FAR 21.117 — Supplemental Type Certificates

FAR 21.117 spells out when the FAA will issue a Supplemental Type Certificate (STC) and what that certificate actually includes. An STC is the FAA's formal approval of a major change to the type design of an already type-certificated product (like installing a new avionics suite, engine, or interior in an existing aircraft model).

The rule is short but important:

• Eligibility: An applicant gets an STC if the FAA finds they meet the requirements of § 21.113 (who may apply and what an STC is for) and § 21.115 (the applicable airworthiness and certification requirements).
• What an STC consists of:
  • The FAA's approval of the change in the type design of the product, and
  • The original type certificate previously issued for that product.

Why it matters operationally: when you fly an aircraft modified under an STC, the STC paperwork (and any flight manual supplement) becomes part of the aircraft's required documents. Pilots should know an STC doesn't replace the original type certificate — it rides along with it.