When an aviation maintenance technician (AMT) performs a major alteration to a certificated aircraft, the paperwork that follows is not a formality — it is a federally mandated process that protects the airworthiness of the aircraft, the rights of the owner, and the legal standing of everyone involved. Understanding exactly who must sign, what must be recorded, when the owner must be notified, and what happens to the paperwork afterward is essential knowledge for any AMT, and it is a reliable source of questions on the FAA General written knowledge test.
This article walks through the entire notification and documentation chain for major alterations: from recognizing what qualifies as a major alteration, to completing FAA Form 337, to the return-to-service signature, to the owner notification requirement, and finally to the distribution of that form to the FAA registry. Every step is grounded in 14 CFR Part 43 and its associated appendices.
What Qualifies as a Major Alteration
Before notification requirements even come into play, you must correctly classify the work. Under 14 CFR Part 43, Appendix A, a major alteration is defined as an alteration not listed in the aircraft, aircraft engine, or propeller specifications that might appreciably affect structural strength, flight characteristics, powerplant operation, flight characteristics, or other airworthiness qualities — and that is not done according to accepted practices or cannot be done by elementary operations. Examples include installing a non-approved engine modification, changing control surface geometry, or adding avionics that affect the aircraft's flight envelope.
Work that does not meet this threshold is either a minor alteration (which requires only a maintenance record entry) or a major repair (which also requires Form 337 but under slightly different circumstances). Getting the classification right is the first gate in the process.
FAA Form 337: The Core Document
Any major alteration — and any major repair — must be documented on FAA Form 337, Major Repair and Alteration. This two-page form captures the aircraft identification, the description of work performed, the parts and materials used, the method of compliance with airworthiness standards, and the signatures that make the aircraft legally airworthy again.
The form must be completed in duplicate. One copy is retained by the aircraft owner as a permanent part of the aircraft maintenance records. The second copy must be forwarded to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after the aircraft is approved for return to service, as specified in 14 CFR Part 43, Appendix B. This 48-hour rule is one of the most commonly tested specifics in AMT exam preparation — it applies to the FAA copy, not to the owner's copy.
Who Fills Out Form 337
The person who performs or supervises the major alteration is responsible for completing the Form 337 accurately. That person is typically a certificated AMT holding an Airframe or Powerplant rating, as appropriate to the work. A certificated repair station may also accomplish and certify major alterations within its ratings. When a Designated Airworthiness Representative (DAR) or an FAA Aviation Safety Inspector approves the data used for the alteration, that approval is reflected in Block 3 of the form.
Approval for Return to Service
Completing Form 337 does not by itself return the aircraft to service. The person who approves the aircraft for return to service must make a maintenance record entry as required by 14 CFR 43.9 and must sign and date Block 7 of Form 337. This signature constitutes the approval for return to service and carries significant legal weight — the signer is certifying that the work was done in accordance with the applicable airworthiness requirements and that the aircraft is airworthy with respect to the alteration performed.
Only persons authorized under 14 CFR 43.3 may approve an aircraft for return to service after a major alteration. These include certificated AMTs with the appropriate rating, certificated repair stations, and, in limited circumstances, manufacturers. A pilot-owner exercising the owner-produced parts provisions does not have authority to approve a major alteration for return to service — that requires a certificated AMT or repair station.
The Owner Notification Requirement
Here is the element that catches many students off guard: even after Form 337 is properly completed and the return-to-service signature is in place, the regulations impose a specific notification duty toward the aircraft owner. Under 14 CFR 43.9(a) and the specific requirements of Part 43, Appendix B, the owner or operator must be provided with a copy of the completed Form 337 so they can maintain it as part of the aircraft's permanent maintenance records. The aircraft owner is legally required to keep this document; it must be available for inspection and must accompany the aircraft when it is sold.
The practical implication is straightforward: before the aircraft leaves the shop, the AMT must hand the owner (or operator) the original completed Form 337 copy designated for the aircraft records. If the owner is not physically present, best practice — and what the regulations contemplate — is that the owner receives written documentation of the alteration and the return-to-service status before the aircraft is operated. The owner's signature is not required on Form 337 itself as a condition of approval, but the owner must be notified and must receive their copy of the form.
This distinction matters on the test: the AMT signs to approve return to service; the owner receives the form and must keep it. The owner does not approve the alteration, but the owner does have a legal obligation to maintain the record and to make it available upon request by the FAA.
Key Numbers and Rules
- 48 hours: The FAA copy of Form 337 must be sent to the FAA Aircraft Registration Branch within 48 hours after the aircraft is approved for return to service (14 CFR Part 43, Appendix B, and Form 337 instructions).
- Two copies: Form 337 is always prepared in duplicate — one for the aircraft owner's records, one for the FAA.
- Permanent record: The owner's copy of Form 337 is a permanent maintenance record that must be transferred with the aircraft upon sale (14 CFR 91.417).
- 14 CFR 43.3 authorization: Only persons authorized under this section may sign Block 7 (return-to-service approval) on Form 337 after a major alteration.
- Appendix A classification: The definition of major vs. minor alteration is found in 14 CFR Part 43, Appendix A — knowing this appendix is crucial for proper form selection.
- 14 CFR 91.417: Owner/operator record-keeping requirements; requires that the Form 337 be available for inspection and transferred with the aircraft.
- No pilot-owner RTS authority: Unlike preventive maintenance under 14 CFR 43.3(g), a pilot-owner cannot approve a major alteration for return to service.
Why This Process Matters
Major alterations can significantly change how an aircraft flies, how much it weighs, or how its systems function. If those changes are not properly documented, a future owner, AMT, or pilot may be completely unaware that the aircraft has been altered from its type-certificated design. Improper alterations that go undocumented have contributed to fatal accidents — the FAA's documentation chain exists precisely to prevent an airworthiness-critical modification from disappearing into the aircraft's history.
From the AMT's career perspective, failing to complete Form 337, failing to forward the FAA copy within 48 hours, or failing to provide the owner with their copy are each violations of 14 CFR Part 43. The FAA may take certificate action against an AMT who consistently fails to meet these documentation requirements, regardless of how technically sound the underlying work was.
Common Test Traps
- Confusing major repairs with major alterations: Both require Form 337, but the classification affects how the regulatory approval data is cited. Know Appendix A definitions cold.
- Misidentifying the 48-hour deadline: The 48-hour rule applies to sending the FAA's copy to Oklahoma City, not to completing the form or notifying the owner. Don't mix up the timing.
- Assuming the owner must sign for approval: The owner does not approve the return to service — the certificated AMT or repair station does. The owner receives the record, not the signature authority.
- Forgetting that Form 337 is permanent: Some students assume maintenance records can be discarded after a period of years. Form 337 is a permanent record that stays with the aircraft indefinitely and transfers to new owners.
- Overlooking the pilot-owner limitation: Under 14 CFR 43.3(g), pilots may perform preventive maintenance and approve it for return to service — but this does NOT extend to major alterations. Any question that implies a pilot-owner signed off a major alteration describes a regulatory violation.
Mastering the Form 337 process — who completes it, who signs it, who gets a copy, and when the FAA copy must be mailed — gives you a solid command of one of the most practically important areas of AMT regulatory knowledge. More importantly, following these procedures faithfully in the field ensures that every aircraft leaving your shop does so with a complete, accurate, and legally sound maintenance history.