How to Complete and Submit FAA Form 337: Filing and Approvals

To complete and submit FAA Form 337 for a major repair or major alteration, fill in each numbered block using data that matches the aircraft’s registration and data plate, describe the work and the FAA-approved data supporting it in Item 8, obtain the return-to-service signature in Item 7 from a person authorized under 14 CFR 43.7, give one signed copy to the aircraft owner, and forward a duplicate to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours of approving the aircraft for return to service.¹ The form travels with the aircraft for life, so every future owner, inspector, and buyer will see what was done and who approved it.

When the Form Is Required

Form 337 documents major repairs and major alterations to the airframe, powerplant, propeller, or appliance of a U.S.-registered aircraft.² A major repair is one that, if done incorrectly, could noticeably affect weight, balance, structural strength, performance, powerplant operation, or flight characteristics, or that falls outside accepted practices. A major alteration is a change not listed in the aircraft, engine, or propeller specifications that could have those same effects.³

Minor repairs and minor alterations do not require a Form 337; they go in the aircraft logbook instead. Appendix A to 14 CFR Part 43 lists the specific work that counts as major, covering wing, tail, fuselage, control system, and landing gear changes; modifications to fuel, electrical, hydraulic, and pressurization systems; engine conversions that change compression ratio or gear ratios; changes to propeller blade or hub design; and installation of avionics not previously approved for the aircraft.⁴ Most glass-panel and ADS-B installations rely on a Supplemental Type Certificate and require the form. When you are unsure whether a job crosses the line, Appendix A is the definitive reference.

Who Can Do the Work and Sign the Form

Under 14 CFR 43.3, the work may be performed by the holder of a mechanic certificate, a repairman certificate holder, a certificated repair station under Part 145, an air carrier operating under Part 121 or 135, or the original manufacturer.⁵ Someone working under the direct supervision of a certificated mechanic can assist, but that supervised person cannot perform the inspection required after major work.

Approving the aircraft for return to service is a separate authorization under 14 CFR 43.7. A mechanic who also holds an Inspection Authorization (IA) can sign it off, as can a certificated repair station or an air carrier’s authorized representative.⁶ The person who turns the wrenches and the person who signs the return-to-service block may be different people; both signatures appear on the form.

Filling Out the Form Block by Block

The form has eight numbered items on the front and a large description area on the back. Blank copies are available as a PDF on the FAA website or through your local Flight Standards District Office.² Before you start, gather the aircraft data plate, the current registration certificate (AC Form 8050-3), and every piece of approved data supporting the work.

Item 1 — Aircraft Identification

Enter the nationality and registration mark (the N-number), serial number, make, model, and series, taken directly from the data plate. The FAA registry validates the form by matching the N-number to the serial number, and a mismatch will get the form kicked back.⁷ If an application to change the N-number is pending but the new registration certificate has not arrived, use the old N-number.

Item 2 — Owner

Enter the owner’s full name and address exactly as shown on the Certificate of Aircraft Registration.⁷ Do not abbreviate or rearrange the name. Discrepancies between the form and the registration are a common reason for rejection.

Item 4 — Type of Work

Check the box for repair or alteration. Use only one line per form. If you performed both a major repair and a major alteration, prepare two separate Form 337s.⁷

Item 5 — Unit Identification

Identify the specific unit repaired or altered: the airframe, a particular engine, the propeller, or an appliance. Fill in only the blocks that apply. For an airframe entry, list the aircraft make, model, and serial number. For a powerplant or propeller, list the manufacturer, part model, and serial number.

Item 6 — Conformity Statement

The person or organization that actually performed the work completes this block. A mechanic enters their name and permanent mailing address. A repair station enters its name, address, certificate number, and the signature of an authorized representative. The block also requires a statement that the work was performed in accordance with approved data and conforms to the applicable regulations.⁷

Item 7 — Approval for Return to Service

The person authorized under 14 CFR 43.7 signs Item 7. This is typically the IA holder or a repair station representative. The signature here is the formal approval that the aircraft is airworthy after the work, and it is separate from any data approval in Item 3.⁷ Before signing, the approver must verify that everything described in Item 8 matches the approved data and that the form is complete.

Item 8 — Description of Work Accomplished

This is the heart of the form and where most problems arise. Write a detailed narrative of what was done on the back of the form, or on additional sheets if needed. Any additional pages must be identified with the aircraft N-number and the date the work was completed.²

Reference every applicable regulatory section and the specific FAA-approved data supporting the work. Approved data includes Supplemental Type Certificates (list the full STC number, such as SA01234CH), production certificates, Technical Standard Order authorizations, and data reviewed under FAA Order 8300.16 or AC 43-210.⁷ Vague references like “per manufacturer’s instructions” without identifying the specific document are insufficient. Describe the location of the repair or alteration relative to the aircraft structure so a future inspector can find and evaluate it.

If the work affects weight and balance, enter the changes in the aircraft’s weight-and-balance records with the date, a signature, and a reference to the Form 337. Including the weight-and-balance computations directly in Item 8 is not required, but many mechanics do so for completeness.⁷

Item 3 and Field Approvals

Item 3 is marked “For FAA Use Only” and comes into play when no previously approved data exists for the modification, which triggers the field approval process. The applicant submits the Form 337 with a supporting data package to the local FSDO. The FSDO evaluates whether the proposed change falls within the scope of a field approval or requires a full Supplemental Type Certificate instead.⁸ A field approval is limited to one serial-numbered aircraft, so the approved data cannot be reused on a different airframe.

When the FAA is satisfied that the data conforms to airworthiness requirements, an Aviation Safety Inspector signs Item 3 with a standard statement identifying whether the approval rests on a review of the data package alone or on a physical inspection, demonstration, or test.⁷ A Designated Airworthiness Representative can also sign Item 3 after reviewing and certifying the alteration data package.

The Extended-Range Fuel Tank Exception

When a major alteration installs an extended-range fuel tank in the passenger compartment or baggage compartment, the standard two-copy rule does not apply. Complete the form in at least triplicate: one copy stays on board the aircraft, one goes to the owner, and one is forwarded to the FAA registry.¹ The onboard copy needs to be immediately available during any ramp check or inspection.

Filing and the 48-Hour Clock

Once the person approving return to service signs Item 7, the 48-hour filing clock starts. One signed copy goes to the aircraft owner. The duplicate goes to the FAA registry at:¹

Federal Aviation Administration
Aircraft Registration Branch, AFB-710
PO Box 25082
Oklahoma City, OK 73125⁹

The FAA also accepts electronic submissions through its E-337 system. To get access, contact your local FSDO or International Field Office and request an industry user account. The FSDO verifies your credentials and submits the account request, and FAA support staff create the account and email login information. Once set up, authenticate through the FAA eForms portal at eforms.faa.gov.¹⁰ If you start a form electronically, finish it electronically. You cannot mix the paper and electronic processes on the same form because the digital signature framework does not carry over.

The mechanic who performed the work also makes a corresponding entry in the aircraft maintenance logbook under 14 CFR 43.9. That entry needs a description of the work (or a reference to the Form 337), the completion date, and the signature, certificate number, and certificate type of the person approving return to service.¹¹

Why Forms Get Rejected

When the Aircraft Registration Branch receives a Form 337 that does not pass validation, it routes the form back through the responsible FAA office to the person who signed Item 7, and ultimately back to the person named in Item 6 for correction and resubmission.⁷ The most frequent problems:

N-number and serial number do not match. The registry validates these two fields against each other, and a transposed digit or outdated N-number stops the form cold.
Attachments larger than 8½ by 11 inches. Oversized pages are returned for correction.
Vague Item 8 descriptions. Failing to reference the specific approved data (STC number, AC, or regulatory section) or not describing where on the aircraft the work was performed.
Incomplete owner information. The name and address in Item 2 must match the Certificate of Aircraft Registration exactly.
Mixing paper and electronic formats on the same form.

Catching these before mailing saves weeks. The person signing Item 7 is responsible for confirming that Item 8 matches the approved data and that the form is complete before approving return to service.⁷

Retention and Penalties

Under 14 CFR 91.417, a Form 337 recording a major alteration is a permanent record that stays with the aircraft for its operational life and transfers to the new owner at sale. A Form 337 recording a major repair is a temporary record retained until the work is repeated, superseded by other work, or for one year, whichever comes first. In practice, most owners keep every Form 337 permanently, because gaps in maintenance history reduce resale value and complicate annual inspections.

Filing is required by law under 49 U.S.C. § 44701, and the form warns that failure to report can result in civil penalties under 49 U.S.C. § 46301.² The civil penalty ceiling for an individual or small business concern is $1,100 per violation; for larger entities, it can reach $75,000 per violation.¹² Beyond fines, performing a major repair without filing the required Form 337, or without following approved data, can result in suspension or revocation of a mechanic’s certificate. If the aircraft is later involved in an incident, a missing Form 337 leaves the mechanic with no documented proof that the work met regulatory standards, and it leaves the owner with an airworthiness question that can ground the aircraft during an annual inspection or kill a sale during a pre-buy evaluation.

1
eCFR. 14 CFR Appendix B to Part 43 – Recording of Major Repairs and Major Alterations
2
Federal Aviation Administration. FAA Form 337 – Major Repair and Alteration
3
eCFR. 14 CFR 1.1 – General Definitions
4
eCFR. 14 CFR Part 43 Appendix A – Major Alterations, Major Repairs, and Preventive Maintenance
5
eCFR. 14 CFR 43.3 – Persons Authorized to Perform Maintenance, Preventive Maintenance, Rebuilding, and Alteration
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eCFR. 14 CFR 43.7 – Persons Authorized to Approve Aircraft, Airframes, Aircraft Engines, Propellers, Appliances, or Component Parts for Return to Service
7
Federal Aviation Administration. AC 43.9-1G – Instructions for Completion of FAA Form 337
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Federal Aviation Administration. Field Approval Process
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Federal Aviation Administration. Form FAA 337 – Major Repair and Alteration
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Federal Aviation Administration. Notice N 8000.337
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eCFR. 14 CFR 43.9 – Content, Form, and Disposition of Maintenance, Preventive Maintenance, Rebuilding, and Alteration Records
12
Office of the Law Revision Counsel. 49 USC 46301 – Civil Penalties