FAA Form 337: Filled-Out Example and Instructions

What is FAA Form 337, and when is it required?

FAA Form 337 is titled "Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance)" (FAA Forms). It is required in addition to the normal logbook entry: "major repairs and major alterations shall be entered on a form, and the form disposed of, in the manner prescribed in appendix B, by the person performing the work" (14 CFR 43.9(d)).

What counts as major comes from 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 "That is not done according to accepted practices or cannot be done by elementary operations." A major alteration is defined the same way for alterations not listed in the aircraft, engine or propeller specifications (14 CFR 1.1). Part 43 Appendix A then lists examples, such as alterations to wings, tail surfaces, fuselage, engine mounts, control systems and landing gear, and repairs to spars and other primary structure (Appendix A).

The FAA gives the form three purposes: a record for owners and operators "of major repairs and major alterations indicating the details and approvals", a copy for the aircraft records at the FAA Aircraft Registration Branch, and a record of the installer's compatibility assessment (AC 43.9-1G, paragraph 6.1).

Who may sign which item on Form 337?

Three items carry signatures, and each means something different. The AC is explicit that Item 3 signatures "count as data approval or completeness of approved data only and do not indicate the approval for return to service for the work in Item 8" (AC 43.9-1G, paragraph 9.3.1).

An A&P mechanic alone cannot sign Item 7 for major work: the airframe rating allows return to service "(excluding major repairs and major alterations)" (14 CFR 65.85). An inspection authorization holder may approve an aircraft for return to service after a major repair or major alteration under part 43, "if the work was done in accordance with technical data approved by the Administrator" (14 CFR 65.95). The holder of a repair station certificate may approve for return to service "as provided in Part 145 of this chapter" (14 CFR 43.7).

How to fill out Form 337, block by block (Items 1 to 8)

The FAA uses "Item" and "Block" for the same thing on this form (AC 43.9-1G, paragraph 8). The table summarises the instructions in AC 43.9-1G, paragraph 8.

For a component part repaired or altered off the aircraft, Items 1, 2 and sometimes 5 are left blank and both copies stay with the part until it is installed; the installer then completes them and sends the FAA copy within 48 hours after the aircraft is approved for return to service (AC 43.9-1G, paragraph 8.2).

Form 337 example: a major repair using approved data

Illustrative only. The aircraft, registration, people, certificate numbers and document references are invented, and bracketed items are placeholders. Follow AC 43.9-1G and your own procedures.

Scenario: a repair station repairs a cracked wing spar cap on a single-engine airplane, N12345 (fictional), using the manufacturer's structural repair manual, which is FAA-approved data for this repair. Because the data is already approved, Item 3 stays blank.

• Item 1: N12345; make, model, series and serial number exactly as on the identification plate and the registration certificate.
• Item 2: owner name and address exactly as on the registration certificate.
• Item 3: blank.
• Item 4: Repair checked, one line only.
• Item 5: Airframe line completed (make, model, serial number); other lines blank.
• Item 6: A, the repair station's certificated name and address; B, repair station; C, its air agency certificate number; D, date completed and the authorized employee's signature.
• Item 7: Approved; repair station box; signature, date, printed name and certificate number.
• Item 8: "Removed left wing outboard access panels. Inspected left wing spar lower cap at wing station [X]; found crack 1.2 inches long. Repaired in accordance with [manufacturer] structural repair manual [document number, revision, chapter and figure]. Installed doubler [part number]. Pre-closure inspection performed and covered areas found satisfactory: [signature, certificate number]. Weight and balance not affected. [Additional sheets: none.]"

Form 337 example: a major alteration that needs a field approval

Illustrative only. When the alteration uses data that is not yet approved, the order of work changes. Under AC 43.9-1G, paragraph 9.2, you complete the form "leaving Items 6 and 7 blank. Both copies of the form and any supplemental data will be sent to the FAA office." If the FAA approves the data, it enters a statement in Item 3 and returns both forms; you then do the work, complete Items 6 and 7, give one copy to the owner and send the other to the registry (AC 43.9-1G).

Scenario: an owner wants a one-off antenna installation on N12345 (fictional) that no supplemental type certificate covers. Items 1, 2, 4 (Alteration) and 5 are completed; Item 8 describes the proposed installation, structure, wiring and the data substantiating it; Items 6 and 7 stay blank until the FAA has entered, in Item 3, the statement for approval by examination or by physical inspection quoted in the AC. The FAA's statement for approval by physical inspection reads in part: "The alteration or repair identified herein complies with the applicable airworthiness requirements and is approved for the above described aircraft, subject to conformity inspection by a person authorized in section 43.7." (AC 43.9-1G, paragraph 8.3.2).

Where to send Form 337, and which copies to keep

The rule is in Appendix B to part 43. Each person performing a major repair or major alteration shall "Execute FAA Form 337 at least in duplicate", "Give a signed copy of that form to the aircraft owner" and "Forward a copy of that form 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." (Appendix B). The FAA forms page gives the mailing address as Aircraft Registration Branch, P.O. Box 25504, Oklahoma City, OK 73125, and P.O. Box 25724 for extended-range fuel tank installations (FAA Forms).

• Repair stations have an alternative for major repairs made under a manual or specifications acceptable to the FAA: they may "Use the customer's work order upon which the repair is recorded", give the owner a signed copy and a maintenance release, and "retain a duplicate copy for at least two years" (Appendix B(b)).
• Extended-range fuel tanks in the passenger or baggage compartment need the form "in at least triplicate", with one copy placed on board the aircraft (Appendix B(d); 91.417(d)).
• The owner keeps "Copies of the forms prescribed by § 43.9(d) of this chapter for each major alteration" among the status records (91.417(a)(2)(vi)), and those records "shall be retained and transferred with the aircraft at the time the aircraft is sold" (91.417(b)(2)).
• The owner must also show Form 337 "for inspection upon request of any law enforcement officer" (91.417(c)).

Can Form 337 be filed electronically?

Yes. "This form can be completed on paper or by using the electronic FAA Form 337" (AC 43.9-1G, paragraph 6.2). Pick one route per form: "If FAA Form 337 is started in the electronic format, it must be completed that way and cannot be completed using the paper process." (paragraph 9.2). Electronic forms are submitted to the registry automatically through the FAA's eForm 337 service.

Common mistakes that get a Form 337 returned

Each of these comes from the instructions in AC 43.9-1G.

• N-number and serial number do not match the registry record. The registry validates the form by matching them; if an N-number change is pending and the form must go in first, "then the old 'N' number must be used" (paragraph 8.1).
• More than one Type line or Unit Identification line on one form. Use a separate form for each unit (paragraphs 8.4 and 8.5).
• Oversized attachments. Attachments to Item 8 must be on 8.5 by 11 inch paper; larger ones mean "the submission will be returned to the person identified in Item 7 for correction and resubmission" (paragraph 8.8.3).
• Item 8 that does not reference the FAA-approved data or the applicable 14 CFR sections, or does not say where on the aircraft the work was done (paragraphs 8.8.1 and 8.8.2).
• No pre-closure statement where the repair is later covered by skin or structure (paragraph 8.8.1).
• Treating an Item 3 signature as a return to service. It approves data only; Item 7 is the return to service (paragraph 9.3).
• Using Form 337 for an aircraft that is not US-registered. "FAA Form 337 is not authorized for use with aircraft not registered in the United States." (paragraph 9.4).
• Changing weight and balance without updating the weight and balance records with the date, signature and a reference to the form (paragraph 8.8.4).

Your 337s are also a data asset

A repair station or operator that has filed Form 337s for years holds a dated record of major repairs and alterations, each tied to the approved data used and the outcome of the inspection. With customer names, N-numbers and serial numbers replaced by consistent tokens (see the de-identification guide), years of these records can be licensed to AI developers building maintenance tools. Who owns which records, and what to leave out, is covered in can you sell aircraft maintenance records and, for stations and operators, in the aviation repair station data guide.

If your company keeps years of maintenance, parts and release records, the guide for repair stations and operators explains what AI developers ask for and who signs. To see what yours could be worth, describe them in six questions; no files or system access needed.