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 allows major aircraft modifications through two primary pathways: Form 337 for one-off repairs or alterations to individual aircraft components, and Supplemental Type Certificates (STCs) for modifications applicable to an entire family of aircraft types.
• STCs are a more robust standard than Form 337s, requiring more extensive engineering and proof of airworthiness, often including supplements to the aircraft's Pilot's Operating Handbook (POH).
• Both Form 337s and STCs verify that modifications meet FAA and mechanic standards; modifications lacking either document, particularly in experimental aircraft, should be investigated due to potentially untested performance parameters.

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