Airworthy PrepFAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), is the form you must complete, in addition to the normal logbook entry, whenever you perform a major repair or a major alteration (AC 43.9-1G, Instructions for Completion of FAA Form 337, para 1; 14 CFR 43.9(d)). The person doing the work executes it at least in duplicate, gives a signed copy to the aircraft owner, and sends a copy to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after the item is approved for return to service (14 CFR part 43, appendix B, para (a)). This guide walks through each block, who signs what, and how the topic shows up on the A&P General test, as of October 2026.
What Form 337 is for
AC 43.9-1G gives the form three purposes (para 6.1): a record for the owner of major repairs and alterations with their approvals, a copy for the aircraft’s file at the FAA Aircraft Registration Branch, and a record of the installer’s compatibility assessment (the NOTICE block above Item 8).
The form is free, as a fillable PDF or through the electronic Form 337 system (E-337) at eforms.faa.gov (paras 6 and 6.2). A form goes in either electronically or on paper, never both (para 9.2, note).
When is a Form 337 required?
A Form 337 is required for every major repair and every major alteration (14 CFR 43.9(d); part 43, appendix B, para (a)). Minor repairs and minor alterations get a logbook entry only.
14 CFR 1.1 defines both terms (14 CFR 1.1):
- A major repair is one that, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics or other qualities affecting airworthiness, or one that is not done according to accepted practices or cannot be done by elementary operations.
- A major alteration is an alteration not listed in the aircraft, engine or propeller specifications that might appreciably affect those same qualities, or that is not done according to accepted practices or cannot be done by elementary operations.
- Minor repairs and minor alterations are simply everything that is not major.
Part 43, appendix A lists examples, such as repairs that splice or reinforce spars (appendix A, para (b)(1)). For how to decide, see our guide to major vs minor alterations under part 43, appendix A.
The form is also tied to return to service: under 14 CFR 43.5(b), no one may approve an item for return to service unless the repair or alteration form authorized by or furnished by the Administrator has been executed in a manner prescribed by the Administrator.
Form 337 vs logbook entry
Keep this straight: a Form 337 does not replace the logbook entry. 43.9(d) says major repairs and major alterations go on the form “in addition to the entry required by paragraph (a)” of 43.9. So a major repair needs both:
| 43.9(a) logbook entry | Form 337 | |
|---|---|---|
| Required for | All maintenance, preventive maintenance, rebuilding and alteration (43.9(a)) | Major repairs and major alterations only (43.9(d)) |
| Who makes it | Each person who does the work (43.9(a)) | The person who performs or supervises the work prepares it (AC 43.9-1G, para 8) |
| Copies | One entry in the item’s maintenance record | At least two: one to the owner, one to the FAA (appendix B, para (a)) |
| Goes to the FAA | No | Yes, within 48 hours of approval for return to service (appendix B, para (a)(3)) |
For what the logbook entry itself must contain, see our guide to 14 CFR 43.9 and 43.11 maintenance record entries.
Weight and balance changes go in the weight and balance records with the date, a signature and a reference to the form (AC 43.9-1G, para 8.8.5).
How to fill out Form 337, block by block
AC 43.9-1G, paragraph 8, covers Items 1 through 8. The FAA uses “Item” and “Block” to mean the same thing (para 8). Here is each one in plain words.
| Item | What goes in it | Source |
|---|---|---|
| 1. Aircraft | Nationality and registration mark (the “N” number) as on the Certificate of Aircraft Registration, plus make, model and serial number from the manufacturer’s identification plate, and the series of the model | AC 43.9-1G, para 8.1 |
| 2. Owner | The owner’s complete name and address as on the registration certificate (or the registration application during a change of ownership) | para 8.2 |
| 3. For FAA Use Only | Data approval or a designee’s statement of completeness, entered by the FAA or an authorized designee. Left alone when you use approved data | paras 8.3, 9.1 |
| 4. Type | A check showing whether the unit was repaired or altered. One “Type” line per form | para 8.4 |
| 5. Unit Identification | Identifies the airframe, powerplant, propeller or appliance worked on. Fill in only the unit you worked on. One line per form | para 8.5 |
| 6. Conformity Statement | A: name and address of the person or agency doing the work. B: kind of agency. C: certificate number (mechanics enter their mechanic certificate number). D: completion date and signature certifying the work complies with part 43. For extended-range fuel tanks installed in the passenger or baggage compartment, check the box provided | paras 8.6.1 to 8.6.5 |
| 7. Approval for Return to Service | “Approved” or “Rejected,” who made the finding, signature, date, printed name and certificate or designation number | paras 8.7.1 to 8.7.3 |
| 8. Description of Work Accomplished | On the back of the form: a clear, concise, legible description of the work, its location, the applicable 14 CFR sections and the FAA-approved data used | paras 8.8.1, 8.8.2 |
A few details worth knowing for Item 8:
- Location matters. The AC’s own example begins by saying the wing was removed, skin removed from the outer 6 feet, and a buckled spar repaired 49 inches from the tip (para 8.8.1).
- Hidden work. If the repair or alteration will be covered by skin or other structure, an authorized person should add a pre-closure statement with a signature and certificate number saying the covered areas were inspected and found satisfactory (para 8.8.1).
- Extra pages. Check “Additional Sheets Are Attached” and use 8 1/2 by 11-inch paper (para 8.8.3).
For a component part worked on off the aircraft, Items 1, 2 and sometimes 5 stay blank and both copies stay with the part. The installer completes them, gives one copy to the owner and sends the other to the FAA within 48 hours after the aircraft is approved for return to service (para 8.2, note).
Who signs Form 337?
Three different signatures mean three different things (AC 43.9-1G, para 9.3):
- Item 3 (FAA inspector or designee): approval of the data, or a designee’s certification that the data is complete. It is not approval for return to service (para 9.3.1 and note).
- Item 6 (the person who did the work): certification that the work complies with all applicable airworthiness requirements and FAA-approved data, for the work in Item 8 only (para 9.3.2).
- Item 7 (the person approving for return to service): the airworthiness determination. A signature here is not an approval of data (para 9.3.3).
Here is the catch for A&P students. A mechanic’s airframe and powerplant ratings allow approval for return to service, excluding major repairs and major alterations (14 CFR 65.85(a), 65.87(a); paragraph (b) of each has a light-sport exception). A holder of an inspection authorization (IA) may inspect and approve for return to service an aircraft or part after a major repair or major alteration, if the work was done under technical data approved by the Administrator (14 CFR 65.95(a)(1)). So a typical mechanic can do the work and sign Item 6, while an IA (or another authorized person such as a repair station under 43.7(c)) signs Item 7.
Before signing Item 7, the approver must make sure all the work in Item 8 matches approved data and the form is complete (AC 43.9-1G, para 8.7.3).
Approved data vs unapproved data
How the form moves depends on the data (AC 43.9-1G, paras 9.1 and 9.2):
- Approved data. Complete everything except Item 3, then distribute the copies as usual.
- Unapproved data. Leave Items 6 and 7 blank and send both copies, with supporting data, to the FAA office. If the FAA approves the data, it records that in Item 3 and returns both copies. You then complete Items 6 and 7 and distribute them.
Where the copies go and how long they are kept
| Who | What happens | Source |
|---|---|---|
| Aircraft owner | Gets a signed copy | Part 43, appendix B, para (a)(2) |
| FAA Aircraft Registration Branch | Gets a copy within 48 hours after approval for return to service. Paper forms go to AFB-710, P.O. Box 25504, Oklahoma City, OK 73125; E-337 forms are submitted automatically | Appendix B, para (a)(3); AC 43.9-1G, para 9.5 |
| On board the aircraft | A copy, but only for extended-range fuel tanks in the passenger or baggage compartment (form made in at least triplicate) | Appendix B, para (d); 14 CFR 91.417(d) |
The owner or operator keeps copies of the forms for each major alteration to the airframe and currently installed engines, rotors, propellers and appliances, and these records transfer with the aircraft when it is sold (14 CFR 91.417(a)(2)(vi) and (b)(2)). When a fuel tank is installed in the passenger or baggage compartment, the owner or operator must also present the on-board Form 337 for inspection if a law enforcement officer asks (91.417(c) and (d)).
One alternative exists. A certificated repair station doing a major repair under a manual or specifications acceptable to the Administrator may use the customer’s work order and give the owner a signed maintenance release instead, keeping a duplicate for at least two years (appendix B, para (b)). That option covers major repairs only, not major alterations.
How the A&P General test asks about Form 337
Form 337 falls under subject I.I of the General test, Regulations, Maintenance Forms, Records, and Publications, which makes up 5 to 10 percent of the 60 questions (Companion Guide to the mechanic ACS, FAA-G-ACS-1, Change 2, ch. 3, pp. 12-13). The Aviation Mechanic ACS, FAA-S-ACS-1 lists these related elements (pp. 14-15):
- AM.I.I.K5: the purpose and use of FAA forms, including Form 337.
- AM.I.I.K6: part 1 terms, including major and minor repair and alteration.
- AM.I.I.K7: criteria and responsibility for deciding whether a repair or alteration is major or minor.
- AM.I.I.R1: completeness or accuracy of documentation.
- AM.I.I.S1 and S2: complete a Form 337, and examine one for accuracy. Skill elements are evaluated on the practical test (Companion Guide, ch. 2, p. 11), part of the oral and practical test.
Try one (an original question)
An A&P mechanic without an inspection authorization completes a major repair to a wing spar using FAA-approved data. Which statement is correct?
A. The mechanic signs Item 7 and returns the aircraft to service, because the data is approved.
B. The mechanic signs Item 6, and a person such as an IA approves the aircraft for return to service in Item 7.
C. A logbook entry is enough, because the data is approved.
Answer: B. The airframe rating’s return-to-service privilege excludes major repairs (14 CFR 65.85(a)), while an IA may approve after a major repair done under approved data (65.95(a)(1)). C is wrong because 43.9(d) requires the form in addition to the logbook entry.
A short study plan
- Read 14 CFR 43.5, 43.9, part 43 appendix B, and the 14 CFR 1.1 definitions.
- Read AC 43.9-1G, paragraphs 6 to 9.
- Read 14 CFR 65.85, 65.87 and 65.95(a) for who may approve.
- Practice on the Regulations, Maintenance Forms, Records, and Publications topic page.
Common questions
Who fills out and signs Form 337?
The person who performs or supervises the work prepares it (AC 43.9-1G, para 8) and signs Item 6. The person approving for return to service, such as an IA, signs Item 7. An FAA inspector or designee signs Item 3 only when data needs approval or certification (paras 9.3.1 to 9.3.3).
When does Form 337 have to reach the FAA?
Within 48 hours after the aircraft or item is approved for return to service (14 CFR part 43, appendix B, para (a)(3)).
Do I still need a logbook entry if I file a Form 337?
Yes. 43.9(d) requires the form in addition to the 43.9(a) maintenance record entry.
Can an A&P mechanic sign off a Form 337?
An A&P can do the work and sign Item 6, but the airframe and powerplant ratings exclude approving major repairs and major alterations for return to service (14 CFR 65.85(a), 65.87(a)). An IA may approve them under 65.95(a)(1).