FAR 21.113 — Supplemental Type Certificate

FAR 21.113 tells you when someone must apply for a Supplemental Type Certificate (STC) before making a major change in type design to an already-certificated product (aircraft, engine, or propeller).

The rule splits into two scenarios:

• If you hold the original Type Certificate (TC): You can either apply for an STC or apply to amend the original TC under subpart D of Part 21.
• If you do NOT hold the TC: You must apply for an STC — amending the TC isn't an option.

This only applies to major changes that aren't so significant they'd require a brand-new TC under § 21.19 (which covers changes so extensive a complete redesign and new certification are needed).

Applications must be made in the form and manner prescribed by the FAA.

Why it matters operationally: As a pilot, when you see an STC referenced in the aircraft's records (think: avionics upgrades, gross weight increases, engine conversions, or auxiliary fuel tanks), you know the modification was formally approved by the FAA. The STC paperwork, including any flight manual supplements, becomes part of the aircraft's required documents.