FAA Form 337 — Major Repair and Alteration

FAA Form 337 is the legal instrument through which a major repair or major alteration performed on a U.S.-registered civil aircraft is officially documented and approved for return to service. Its requirement derives from 14 CFR §43.9(d), which states that a person who performs a major repair or major alteration shall record that work on a form and in the manner prescribed by the Administrator — that form being the FAA Form 337 (OMB No. 2120-0020). Part 43 Appendix B defines precisely what must be captured: aircraft identification (make, model, registration, serial number), description of the work performed, the approval reference, and the signature of the approving person.

The threshold event that triggers Form 337 is a finding that work falls within the definition of "major repair" or "major alteration" as set out in Part 43 Appendix A. Appendix A lists approximately 40 specific categories. Major alterations to an airframe include changes to the basic design of the fuel system; changes to the wing or fixed or movable control surface by removing, installing, or altering structural components; changes to the basic design of the landing gear; and installation of a powerplant other than originally certificated. Major repairs to an airframe include any repairs to box beams, monocoque or semi-monocoque wing or control surfaces, or primary structural components of the fuselage beyond allowable damage limits in the manufacturer's Structural Repair Manual. For powerplant major repairs, the list includes separation or disassembly of a crankcase or crankshaft of a reciprocating engine, repairs to engine mounts, and any repair to contra-rotating propeller reduction gears. Any major repair to a propeller blade or hub is also captured. If work falls outside these defined major categories, it is a minor repair or minor alteration and does not require a Form 337 — it is documented only in the aircraft's maintenance records under §43.9.

The authority to sign Form 337 depends on the approval basis. Block 3 of the form identifies the type of approval: the work may be approved by an FAA Inspector (field approval), by a Designated Airworthiness Representative (DAR) authorized for that category of work, or pursuant to an already-approved data package — such as an STC, an FAA-Approved Manufacturer's Repair Manual, or an FAA-Issued Advisory Circular. When a repair station holds appropriate ratings, it may approve major repairs within its rating scope without requiring a field approval. An A&P holding an Inspection Authorization (IA) may approve and sign Form 337 for major repairs that the IA performed — but an IA cannot field-approve a major alteration without FAA inspector or DAR involvement when no pre-approved data exists. Specifically, for major alterations without pre-existing approved data, the IA submits the Form 337 with supporting engineering data to the local FSDO; the FAA inspector signs Block 3 to grant the field approval, after which the IA signs Block 7 as the approving mechanic for return to service. When the alteration is performed under an STC, the STC number is entered in Block 1, and the STC's approved data is the authorization — no field approval is needed.

Distribution of the completed Form 337 is strictly defined in Appendix B. The original is forwarded to the FAA Aircraft Registration Branch, AFS-750, in Oklahoma City, Oklahoma, within 48 hours of the return to service. This copy becomes a permanent part of the aircraft's official FAA record and is associated with the aircraft's N-number and serial number in perpetuity — it travels with the aircraft's legal identity regardless of ownership changes. One copy is retained in the aircraft's permanent maintenance records by the owner or operator. For work performed by a repair station, the repair station retains a third copy. The Form 337 is thus unique among maintenance documents in that it creates a federal record filed with the FAA Registry, not merely an entry in the aircraft's logbook.

The EASA framework addresses the same territory through a structurally different mechanism. A major change to a type design (equivalent to a major alteration) is approved through EASA Part-21 Subpart D (Major Changes to Type Design by the TC holder) or Subpart E (Supplemental Type Certificate, equivalent to FAA STC). The documentation of the approved change lives in the aircraft's technical records under Part-M M.A.305 (Aircraft Maintenance Record System), which requires retention of all modification records for the operational life of the aircraft plus two years. EASA Form 1 documents the release of components into service but does not serve the major-repair approval function — that role belongs to the Certifying Staff CRS issued by the approved Part-145 or Part-M subcontracted organization after the modification. There is no direct EASA equivalent of the Form 337 as a single FAA-filed document; the EASA framework distributes the same information across the STC/major change approval data, the work order, and the CRS.

For flight schools, Form 337 events most commonly arise from three scenarios: prop strikes, structural damage from ground handling or hard landings, and STC-based avionics upgrades. A prop strike on a training aircraft requires inspection of the engine crankshaft and propeller in accordance with the engine manufacturer's service instructions; if the inspection reveals damage or if the engine manufacturer specifies teardown as mandatory after any prop strike regardless of apparent damage, the subsequent repair is a major repair requiring Form 337. Schools that misclassify a prop strike event as a minor repair — skipping the Form 337 requirement — are creating an undocumented major repair in the aircraft's history, which will surface during any pre-purchase inspection and can render the aircraft unsaleable or dramatically reduce its market value.

For avionics upgrades, most modern glass cockpit installations (Garmin G3X, G500, G1000 NXi, Avidyne IFD series) are performed under an existing STC that covers the specific airframe-avionics combination. The installer documents the work on Form 337, references the STC in Block 1, and files the form with Oklahoma City. Schools that have accumulated Form 337-based modifications without maintaining a centralized log of those forms frequently discover missing forms during aircraft refinancing or resale — at which point the forms must be reconstructed from the FAA Registry (obtainable via FOIA request to AFS-750) or, if the work was performed years earlier by an organization that no longer exists, may be effectively irretrievable.

Aviatize's digital data and records module maintains a dedicated Form 337 repository for each aircraft in the fleet, associating every major repair and major alteration document with the aircraft record by N-number, serial number, and work order. When a work order is closed for a task that the system's maintenance control module has flagged as a potential major repair or alteration — based on ATA chapter, damage type, or technician classification — the workflow prompts the technician to confirm whether Form 337 documentation is required and to attach the completed form before the work order can be finalized. This prevents the common failure mode of a technician completing the repair, signing off the logbook entry, and neglecting the Form 337 filing deadline until days or weeks later.

For pre-purchase diligence and aircraft lifecycle management, Aviatize's compliance and auditing tools generate a complete modification history for each aircraft, listing every Form 337 on file, the associated STC or approval basis, the approving IA or DAR, and the filing date. This report is exportable as a single document for buyers, lenders, or FSDO inspectors, replacing a manual search through paper logbooks and supplemental files. The platform also cross-references each Form 337 against the associated STC's continued airworthiness requirements — for example, if an STC-based engine modification imposes an additional inspection interval or Airworthiness Limitation, that requirement is tracked in maintenance control alongside the standard manufacturer intervals.