Definition
A Supplemental Type Certificate (STC) is an FAA-issued approval that authorizes a major change in design to an aircraft, engine, or propeller after its original Type Certificate has been issued. The STC permits a modification — such as installing a different engine, adding a new avionics package, or changing a structural component — while keeping the aircraft legally airworthy. The STC documents what the modification is, what models it applies to, and the conditions under which it is approved.
Plain English
An official FAA approval that lets someone make a significant change to an aircraft without having to certify the entire aircraft from scratch. It says, in effect, 'this specific modification is approved for this specific aircraft.'
Context Anchor
Seen in maintenance records, aircraft modification paperwork, equipment installations, and discussions about whether a change to an aircraft is approved.
Derivation
Supplemental' comes from Latin supplementum, meaning 'something added to complete or fill in.' An STC supplements the original Type Certificate — it adds approved changes on top of the aircraft's original approval rather than replacing it.
Why Pilots Care
Ensures any approved modification keeps the aircraft legal and airworthy.
Intuition Check
Do not read “supplemental” as meaning optional or informal. Here it means the approval is added to the original aircraft design approval and is still an official FAA approval.
Example Sentence 1
The owner installed a more powerful engine under an STC, and the modification was recorded in the aircraft's permanent records.
Example Sentence 2
The mechanic confirmed the modification was covered by a valid STC.