mentally draw horizontal and vertical lines from the letters and numerals specified; the point where these lines intersect is the area sought. ##### Station Numbers and Location Identification on Aircraft A numbering system is used on large assemblies for aircraft to locate stations, such as fuselage frames. Fuselage station 185 indicates a location that is 185-inches from the datum of the aircraft. The measurement is usually taken from the nose or zero station, but in some instances, it may be taken from the firewall or some other point chosen by the manufacturer. Just as forward and aft locations on aircraft are made by reference to the datum, locations left and right of the aircraft's longitudinal axis are made by reference to the buttock line and are called butt stations. Vertical locations on an airplane are made in reference to the waterline. The same station numbering system is used for wing and stabilizer frames. The measurement is taken from the centerline or zero station of the aircraft. shows use of the fuselage stations (FS), waterline locations (WL), and left and right buttock line locations (RBL and LBL). ##### Allowances and Tolerances When a given dimension on a print shows an allowable variation, the plus (+) Figure indicates the maximum, and the minus (-) Figure the minimum allowable variation. The sum of the plus and minus allowance figures is called tolerance. For example, using 0.225 + 0.0025 -0.000 5, the plus and minus figures indicate the part is acceptable if it is not more than 0.002 5 larger than the 0.225 given dimension, or not more than 0.000 5 smaller than the 0.225 dimension. Tolerance in this example is 0.003 0 (0.002 5 max plus 0.000 5 min). If the plus and minus allowances are the same, you will find them presented as 0.225 ± 0.002 5. The tolerance would then be 0.005 0. Allowance can be indicated in either fractional or decimal form. When very accurate dimensions are necessary, decimal allowances are used. Fractional allowances are sufficient when precise tolerances are not required. Standard tolerances of -0.010 or -1/32 may be given in the title block of many drawings, to apply throughout the drawing. ##### Finish Marks Finish marks are used to indicate the surface that must be machine finished. Such finished surfaces have a better appearance and allow a closer fit with adjoining parts. During the finishing process, the required limits and tolerances must be observed. Do not confuse machined finishes with those of paint, enamel, chromium plating, and similar coating. ##### Scale Some drawings are made the same size as the drawn part; reflecting a scale of 1:1. Other scales may be used. However, when drawings are made on a computer, drawing sizes may be easily increased (zoom in) or decreased (zoom out). Some electronic printers have the same capability. Furthermore, when a 1:1 copy of a print is made, the copy size may differ slightly from that of the original. For accurate information, refer to the dimensions shown on the drawing. ##### Application When shown near or in the title block, application may refer to a specific aircraft, assembly, sub-assembly or unique application. For example, in Figure 5-3A the title block indicates the bracket assembly is for a Roll Servo installation for an S-Tec Auto Pilot installation. If this drawing pertained to a B95 Aircraft equipped with an Aero-Tech air conditioning system and the bracket illustrated was unique to that installation, the title block would provide that application information. The title block may indicate Bracket Assy., Roll Servo, with Aero-Tech air conditioner (Model AT103-1) installed. #### Methods of Illustration ##### Applied Geometry Geometry is the branch of mathematics that deals with lines, angles, figures, and certain assumed properties in space. Applied geometry, as used in drawings, makes use of these properties to accurately and correctly represent objects graphically. In the past, draftsmen utilized a variety of instruments with various scales, shapes, and curves to make their drawings. Today, computer software graphics programs show drawings at nearly any scale, shape, and curve imaginable, outdating the need for additional instruments. Several different methods are used to illustrate objects graphically. The most common are orthographic projections, pictorial drawings, diagrams, and flowcharts. ##### Orthographic Projection Drawings to show the exact size and shape of all the parts of complex objects, several views are necessary. This is the system used in orthographic projection. In orthographic projection, there are six possible views of an object, because all objects have six sides-front, top, bottom, rear, right side, and left side. shows an object placed in a transparent box, hinged at the edges. The projections on the sides of the box are the views as seen looking straight at the object through each side. If the outlines of the object are drawn on each surface of the box, and the box is then opened to lay flat , the result is a six-view orthographic projection. It is seldom necessary to show all six views to portray an object clearly; therefore, only those views necessary to illustrate the required characteristics of the object are drawn. One, two, and three-view drawings are the most common. Regardless of the number of views used, the arrangement is generally as shown in . ##### Detail View A detail view shows only a part of the object, but in greater detail and to a larger scale than the principal view. The part that is shown in detail elsewhere on the drawing is usually encircled by a heavy line on the principal view. The principal view shows the complete object, while the detail view is an enlarged drawing of a portion of the object. ##### Pictorial Drawings A pictorial drawing is like a photograph. It shows an object as it appears to the eye, but it is not satisfactory for showing complex forms and shapes. Pictorial drawings are useful in showing the general appearance of an object and are used extensively with orthographic projection drawings. Pictorial drawings are used in Aircraft Maintenance Manuals (AMM), Structural Repair Manuals (SRM), and Illustrated Parts Catalogues (IPC). Three types of pictorial drawings used frequently by aircraft engineers and technicians are: perspective, isometric, oblique, and exploded view. ##### Perspective Drawings A perspective view shows an object as it appears to an observer. It most closely resembles the way an object would look in a photograph. Because of perspective, some of the lines of an object are not parallel and therefore the actual angles and dimensions are not accurate. ##### Isometric Drawings An isometric drawing uses a combination of views from an orthographic projection and tilts the object forward so that portions of all three views can be seen in one view. This provides the observer with a three-dimensional view of the object. Unlike a perspective drawing where lines converge and dimensions are not true, lines in an isometric drawing are parallel and dimensioned as they are in an orthographic projection. ##### Oblique Drawings An oblique view is like an isometric view, except for one distinct difference. In an oblique drawing, two of the three drawing axes are always at right angles to each other. ##### Exploded View Drawings An exploded view drawing is a pictorial drawing of two or more parts that fit together as an assembly. The view shows the individual parts and their relative position to the other parts before they are assembled. ##### Zone Numbers Zone numbers on drawings are like the numbers and letters printed on the borders of a map. They help locate a point. To find a point, mentally draw horizontal and vertical lines from the letters and numerals specified; the point where these lines intersect is the area sought. ##### Station Numbers and Location Identification on Aircraft A numbering system is used on large assemblies for aircraft to locate stations, such as fuselage frames. Fuselage station 185 indicates a location that is 185-inches from the datum of the aircraft. The measurement is usually taken from the nose or zero station, but in some instances, it may be taken from the firewall or some other point chosen by the manufacturer. Just as forward and aft locations on aircraft are made by reference to the datum, locations left and right of the aircraft's longitudinal axis are made by reference to the buttock line and are called butt stations. Vertical locations on an airplane are made in reference to the waterline. The same station numbering system is used for wing and stabilizer frames. The measurement is taken from the centerline or zero station of the aircraft. shows use of the fuselage stations (FS), waterline locations (WL), and left and right buttock line locations (RBL and LBL). ##### Allowances and Tolerances When a given dimension on a print shows an allowable variation, the plus (+) Figure indicates the maximum, and the minus (-) Figure the minimum allowable variation. The sum of the plus and minus allowance figures is called tolerance. For example, using 0.225 + 0.0025 -0.000 5, the plus and minus figures indicate the part is acceptable if it is not more than 0.002 5 larger than the 0.225 given dimension, or not more than 0.000 5 smaller than the 0.225 dimension. Tolerance in this example is 0.003 0 (0.002 5 max plus 0.000 5 min). If the plus and minus allowances are the same, you will find them presented as 0.225 ± 0.002 5. The tolerance would then be 0.005 0. Allowance can be indicated in either fractional or decimal form. When very accurate dimensions are necessary, decimal allowances are used. Fractional allowances are sufficient when precise tolerances are not required. Standard tolerances of -0.010 or -1/32 may be given in the title block of many drawings, to apply throughout the drawing. ##### Finish Marks Finish marks are used to indicate the surface that must be machine finished. Such finished surfaces have a better appearance and allow a closer fit with adjoining parts. During the finishing process, the required limits and tolerances must be observed. Do not confuse machined finishes with those of paint, enamel, chromium plating, and similar coating. ##### Scale Some drawings are made the same size as the drawn part; reflecting a scale of 1:1. Other scales may be used. However, when drawings are made on a computer, drawing sizes may be easily increased (zoom in) or decreased (zoom out). Some electronic printers have the same capability. Furthermore, when a 1:1 copy of a print is made, the copy size may differ slightly from that of the original. For accurate information, refer to the dimensions shown on the drawing. ##### Application When shown near or in the title block, application may refer to a specific aircraft, assembly, sub-assembly or unique application. For example, in Figure 5-3A the title block indicates the bracket assembly is for a Roll Servo installation for an S-Tec Auto Pilot installation. If this drawing pertained to a B95 Aircraft equipped with an Aero-Tech air conditioning system and the bracket illustrated was unique to that installation, the title block would provide that application information. The title block may indicate Bracket Assy., Roll Servo, with Aero-Tech air conditioner (Model AT103-1) installed. #### Methods of Illustration ##### Applied Geometry Geometry is the branch of mathematics that deals with lines, angles, figures, and certain assumed properties in space. Applied geometry, as used in drawings, makes use of these properties to accurately and correctly represent objects graphically. In the past, draftsmen utilized a variety of instruments with various scales, shapes, and curves to make their drawings. Today, computer software graphics programs show drawings at nearly any scale, shape, and curve imaginable, outdating the need for additional instruments. Several different methods are used to illustrate objects graphically. The most common are orthographic projections, pictorial drawings, diagrams, and flowcharts. ##### Orthographic Projection Drawings to show the exact size and shape of all the parts of complex objects, several views are necessary. This is the system used in orthographic projection. In orthographic projection, there are six possible views of an object, because all objects have six sides-front, top, bottom, rear, right side, and left side. shows an object placed in a transparent box, hinged at the edges. The projections on the sides of the box are the views as seen looking straight at the object through each side. If the outlines of the object are drawn on each surface of the box, and the box is then opened to lay flat , the result is a six-view orthographic projection. It is seldom necessary to show all six views to portray an object clearly; therefore, only those views necessary to illustrate the required characteristics of the object are drawn. One, two, and three-view drawings are the most common. Regardless of the number of views used, the arrangement is generally as shown in . ##### Detail View A detail view shows only a part of the object, but in greater detail and to a larger scale than the principal view. The part that is shown in detail elsewhere on the drawing is usually encircled by a heavy line on the principal view. The principal view shows the complete object, while the detail view is an enlarged drawing of a portion of the object. ##### Pictorial Drawings A pictorial drawing is like a photograph. It shows an object as it appears to the eye, but it is not satisfactory for showing complex forms and shapes. Pictorial drawings are useful in showing the general appearance of an object and are used extensively with orthographic projection drawings. Pictorial drawings are used in Aircraft Maintenance Manuals (AMM), Structural Repair Manuals (SRM), and Illustrated Parts Catalogues (IPC). Three types of pictorial drawings used frequently by aircraft engineers and technicians are: perspective, isometric, oblique, and exploded view. ##### Perspective Drawings A perspective view shows an object as it appears to an observer. It most closely resembles the way an object would look in a photograph. Because of perspective, some of the lines of an object are not parallel and therefore the actual angles and dimensions are not accurate. ##### Isometric Drawings An isometric drawing uses a combination of views from an orthographic projection and tilts the object forward so that portions of all three views can be seen in one view. This provides the observer with a three-dimensional view of the object. Unlike a perspective drawing where lines converge and dimensions are not true, lines in an isometric drawing are parallel and dimensioned as they are in an orthographic projection. ##### Oblique Drawings An oblique view is like an isometric view, except for one distinct difference. In an oblique drawing, two of the three drawing axes are always at right angles to each other. ##### Exploded View Drawings An exploded view drawing is a pictorial drawing of two or more parts that fit together as an assembly. The view shows the individual parts and their relative position to the other parts before they are assembled. ##### Zone Numbers Zone numbers on drawings are like the numbers and letters printed on the borders of a map. They help locate a point. To find a point, mentally draw horizontal and vertical lines from the letters and numerals specified; the point where these lines intersect is the area sought. ##### Station Numbers and Location Identification on Aircraft A numbering system is used on large assemblies for aircraft to locate stations, such as fuselage frames. Fuselage station 185 indicates a location that is 185-inches from the datum of the aircraft. The measurement is usually taken from the nose or zero station, but in some instances, it may be taken from the firewall or some other point chosen by the manufacturer. Just as forward and aft locations on aircraft are made by reference to the datum, locations left and right of the aircraft's longitudinal axis are made by reference to the buttock line and are called butt stations. Vertical locations on an airplane are made in reference to the waterline. The same station numbering system is used for wing and stabilizer frames. The measurement is taken from the centerline or zero station of the aircraft. shows use of the fuselage stations (FS), waterline locations (WL), and left and right buttock line locations (RBL and LBL). ##### Allowances and Tolerances When a given dimension on a print shows an allowable variation, the plus (+) Figure indicates the maximum, and the minus (-) Figure the minimum allowable variation. The sum of the plus and minus allowance figures is called tolerance. For example, using 0.225 + 0.0025 -0.000 5, the plus and minus figures indicate the part is acceptable if it is not more than 0.002 5 larger than the 0.225 given dimension, or not more than 0.000 5 smaller than the 0.225 dimension. Tolerance in this example is 0.003 0 (0.002 5 max plus 0.000 5 min). If the plus and minus allowances are the same, you will find them presented as 0.225 ± 0.002 5. The tolerance would then be 0.005 0. Allowance can be indicated in either fractional or decimal form. When very accurate dimensions are necessary, decimal allowances are used. Fractional allowances are sufficient when precise tolerances are not required. Standard tolerances of -0.010 or -1/32 may be given in the title block of many drawings, to apply throughout the drawing. ##### Finish Marks Finish marks are used to indicate the surface that must be machine finished. Such finished surfaces have a better appearance and allow a closer fit with adjoining parts. During the finishing process, the required limits and tolerances must be observed. Do not confuse machined finishes with those of paint, enamel, chromium plating, and similar coating. ##### Scale Some drawings are made the same size as the drawn part; reflecting a scale of 1:1. Other scales may be used. However, when drawings are made on a computer, drawing sizes may be easily increased (zoom in) or decreased (zoom out). Some electronic printers have the same capability. Furthermore, when a 1:1 copy of a print is made, the copy size may differ slightly from that of the original. For accurate information, refer to the dimensions shown on the drawing. ##### Application When shown near or in the title block, application may refer to a specific aircraft, assembly, sub-assembly or unique application. For example, in Figure 5-3A the title block indicates the bracket assembly is for a Roll Servo installation for an S-Tec Auto Pilot installation. If this drawing pertained to a B95 Aircraft equipped with an Aero-Tech air conditioning system and the bracket illustrated was unique to that installation, the title block would provide that application information. The title block may indicate Bracket Assy., Roll Servo, with Aero-Tech air conditioner (Model AT103-1) installed. #### Methods of Illustration ##### Applied Geometry Geometry is the branch of mathematics that deals with lines, angles, figures, and certain assumed properties in space. Applied geometry, as used in drawings, makes use of these properties to accurately and correctly represent objects graphically. In the past, draftsmen utilized a variety of instruments with various scales, shapes, and curves to make their drawings. Today, computer software graphics programs show drawings at nearly any scale, shape, and curve imaginable, outdating the need for additional instruments. Several different methods are used to illustrate objects graphically. The most common are orthographic projections, pictorial drawings, diagrams, and flowcharts. ##### Orthographic Projection Drawings to show the exact size and shape of all the parts of complex objects, several views are necessary. This is the system used in orthographic projection. In orthographic projection, there are six possible views of an object, because all objects have six sides-front, top, bottom, rear, right side, and left side. shows an object placed in a transparent box, hinged at the edges. The projections on the sides of the box are the views as seen looking straight at the object through each side. If the outlines of the object are drawn on each surface of the box, and the box is then opened to lay flat , the result is a six-view orthographic projection. It is seldom necessary to show all six views to portray an object clearly; therefore, only those views necessary to illustrate the required characteristics of the object are drawn. One, two, and three-view drawings are the most common. Regardless of the number of views used, the arrangement is generally as shown in . ##### Detail View A detail view shows only a part of the object, but in greater detail and to a larger scale than the principal view. The part that is shown in detail elsewhere on the drawing is usually encircled by a heavy line on the principal view. The principal view shows the complete object, while the detail view is an enlarged drawing of a portion of the object. ##### Pictorial Drawings A pictorial drawing is like a photograph. It shows an object as it appears to the eye, but it is not satisfactory for showing complex forms and shapes. Pictorial drawings are useful in showing the general appearance of an object and are used extensively with orthographic projection drawings. Pictorial drawings are used in Aircraft Maintenance Manuals (AMM), Structural Repair Manuals (SRM), and Illustrated Parts Catalogues (IPC). Three types of pictorial drawings used frequently by aircraft engineers and technicians are: perspective, isometric, oblique, and exploded view. ##### Perspective Drawings A perspective view shows an object as it appears to an observer. It most closely resembles the way an object would look in a photograph. Because of perspective, some of the lines of an object are not parallel and therefore the actual angles and dimensions are not accurate. ##### Isometric Drawings An isometric drawing uses a combination of views from an orthographic projection and tilts the object forward so that portions of all three views can be seen in one view. This