Airframe Technology: Blueprint Reading
In the world of aircraft maintenance and construction, blueprints are regularly used to perform a variety of tasks. The basic purpose of the blueprint is to describe parts and repairs in a graphical fashion and provide a host of design information to the end user regarding dimensions, processes, specifications, tolerances, and more. The aircraft maintenance technician, AMT, must be able to interpret this graphic, determine drawing orientation, the correct drawing affectivity and revision status, and all of the applicable note information.
Before we get started, when drawings were first reproduced in the mid 1800s, early technology would produce a document having a blue background with white lines and writing. Those traditional methods have given way to more modern methods — yet the term “blueprint” is still used today. For consistency this article will use the word drawing.
Types of drawings
Drawings can be found in many documents like service bulletins, maintenance and structural repair manuals, engineering instructions, and supplemental type certificate (STC) installation instructions. Production drawings used during new manufacture of aircraft and components are sometimes broken down into three different groupings:
Detail drawings which provide information for single parts, assembly drawings which show two or more parts joined together, and installation drawings which provide information regarding the component/assembly’s location on the aircraft.
Affectivity
When using a drawing it is essential the correct affectivity and latest revision to the drawing is utilized. Typically, aircraft production line numbers are used for affectivity.
Many aircraft have had long production runs and it’s essential the correct aircraft affectivity is applied to the correct information in the drawing. Also you need to ensure that any amendments noted on the drawing will be applied if applicable. As an example, heat treat condition or material thickness may have changed from one aircraft to another during production. Today, drawings may be accessed through web-based systems directly from manufacturers, which ensure the correct revision is used.
The drawing tree
A drawing tree is a system used to locate a specific part or assembly on a series of drawings. In the drawing title block (which is explained later) for a detail part, a part number is given. Once the part is fabricated and identified on the drawing, a next higher assembly (NHA) number is noted. The NHA drawing shows the assembly or installation of that detail part on the NHA. This drawing also has a NHA number that correlates to a larger assembly. This drawing tree concept continues until all the assemblies are installed onto their NHA to create a complete aircraft or assembly. The drawing sheet for a complete aircraft is known as the top drawing and will show major assemblies such as the left wing, right wing, fuselage, and empennage. Mastering the use of the drawing tree system will assist you in easily going from the top drawing to a detail part or vice versa.
Drawing layout
The typical drawing will have a title block in the lower right hand corner of the drawing. The title block will contain information such as drawing title, drawing number, measurement scale, aircraft type, revision status, sheet/page number, approving engineer, and the name of the draftsperson. The drawing may have a quadrant system layout with numbers across the bottom of the drawing and letters up the side. Like a road map these quadrants help to quickly locate specific areas of the drawing.
Most drawings will provide the best view of the component, repair, or assembly. The drawing may be of the top looking down, from the front looking back, or from the bottom looking up. The user must take time to study the drawing and determine which view is being presented. There may be orientation keys such as arrows that point forward, aft, up, down, inboard, or outboard. Drawings will also give aircraft station location information. Manufacturers may vary in this detail but standards do exist like fuselage and wing stations start at the front and go aft. Buttock line stations for fuselage and engine nacelle centers go from left to right. Vertical stations or water lines run up and down.
In order to provide further detail of a specific view, the draftsperson may insert a cutting plane or a cross-sectional view of the part. In complex multi-page drawings these cutting planes will have grid numbers to locate them on the individual drawing sheet.
Lines and symbols
Many different types of lines are used on drawings. A solid line indicates the edge of a part. A single dashed line indicates the edge of a part that is hidden. A long dashed and short dashed line may indicate a break line, a center line, or a phantom line showing reference. A closely dashed line called a stitch line indicates stitching or sewing. A thin solid line with freehand zigzags indicates a break to reduce the part size to fit the sheet. A thick solid line with arrowheads indicates a cutting plan view. Dimension lines are thin lines with arrows on each used to indicate distance measured. A leader line points to the edge of a part for dimension measurements. Extension lines are thin unbroken lines used to indicate the extent of dimensions. Center lines or C/L will indicate the center of a part, engine, component, or fuselage.
Helpful Hint: When viewing a complex drawing use different colored pencils to outline the different parts on the drawing and how they stack up.
Fastener symbols
Rather than clutter a drawing with a long list of different fasteners, a fastener quadrant may be used. In the upper left corner of the fastener quadrant is a note or fastener code such as XCM. This code is normally shown on the title page of the drawing such as XCM=NAS1097 AD rivet. In the upper right corner of the fastener quadrant is fastener diameter in 1/32 inch. In the lower right corner of the quadrant is fastener length in 1/16 inch. In the lower left corner of the quadrant dimpling and countersinking information is detailed. Fastener head placement will be shown such as N = near side, F = far side, blank = your choice. It is assumed that on a drawing with a long run of fasteners, unmarked fasteners in the middle of the run are the same as the marked fasteners on either end of that run.
Helpful Hint: To prevent drilling the wrong size hole and installing the incorrect fastener use colored pencils to mark the different type/size fasteners on the drawing.
Process information and parts list
Process information will also be given which may include details like heat treat specifications, corrosion protection, drawing tolerances, edge sealant information, fastener installation information, part marking, and surface finish requirements. A parts list may be included to assist with identifying which parts are needed for the assembly or installation, and may also provide the number of individual parts needed for a higher assembly.
Conclusion
In aviation maintenance the end user of a drawing is generally the AMT. The drawing is a document that takes information from a designer or manufacturer and provides it to you. You must correctly interpret this drawing in order to fabricate, produce, or repair aircraft and components. A drawing can be as simple as fabricating a single sheetmetal part, or as complex as putting together hundreds of individual parts. It is critical to follow all notes and process information that apply to your specific aircraft or component. Not adhering to all of the drawing notes has been linked to aircraft being damaged or even lost. When you find drawing information incorrect or missing, take steps to communicate your findings back to the drawing originator for correction.
Marty Holzer is a certificated AMT. For the last 24 years he has worked in aircraft manufacturing, MRO, and air carrier line maintenance. Now working as a technical instructor for a major air carrier, Holzer has developed and taught structural repair courses which include reading and interpreting drawings for structural repair on commercial and regional aircraft.
