FAR 21.120 — STC Written Permission
FAR 21.120 requires supplemental type certificate (STC) holders to provide FAA-acceptable written permission before others can use the STC to alter aircraft.
FAR 21.120 is a short but important rule that governs how a Supplemental Type Certificate (STC) can be shared with others performing alterations.
An STC is FAA approval for a major design change to a previously type-certificated aircraft, aircraft engine, or propeller. The STC is owned by the holder, but in practice, mechanics, repair stations, and shops often need to use someone else's STC to install that modification on a customer's aircraft.
This section requires that:
- If an STC holder allows another person to use the STC to alter an aircraft, engine, or propeller,
- The STC holder must provide that person with written permission acceptable to the FAA.
Why it matters operationally: Before a maintenance provider installs an STC modification they don't own, they need documented permission from the STC holder. Without that paperwork, the alteration isn't properly authorized, the logbook entry and Form 337 can be questioned, and the aircraft's airworthiness may be in doubt at the next inspection.