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:

• Form 337 and Supplemental Type Certificates (STCs) are the FAA's primary pathways for major aircraft modifications, with 337s for individual airframes and STCs for broader aircraft families.
• STCs represent a more robust approval standard, demanding greater engineering and proof, often necessitating flight manual updates.
• Both certifications confirm modifications meet FAA and mechanic standards; the absence of either for major changes is a cause for concern.

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