The question
Every buyer of a legacy airframe asks the same thing: has it been damaged, and was the repair done right? The logbook narrative rarely volunteers the answer. But major structural repairs and major alterations leave a specific, findable artifact — FAA Form 337 — and its presence, absence, or thin data basis is one of the clearest signals an aircraft's records give.
What "major" means, and why it triggers a 337
"Major" is not a judgment call; it is defined. Part 43, Appendix A sorts repairs and alterations into major and minor by airframe, powerplant, propeller, and appliance. Appendix A lists airframe major items such as work on box beams and engine mounts, and powerplant major repairs such as the "separation or disassembly of a crankcase or crankshaft of a reciprocating engine." When work meets the major threshold, 14 CFR 43.9(d) requires it to be entered on a form disposed of as prescribed in Appendix B — FAA Form 337. A minor repair generates only a logbook entry; a major one generates a 337. The type of paperwork itself tells you how the mechanic classified the work.
What the form should tell you
A properly executed 337 identifies the aircraft, the work performed, and — critically — the approved data the work was based on: a Supplemental Type Certificate, manufacturer data, or data field-approved by the FSDO and recorded on the form. That approved-data basis is the difference between a documented, authorized repair and an assertion. For a damage repair, the 337 is where a buyer learns that a wing spar, a firewall, or a control surface was repaired — and on what authority.
Where the records betray the history
These records are supposed to travel with the aircraft: 91.417(a)(2)(vi) requires copies of the 337 for every major alteration, and 91.417(b)(2) requires the (a)(2) records to transfer at sale. So the failure modes are legible. A visible modification — an STC antenna, a gross-weight increase, an engine conversion — with no matching 337 is a gap. A 337 for a structural repair with a vague or missing data basis is a different gap. And a repaint or fresh interior that postdates the last logbook damage entry is worth a question, because new cosmetics are where damage history goes to hide.
What cannot be concluded
Reading the 337 package and cross-checking it against the airframe flags where major work is undocumented, where the approved-data basis is unclear, or where the physical aircraft and the paperwork disagree. It does not certify any repair or alteration as airworthy and it does not approve a return to service. Whether a given alteration is acceptable, and whether the aircraft conforms to its type design, is for the responsible IA and, where required, the FAA to determine against current data.
What to request or verify next
- The complete 337 file, and a walk of the airframe to match every visible modification and repair to a form.
- The approved-data basis on each 337 — STC number, manufacturer data reference, or field approval — not just the description block.
- The 8130-3 or shop records behind any major component repair, for parts traceability.
- A written explanation for any major repair implied by the aircraft's condition but absent from the records.