Every aircraft drawing is more than a picture — it is a complete technical document that tells a mechanic not only what a part looks like, but exactly what materials and components are needed to build or assemble it. Two closely related features carry that information: the bill of materials (BOM) and the parts list. Together they transform a graphic representation into an actionable manufacturing or maintenance instruction. For AMT candidates, understanding how to read and use these lists is a fundamental skill tested on the FAA General knowledge exam and applied every day in the hangar.
Although the two terms are sometimes used interchangeably, there is a meaningful distinction. A bill of materials typically describes the raw stock — metal alloy and temper, sheet thickness, tubing diameter, fabric weight, hardware specifications, and similar starting materials — needed to fabricate the part shown on the drawing. A parts list (also called a parts breakdown) tends to list finished, individually identified components that are assembled together to produce the final assembly shown. On many aircraft drawings you will find one or both of these lists presented as a tabular block, usually in the lower right area of the drawing sheet or immediately adjacent to the title block.
How Bills of Materials and Parts Lists Are Organized
Both documents are structured as tables with a consistent set of columns, making it easy to locate information quickly even on a complex drawing. The most common columns you will encounter are:
- Item number (or find number): A sequential number, often circled or flagged on the drawing view itself, that links a bubble on the graphic directly to a row in the list. When a mechanic needs to identify a specific fastener or bracket shown in an assembly view, the item number is the bridge between the picture and the data.
- Part number: The manufacturer's identifying number for that specific component. This is the number used when ordering a replacement part or verifying an approved part in the aircraft's maintenance records.
- Nomenclature or description: A plain-language name for the part or material (for example, "Bracket, aileron bellcrank" or "Sheet, aluminum alloy 2024-T3, 0.040 in."). Standard abbreviations drawn from military and industry standards are common here.
- Material specification: On a BOM in particular, this column identifies the exact specification the raw material must meet — for instance, a reference to an aluminum alloy specification or a steel specification (the exact specification number varies by manufacturer and part). Knowing the specification matters because substituting the wrong alloy can compromise structural integrity.
- Quantity required: How many units of that item are needed for one complete assembly. This tells a mechanic how many rivets of a given type to pull, how many inches of tubing to cut, or how many washers to obtain.
- Unit of measure: Tells whether quantity means pieces, feet, square inches, pounds, or another unit. Missing or misreading the unit of measure is a classic source of shop errors.
- Reference or zone: On larger drawing sheets divided into a grid (similar to a road map), the zone column directs the reader to the area of the sheet where the item appears, saving search time.
On assembly drawings the parts list may also include a next-higher assembly reference, showing which larger assembly the current drawing feeds into. This is important for traceability and for understanding the functional purpose of each component.
Reading Callout Balloons and Matching Them to the List
On an assembly drawing, each component is tagged with a balloon callout — a circle containing the item number, connected by a leader line to the part on the drawing. The mechanic reads the item number in the balloon, locates that same number in the parts list, and instantly has the part number, description, material, and quantity needed. This system allows a complex assembly with dozens of parts to remain readable because the drawing view itself stays clean; all the text data lives in the list.
When a dimension or note on the drawing references a specific material — for example, "Material per item 3" — the mechanic traces that reference to item 3 in the BOM to find the complete material specification. This layered reference system means the drawing, the BOM, and the parts list must always be read as a unit, never in isolation.
Why Bills of Materials and Parts Lists Matter
From a regulatory standpoint, using correct materials and approved parts is not optional — it is a requirement for airworthiness under 14 CFR 43.13 and the applicable aircraft maintenance manual. A bill of materials gives an AMT the documented, manufacturer-approved basis for selecting raw stock. If a mechanic fabricates a replacement bracket from an aluminum alloy that is stronger or weaker than specified, the part may not perform as designed under flight loads, even if it looks identical. The BOM removes guesswork by specifying the exact alloy, temper, and thickness the original designer required.
Parts lists are equally critical for assembly work. Installing a fastener of the wrong grip length, a bearing with the wrong bore diameter, or a seal made of an incompatible material are all errors that a careful review of the parts list prevents. These lists also support the return-to-service documentation process: when an AMT signs off maintenance in a logbook, the parts list provides the part numbers and descriptions needed for an accurate and legally sufficient entry.
Beyond individual maintenance events, bills of materials and parts lists are the foundation of material traceability. If a manufacturer issues a service bulletin or airworthiness directive affecting a specific part number, maintenance personnel can use the parts list to quickly determine whether the affected part is installed on a given aircraft.
Key Numbers and Rules
- The parts list or BOM is commonly located near the title block, often in the lower right area of the drawing sheet or on a separate sheet, but the exact location varies by manufacturer and drawing standard — check the title block area first.
- Item numbers on the drawing balloon and in the list must match exactly; a discrepancy suggests a revision error and should be resolved before work proceeds.
- Quantity values apply to one complete assembly as shown on the drawing; if two of those assemblies are needed for the aircraft, quantities must be doubled.
- Material specifications referenced in a BOM are legal requirements for fabricated parts under FAA-accepted data; substituting an unlisted material requires separate engineering approval or an approved equivalent specification.
- Revision letters on the BOM and on the drawing title block must agree; if the BOM shows revision C and the drawing title shows revision D, one document is out of date and the discrepancy must be resolved.
- When ordering parts, always use the part number from the parts list, not just the item number — item numbers are drawing-internal references and have no meaning outside that specific drawing.
Practical Application in the Hangar
When an AMT receives a drawing package for a fabrication job, the recommended workflow is: (1) read the title block to confirm the drawing number, revision level, and scale; (2) scan the BOM or parts list to understand what materials and components are involved before studying the views; (3) check that all materials and parts are on hand and match the listed specifications; (4) then study the graphic views and dimensions in detail. Starting with the parts list gives context that makes the views easier to interpret.
For sheet metal fabrication, the BOM will specify the aluminum alloy and temper (such as 2024-T3 or 6061-T6), the sheet thickness in decimal inches, and often the surface treatment or finish required. These details cannot be assumed from the drawing view alone — only the BOM carries them. Similarly, for welded steel tube structures, the BOM will specify the steel alloy (for example, 4130 chromoly) and tube dimensions needed for each member.
Common Test Traps
- Confusing item numbers with part numbers: Item numbers are internal drawing references; part numbers are used for procurement and logbook entries. The FAA exam may present a scenario where selecting the item number instead of the part number would be an error.
- Ignoring the revision level: A drawing and its BOM must be at the same revision. Test questions sometimes describe a mismatch scenario to see if you recognize it as a problem.
- Misreading quantity vs. unit of measure: A BOM might list "36" with a unit of "inches" for a piece of tubing — not 36 pieces of tubing. Confusing the two leads to serious shop errors and is a common test scenario.
- Assuming a visually similar part is interchangeable: The parts list specifies the exact approved part number. A part that looks the same but carries a different part number has not been approved for that installation without additional engineering review.
- Overlooking next-assembly references: Some questions test whether you understand that a parts list may reference a higher assembly, and that the drawing you are reading is itself a component of a larger assembly drawing package.