Direct Answer — FAA Form 337 in Four Sentences
FAA Form 337 is required by 14 CFR §43.9(d) whenever a major repair or major alteration — as defined in §1.1 and illustrated in Appendix A to Part 43 — is performed on a US-registered aircraft, airframe, engine, propeller, or appliance. Appendix B to Part 43 requires the person performing the work to execute the form at least in duplicate, give a signed copy to the aircraft owner, and forward a copy to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after the product is approved for return to service. Approval for return to service after a major repair or alteration is a separate authority question under §43.7 — typically an IA acting under §65.95(a)(1) with FAA-approved technical data, a Part 145 repair station, or an air carrier under Part 121/135. Retention is asymmetric: under §91.417(a)(2)(vi), the owner's copy for each major alteration is a permanent record that transfers with the aircraft at sale, while major-repair documentation sits in the §91.417(a)(1) one-year-or-superseded bucket.
Sources: 14 CFR §43.9(d), Appendix B to Part 43, Appendix A to Part 43, 14 CFR §91.417.
The Form 337 is one of the few maintenance documents with a federally mandated chain of custody: one copy to the owner, one to the FAA, on a 48-hour clock. It is also one of the documents most often missing when an aircraft changes hands — and a missing alteration 337 is not a paperwork inconvenience, it is a gap in the §91.417(a)(2) permanent record set that the seller is required to transfer. FAA civil penalties for recordkeeping violations under 49 U.S.C. § 46301 reach $75,000 per violation under 14 CFR §13.301 — but the more common cost is commercial: stalled prebuys, devalued airframes, and conformity questions an operator cannot answer from its own files.
What Makes a Repair or Alteration "Major" — §1.1 Definitions + Appendix A Examples
The Form 337 obligation turns entirely on classification. The definitions live in 14 CFR §1.1; the worked examples live in Appendix A to Part 43.
Major Repair — 14 CFR §1.1
§1.1 defines a major repair as "a repair: (1) That, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) That is not done according to accepted practices or cannot be done by elementary operations."
The conditional is the tell: a repair is major if improper performance could appreciably affect airworthiness qualities — the test looks at downside risk, not whether the repair was actually done well.
Major Alteration — 14 CFR §1.1
§1.1 defines a major alteration as "an alteration not listed in the aircraft, aircraft engine, or propeller specifications— (1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) That is not done according to accepted practices or cannot be done by elementary operations."
Two differences from the repair definition: the alteration must not be listed in the product specifications, and the airworthiness-effect test is direct — no "if improperly done" conditional.
Appendix A to Part 43 converts those abstract definitions into lists. Paragraph (a) enumerates major alterations by product — airframe, powerplant, propeller, and appliance. Paragraph (b) does the same for major repairs. Paragraph (c) is a separate enumerated list of preventive-maintenance tasks (which never require a 337). A sampling from the lists, quoted from the appendix:
Airframe — major alterations
Appendix A(a)(1)Alterations of the following parts and alterations of the following types, when not listed in the aircraft specifications issued by the FAA:
- Wings
- Tail surfaces
- Fuselage
- Engine mounts
- Control system
- Landing gear
Airframe — major repairs
Appendix A(b)(1)Repairs to structural members including, among others:
- Box beams
- Monocoque or semimonocoque wings or control surfaces
- Wing stringers or chord members
- Spars
- Spar flanges
- Members of truss-type beams
Powerplant — major repairs
Appendix A(b)(2)Includes, among others:
- Separation or disassembly of a crankcase or crankshaft of a reciprocating engine equipped with an integral supercharger
- Separation or disassembly of a crankcase or crankshaft of a reciprocating engine equipped with other than spur-type propeller reduction gearing
The classification call is made before the work, and it drives everything downstream
Classify a job as minor and you owe only a §43.9(a) maintenance record entry. Classify it as major and you owe a Form 337 executed in duplicate, an owner copy, an FAA copy on a 48-hour clock, FAA-approved technical data behind the return to service, and — for alterations — a permanent record that follows the aircraft for life. When the call is genuinely borderline, in practice operators resolve it with their FSDO or an IA before the work begins, because reclassifying after the fact means reconstructing a disposition chain that should have started at return to service.
One more boundary worth drawing: the Form 337 documents repairs and alterations, not inspections. Annual and 100-hour inspections are recorded under §43.11 — §43.9(c) expressly excludes inspections performed in accordance with Part 91, Part 125, §135.411(a)(1), or §135.419 from §43.9 — so an annual inspection entry never takes a 337. But a major repair performed to correct a discrepancy the annual found does.
Who Executes the Form — and Who May Approve the Work for Return to Service
Two different duties, often held by two different people. Collapsing them is the most common conceptual error around the 337.
Duty 1 — Executing and disposing of the form: the person performing the work
§43.9(d) requires that major repairs and major alterations be "entered on a form, and the form disposed of, in the manner prescribed in appendix B, by the person performing the work." The execution and disposition duty — fill out the form, give the owner a signed copy, get the FAA its copy within 48 hours — sits with the performer, not the owner. Owners still carry the retention side under §91.417, which is why the owner's signed copy matters: it is the artifact the owner must keep and eventually transfer.
Duty 2 — Approving the work for return to service: a §43.7 authority
§43.7 lists who may approve a product for return to service. For major repairs and major alterations, the realistic set is:
- An inspection authorization holder — §65.95(a)(1) lets an IA inspect and approve for return to service any aircraft or related part or appliance after a major repair or major alteration done in accordance with Part 43, "if the work was done in accordance with technical data approved by the Administrator" (with an exception for aircraft maintained under a Part 121 continuous airworthiness program). Note the approved-data condition: the IA's signature rides on the data, not in place of it. See IA renewal requirements.
- A certificated repair station — under §43.7(c), as provided in Part 145. Repair stations also hold the Appendix B(b) work-order privilege for major repairs, covered below. See Part 145 recordkeeping requirements.
- An air carrier — under §43.7(e), as provided in Part 121 or 135, as applicable.
- A manufacturer — under §43.7(d), for products it worked on under §43.3(j), using technical data approved by the Administrator.
- Certain Canadian persons — §43.17 authorizes Transport Canada license holders and Approved Maintenance Organizations to work on US-registered aircraft in Canada. Under that section, individual license holders cannot approve major repairs or major alterations; an AMO with an appropriate Transport Canada-approved quality system can, using FAA-approved technical data, and the Form 337 then follows the Appendix B(c) path.
Who cannot: a mechanic certificate alone
An A&P may perform a major repair or major alteration, but the mechanic ratings' return-to-service privileges in Part 65 exclude approving that work. §65.85(a) states the airframe-rating privilege applies to maintenance or alteration "(excluding major repairs and major alterations)" — that parenthetical is the whole point. The typical small-shop pattern is a two-signature job: the performing mechanic executes the 337 as the person who did the work, and an IA inspects and approves the product for return to service against the approved data.
Data approval is not return-to-service approval
A major repair or alteration needs FAA-approved technical data behind it — an STC, manufacturer-approved data, AD instructions, or, where no approved data exists, a field approval, which in FAA practice is an FSDO airworthiness inspector's approval of the data described on the Form 337 itself. None of those data approvals returns the aircraft to service. The return to service is a separate act by a §43.7-authorized person, and §43.5 makes both the record entry and the executed repair-or-alteration form preconditions: §43.5(b) requires that "The repair or alteration form authorized by or furnished by the Administrator has been executed in a manner prescribed by the Administrator," and §43.5(c) requires that any resulting changes to operating limitations or flight data in the approved flight manual be revised and set forth as prescribed in §91.9. In practice that is also the moment to update weight and balance records — alterations routinely change empty weight and CG, and an outdated W&B sheet is the classic companion finding to an alteration 337.
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FileFlo classifies Form 337s, work orders, and maintenance releases the day they enter your records, indexes them by aircraft, and flags the permanent-record set §91.417(a)(2) says must transfer at sale — so the gap surfaces now, not at prebuy. See the full Aviation compliance coverage.
Where the Copies Go — the Four Appendix B Disposition Paths
Appendix B to Part 43 prescribes four distinct disposition flows. Which one applies depends on who did the work and what was installed.
Standard path — Appendix B(a)
Applies to
Each person performing a major repair or major alteration
Form
FAA Form 337 at least in duplicate
Copy flow
- Signed copy to the aircraft owner
- Copy forwarded to the FAA Aircraft Registration Branch, Oklahoma City, Oklahoma
- FAA copy due within 48 hours after the product is approved for return to service
Repair-station work order — Appendix B(b)
Applies to
Certificated repair station, major REPAIRS only, made per a manual or specifications acceptable to the Administrator
Form
Customer's work order in place of Form 337
Copy flow
- Signed copy of the work order to the aircraft owner
- Duplicate retained by the repair station at least 2 years from return-to-service approval
- Maintenance release furnished, identifying the product and certifying regulatory compliance
Work approved under §43.17 — Appendix B(c)
Applies to
Major repair or major alteration by persons authorized in §43.17 (certain Canadian maintenance personnel and organizations)
Form
FAA Form 337 at least in duplicate
Copy flow
- Completed copy given to the aircraft owner
- Copy forwarded to the FAA
- FAA copy due within 48 hours after the work is inspected
Extended-range fuel tanks — Appendix B(d)
Applies to
Fuel tanks installed within the passenger compartment or a baggage compartment
Form
FAA Form 337 in at least triplicate
Copy flow
- One copy placed aboard the aircraft (kept on board per §91.417(d))
- One copy to the aircraft owner
- One copy to the FAA within 48 hours after the work is inspected
Two details worth reading closely
- The 48-hour trigger is not the same in every path. Under Appendix B(a)(3) the FAA copy is due "within 48 hours after the aircraft, airframe, aircraft engine, propeller, or appliance is approved for return to service." Under B(c) and B(d) — the §43.17 path and the fuel-tank path — the clock runs "within 48 hours after the work is inspected." A shop that templates one cover process for all four paths will eventually be early on one clock and late on another.
- The work-order substitute is a major-repairs-only privilege. Appendix B(b) opens with major repairs "made in accordance with a manual or specifications acceptable to the Administrator" — a certificated repair station may then use the customer's work order in place of the 337, give the owner a signed copy, keep a duplicate at least two years, and furnish a maintenance release certifying the product was "repaired and inspected in accordance with current Regulations of the Federal Aviation Agency and is approved for return to service." (Yes, the appendix still says Federal Aviation Agency — the text predates the FAA's modern name.) There is no parallel privilege for major alterations: an alteration takes a Form 337, full stop.
The FAA-bound copy is what builds the aircraft's file at the Registry — in practice, the Aircraft Registration Branch adds forwarded 337s to the aircraft records file, which anyone can order copies of. That file is the reason undocumented alterations get caught: a prebuy team pulls the FAA records, walks the airframe, and asks why the STOL kit on the wing has no corresponding form in either file. It is also a safety net — if the owner's copy is lost, the FAA's copy usually still exists. But relying on the Registry as your filing system inverts the obligation: the operator's records are what an inspector asks for first, and §91.417(c) requires making them available upon request.
How Long Form 337s Live — the §91.417 Asymmetry
The retention rule treats the same form differently depending on what it documents. This is the single most misunderstood fact about the 337.
Major ALTERATION 337s — permanent
§91.417(a)(2)(vi) + (b)(2): life of aircraft, transferred at sale
§91.417(a)(2)(vi) lists "Copies of the forms prescribed by § 43.9(d)" for each major alteration to the airframe and currently installed engines, rotors, propellers, and appliances among the records that — per (b)(2) — "shall be retained and transferred with the aircraft at the time the aircraft is sold." The alteration 337 sits in the same permanent tier as life-limited parts status, time since overhaul, and current AD status — the six-item (a)(2) set that defines the aircraft's permanent record.
Major REPAIR 337s — the short tier (on paper)
§91.417(a)(1) + (b)(1): until repeated or superseded, or 1 year
Romanette (vi) says major alteration — it does not say major repair. Repair documentation lives in the §91.417(a)(1) bucket, which (b)(1) says "shall be retained until the work is repeated or superseded by other work or for 1 year after the work is performed." That is the regulatory floor — not sensible practice. A major repair to a spar is exactly what the next buyer's structures DER wants to see, and discarding the form at the one-year mark saves nothing while costing provenance. Treat every 337 as permanent; let the regulation's floor stay a floor.
Special cases that change the retention picture
- Cabin fuel tanks ride along. §91.417(d): when a fuel tank is installed within the passenger compartment or a baggage compartment pursuant to Part 43, "a copy of FAA Form 337 shall be kept on board the modified aircraft by the owner or operator." That copy effectively lives as long as the installation does.
- Repair stations keep their own trail. Under the Appendix B(b) work-order path, the station retains its duplicate work order at least two years from return-to-service approval — separate from, and in addition to, whatever the owner keeps. Part 145 stations carry further records duties of their own; see what inspectors ask for in a Part 145 audit binder.
- CAMP operators live under a different rule. §91.401(b) provides that §§91.405, 91.409, 91.411, 91.417, and 91.419 "do not apply to an aircraft maintained in accordance with a continuous airworthiness maintenance program as provided in part 121, 129, or §§ 91.1411 or 135.411(a)(2) of this chapter." Those operators keep equivalent maintenance records under their air-carrier rules — for Part 135 CAMP operators, the parallel record requirements covered in Part 135 CAMP maintenance recordkeeping — so the retention clock lives there rather than in §91.417.
For a Part 135 certificate holder, the 337 set is also part of the surveillance surface: when an inspector works the airworthiness side of a Part 135 surveillance audit, reconciling installed configuration against the alteration record — 337s, STC paperwork, and the ARROW documents — is routine. The question is never whether you once executed the form. It is whether you can produce it, for this tail, in minutes.
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Get Your Readiness ScoreThe Five Ways 337 Records Go Wrong
Each of these is an administrative failure, not a maintenance failure — which is exactly why disciplined document handling prevents all five.
The work-order shortcut applied to an alteration
Appendix B(b) lets a repair station substitute the customer’s work order for major repairs only. A station that treats an avionics-suite alteration the same way leaves the aircraft with no 337 for a major alteration — a permanent-record gap under §91.417(a)(2)(vi) that the owner inherits, usually without knowing it, and that surfaces years later at prebuy.
The owner copy that never made it to the records
Appendix B(a)(2) requires a signed copy to the aircraft owner — but nothing forces the owner to file it. Forms get left in seat-back pockets, shop folders, and email attachments. At sale, §91.417(b)(2) requires the (a)(2) records to transfer with the aircraft; reconstructing from the FAA file works in practice but adds weeks to a transaction and signals sloppy records to the buyer.
The 48-hour FAA copy that slipped
The disposition duty — including forwarding the FAA copy within 48 hours of return to service under Appendix B(a)(3) — belongs to the person performing the work, per §43.9(d). Shops that batch their FAA submissions weekly are structurally late on every form. The owner’s exposure is indirect but real: an aircraft records file at the Registry that is missing alterations the logbooks show.
Data approval mistaken for return to service
A field-approved 337 or an STC is approval of data — it is not the return to service. §43.5 requires the record entry, the executed form, and any flight-manual operating-limitation revisions (per §91.9) before any person may approve the product for return to service. An aircraft flying on approved data with no return-to-service signature has skipped the step the data existed to support.
The companion records left stale
A major alteration that changes empty weight, CG, or equipment lists ripples into weight and balance records, equipment lists, and — where limitations change — the flight manual under §43.5(c). The 337 gets executed; the companion documents lag. An inspector who finds a fresh alteration 337 next to a five-year-old W&B sheet has found two findings, not one.
Where FileFlo fits — the document layer, honestly scoped
FileFlo does not prepare your Form 337, approve your data, or return your aircraft to service — those are acts of certificated people. What FileFlo does is make sure the paper trail those people create stays provable: it classifies 337s, work orders, and maintenance releases as they enter your records, indexes them to the right tail, separates the permanent §91.417(a)(2) set from the one-year (a)(1) tier, and shows you — before an inspector or a buyer does — which alterations in your logbooks have no matching form on file.
When the FSDO letter or the prebuy request arrives, the answer is an organized, audit-ready export instead of a week of folder archaeology.
Frequently Asked Questions
What is FAA Form 337 and when is it required?
FAA Form 337 is the form that documents major repairs and major alterations to US-registered aircraft, airframes, aircraft engines, propellers, and appliances. The requirement comes from 14 CFR §43.9(d): major repairs and major alterations must be entered on a form, and the form disposed of, in the manner prescribed in Appendix B to Part 43, by the person performing the work. Appendix B(a) requires the form to be executed at least in duplicate — a signed copy goes to the aircraft owner, and a copy is forwarded to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after the product is approved for return to service. Minor repairs and minor alterations do not require a Form 337; they are documented with an ordinary §43.9(a) maintenance record entry.
What is the difference between a major repair and a major alteration?
Both terms are defined in 14 CFR §1.1, and the definitions are parallel but not identical. A major repair is a repair that, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness — or 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, aircraft engine, or propeller specifications that might appreciably affect those same airworthiness qualities, or that is not done according to accepted practices or cannot be done by elementary operations. Appendix A to Part 43 then lists concrete examples of each, organized by airframe, powerplant, propeller, and appliance. The distinction carries a real records consequence: under §91.417(a)(2)(vi), Form 337 copies for major alterations are part of the aircraft permanent records and transfer with the aircraft at sale, while major-repair documentation falls into the §91.417(a)(1) bucket with a shorter regulatory retention floor.
Who fills out and signs FAA Form 337?
14 CFR §43.9(d) places the execution duty on the person performing the work. Approval for return to service is a separate authority question governed by §43.7: holders of an inspection authorization may approve an aircraft for return to service after a major repair or major alteration under §65.95(a)(1) if the work was done in accordance with technical data approved by the Administrator (with an exception for aircraft maintained under a Part 121 continuous airworthiness program); certificated repair stations may approve under Part 145; air carriers may approve as provided in Part 121 or 135; and manufacturers may approve work they performed under §43.3(j). A mechanic certificate alone is not sufficient — the airframe-rating privilege in §65.85(a) expressly excludes major repairs and major alterations from the mechanic's return-to-service authority.
Where do the copies of FAA Form 337 go?
Under Appendix B(a) to Part 43, the person performing a major repair or major alteration executes the form at least in duplicate, gives a signed copy to the aircraft owner, and forwards a copy to the FAA Aircraft Registration Branch in Oklahoma City, Oklahoma within 48 hours after the aircraft, airframe, aircraft engine, propeller, or appliance is approved for return to service. For work performed by persons authorized under §43.17 (certain Canadian maintenance personnel and organizations working on US-registered aircraft in Canada), Appendix B(c) requires a completed copy to the owner and a copy to the FAA within 48 hours after the work is inspected. For extended-range fuel tanks installed within the passenger compartment or a baggage compartment, Appendix B(d) requires execution in at least triplicate, with one copy placed aboard the aircraft, one to the owner, and one to the FAA within 48 hours after the work is inspected.
How long must Form 337 copies be kept?
It depends on whether the form documents an alteration or a repair. Under §91.417(a)(2)(vi), copies of the forms prescribed by §43.9(d) for each major alteration to the airframe and currently installed engines, rotors, propellers, and appliances are part of the records that must be retained and transferred with the aircraft at the time the aircraft is sold — effectively the life of the aircraft. Major-repair documentation falls under §91.417(a)(1), which requires retention until the work is repeated or superseded by other work or for 1 year after the work is performed. In practice, almost every operator and prebuy inspector treats every Form 337 as a permanent record: the FAA retains the forwarded copy in the aircraft's records file, and a gap between the FAA file and the aircraft's own records is exactly what stalls a sale. Note also that §91.417 does not apply to aircraft maintained under a continuous airworthiness maintenance program per §91.401(b) — those operators keep equivalent records under their air-carrier rules instead.
Can a repair station use a work order instead of Form 337?
Yes — but only for major repairs, never for major alterations. Appendix B(b) to Part 43 allows a certificated repair station, for major repairs made in accordance with a manual or specifications acceptable to the Administrator, to use the customer's work order upon which the repair is recorded in place of the Form 337. The repair station must give the aircraft owner a signed copy of the work order, retain a duplicate copy for at least two years from the date of approval for return to service, and furnish a maintenance release that identifies the product and certifies the work was done in accordance with current FAA regulations. If the work is a major alteration, the work-order substitute is not available and a Form 337 must be executed and disposed of under Appendix B(a).
Does a Form 337 ever have to be carried on board the aircraft?
In one specific case, yes. When a fuel tank is installed within the passenger compartment or a baggage compartment pursuant to Part 43, §91.417(d) requires that a copy of FAA Form 337 be kept on board the modified aircraft by the owner or operator. That is why Appendix B(d) requires the form for extended-range fuel tank installations to be executed in at least triplicate instead of the usual duplicate — the third copy lives in the aircraft. For all other major repairs and alterations, the owner's copy belongs with the aircraft maintenance records, not in the cockpit.
What happens if a major alteration was never documented on a Form 337?
It creates a permanent-record gap that compounds over time. §43.5(b) makes execution of the repair or alteration form a precondition to approving the product for return to service, and §91.417(a)(2)(vi) makes the major-alteration form part of the records that transfer at sale — so an undocumented alteration is both a maintenance-release problem and a records problem. In practice these gaps surface during prebuy inspections, conformity reviews, and FSDO records reviews, when an installed modification has no corresponding paperwork in either the aircraft records or the FAA aircraft records file. Resolving one is not a do-it-yourself exercise: it typically requires an appropriately rated person — an IA or repair station — to evaluate the installation against approved data and document it correctly. The cheaper path is structural: index every 337 the day it is executed, and reconcile your set against the FAA's file before a transaction, not during one.
Related Aviation Compliance Reading
14 CFR §43.9 Maintenance Record Entry Requirements
The 6 data points every maintenance entry must contain
Part 91 Aircraft Records Requirements
The full §91.417 record set, including the (a)(2) permanent tier
Airworthiness Directive Compliance Records
AD status — another life-of-aircraft (a)(2) record
Aircraft Weight & Balance Records
The companion document every major alteration touches
Part 145 Repair Station Recordkeeping
Work orders, maintenance releases, and retention on the shop side
ARROW Documents Explained
Airworthiness certificate, registration, and what rides in the aircraft
Every 337, indexed, before anyone asks for it
FileFlo is a compliance document intelligence platform: it classifies aviation maintenance documents against the governing CFR — Form 337s under §43.9(d) and Appendix B, the §91.417(a)(2) permanent record set, repair-station work orders and maintenance releases — tracks what must transfer at sale, and generates inspector-format audit binders on demand. It proves your records; your IA, repair station, and FSDO still run the aviation. Starter $89/month · Professional $299/month · 5-day free trial, no credit card required.