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 permits major aircraft modifications primarily through two pathways: a Form 337 for one-off alterations to an individual aircraft, or a Supplemental Type Certificate (STC) which provides blanket approval for an entire family of aircraft types.
• STCs represent a more robust standard than Form 337s, requiring more extensive engineering and often including supplements to the aircraft's pilot's operating handbook or flight manual.
• Both Form 337s and STCs certify that modifications meet legal and airworthiness standards set by mechanics and the FAA; the absence of either on a certified aircraft should prompt a thorough investigation.

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