Aircraft drawings contain various types of information that help identify components, locate parts, interpret dimensions, and understand manufacturing or maintenance requirements. This information may include revision details, notes, zone numbers, aircraft station references, tolerances, surface finish requirements, and drawing scales. Understanding these elements helps aviation maintenance technicians interpret drawings correctly and use the appropriate technical information during maintenance and repair.
Revision Block
Revisions to a drawing may be necessary because of changes in dimensions, design, materials, or other technical requirements. Revision information is usually recorded in ruled columns located near the title block or in another designated area of the drawing. When an approved drawing is revised, the changes must be properly documented and incorporated into the applicable controlled copies.
When a drawing contains revisions, the affected areas may be identified by letters, numbers, or other revision symbols. These symbols are listed in the revision block, which provides information about the changes. [Figure 1-D]
A revision block typically contains the revision identification symbol, date, description of the change, authorization for the change, and the name or identification of the person responsible for making the revision. The title block may also contain a space for the current revision designation to distinguish the revised drawing from its previous versions.
Notes
Notes are added to drawings when additional information is required to clarify the design, construction, or application of an object. Some notes specify methods of attachment or construction. Others provide alternative configurations, identify available modifications, or explain requirements that cannot be shown clearly using conventional drawing methods.
Notes may be placed alongside the item to which they refer. If they are lengthy or would cause the drawing to become crowded, they may be placed elsewhere on the drawing and identified by letters or numbers. Figure 1-E illustrates one method of presenting notes.
When a note refers to a specific part, a leader line with an arrowhead connects the note to the part. If a note applies to more than one part, its wording must clearly identify the items to which it applies. When several notes are included, they are generally grouped together and numbered consecutively.
Zone Numbers
Zone numbers on drawings function similarly to the numbers and letters printed along the borders of a map. They help locate a specific point or feature on a drawing.
To locate a point, use the specified horizontal and vertical grid references. The intersection of the corresponding grid lines identifies the area being referenced. Figure 1-F shows zone numbers on a drawing.
The same method is used to locate parts, sections, and views on large drawings, particularly assembly drawings. Parts identified by numbers in the title block or bill of material can be located by finding the corresponding item or zone references on the drawing. On some drawings, zone numbers along the lower border are arranged from right to left.
Station Numbers and Location Identification on Aircraft
Aircraft station numbers provide a reference system for identifying the locations of structural components and other parts on an aircraft. They are commonly used to identify fuselage frames, wing structures, and other major assemblies.
A fuselage station (FS) identifies a longitudinal location measured from a designated aircraft datum. For example, Fuselage Station 185 may indicate a location 185 inches from the aircraft datum. The datum is established by the aircraft manufacturer and may be located at the nose, firewall, or another specified reference point.
Locations forward and aft of the datum are identified using fuselage station references. Locations to the left and right of the aircraft’s longitudinal axis are identified using buttock line references, commonly designated as left buttock line (LBL) and right buttock line (RBL). Vertical locations are identified using waterline (WL) references.
Similar station-numbering systems may be used for wing and stabilizer structures. These measurements are generally referenced to a designated centerline or zero station, as specified by the manufacturer.
Figure 2 illustrates the use of fuselage stations (FS), waterline locations (WL), and left and right buttock line locations (LBL and RBL).
Allowances and Tolerances
Dimensions on aircraft drawings may include allowable variations to account for manufacturing requirements and ensure proper fit and function.
When a dimension includes plus (+) and minus (−) values, the plus value indicates the maximum permitted increase above the nominal dimension, while the minus value indicates the maximum permitted decrease below it. The total permissible dimensional range is called the tolerance.
For example, consider the dimension: 0.225 +0.0025 / −0.0005
The maximum acceptable dimension is: 0.225 + 0.0025 = 0.2275
The minimum acceptable dimension is: 0.225 − 0.0005 = 0.2245
The total tolerance is: 0.2275 − 0.2245 = 0.0030
Therefore, the acceptable dimensional range is 0.2245 to 0.2275.
If the plus and minus allowances are equal, the dimension may be expressed as: 0.225 ± 0.0025
The maximum dimension is 0.2275, and the minimum dimension is 0.2225. The total tolerance is 0.0050.
Allowances may be expressed in fractional or decimal form. Decimal dimensions are commonly used when close dimensional control is required, while fractional dimensions may be used where the specified precision permits them.
Some drawings include general tolerances in the title block. These tolerances apply to dimensions that do not have individually specified tolerances, subject to the drawing’s stated requirements.
Finish Marks
Finish marks or surface-finish symbols indicate the required condition of a surface. They may specify surfaces that require machining or other finishing operations to achieve the required dimensions, surface quality, or fit with adjoining components.
During the finishing process, the specified dimensional limits and tolerances must be maintained. Surface-finish requirements may be important for components that require close fits, proper operation, or reduced friction.
Machined surface finishes should not be confused with protective or decorative coatings such as paint, enamel, or chromium plating. These treatments serve different purposes and are specified using the applicable drawing requirements.
Scale
The scale of a drawing indicates the relationship between the size of the representation and the actual size of the object.
Some drawings are produced at full scale, or 1:1, meaning that the drawing represents the object at its actual size. Other drawings may use a reduced or enlarged scale, depending on the size and complexity of the object.
Computer-based drawings can be enlarged or reduced when viewed on a screen. Printed copies may also be resized by printers or other reproduction equipment. Consequently, the physical size of a printed drawing may not accurately represent the original scale.
For accurate measurements, technicians must refer to the dimensions specified on the drawing rather than relying solely on measurements taken from the printed or displayed image. Figure 1-H illustrates the use of drawing scale.
Application
The application information shown in or near the title block identifies the aircraft, assembly, subassembly, or specific configuration for which a drawing is intended.
For example, Figure 1-A shows a title block indicating that a bracket assembly is intended for a roll servo installation in an S-TEC autopilot system.
If the same bracket were designed specifically for a B95 aircraft equipped with an Aero-Tech air-conditioning system, the title block could identify that particular application. The drawing might include an application description such as:
Bracket Assy., Roll Servo, with Aero-Tech Air Conditioner (Model AT103-1) Installed
Application information helps ensure that the drawing is used for the correct aircraft or equipment configuration.
