FAA Form 337 Explained: Recording Major Repairs and Alterations

What Form 337 is for

Form 337 documents work classified as a major repair or major alteration. Minor repairs and alterations are recorded in the maintenance records without a 337.

FAA AC 43.9-1G describes three purposes for the form: a record for the owner, a record for the FAA’s aircraft records, and a place to record the installer’s compatibility assessment.

Who completes it and where it goes

The person who performs or supervises the major repair or alteration prepares the form, at least in duplicate. A signed copy goes to the owner, and a copy goes to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after return to service.

The form can be filed on paper or electronically, but not both. Some installations, such as extended-range fuel tanks in the passenger or baggage compartment, require additional copies, including one carried on board.

When a repair station uses a work order instead

For major repairs performed under an acceptable manual, a certificated repair station may use a work order together with a maintenance release instead of Form 337. Owners should still receive and keep the documentation that describes the work.

Why parts buyers should care

When a component or aircraft changes hands, missing 337s and incomplete alteration records are a common source of delays. Ask for the alteration and repair history along with the part’s release documentation, particularly for structural, avionics, and fuel-system items.

If a sourcing request involves an altered configuration, share the relevant 337 or approved data with the supplier so the replacement part matches the aircraft as it actually exists.