Major Repair And Alteration

For certified aircraft, the FAA permits major modifications via many regulatory pathways, but the two most common in the GA fleet are via a Form 337 or a supplemental type certificate (STC). A Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance) is the documentation required for one-off repairs or modifications to an individual airframe, engine or prop. Supplemental Type Certificate (STC) modifications are more of a blanket approval designed to cover an entire family of aircraft types.

Key Takeaways:

• The FAA primarily permits major aircraft modifications through two regulatory pathways: Form 337 for one-off repairs or alterations to individual aircraft, and Supplemental Type Certificates (STCs) for blanket approval across entire families of aircraft types.
• STCs represent a more robust standard than Form 337s, requiring more extensive engineering, proof of airworthiness, and often an accompanying supplement to the aircraft's pilot's operating handbook (POH).
• Both Form 337s and STCs confirm that modifications meet the legal requirements and standards of both the mechanic and the FAA; modifications lacking either of these documents should be thoroughly investigated for compliance.

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