Definition
Supplemental Type Certificates are FAA approvals authorizing a major modification or alteration to an aircraft, engine, or propeller from its original type-certificated design. An STC documents that the change has been engineered, tested, and approved as airworthy, and it identifies the specific product the modification applies to.
Plain English
An STC is an official FAA approval that lets someone change an aircraft from how it was originally built and certified — for example, installing a different engine, a new avionics package, or modified wingtips — and confirms the change is safe and legal.
Context Anchor
Pilots encounter STCs in aircraft records, installed equipment paperwork, flight manual supplements, and discussions about whether a modified aircraft is legal to fly.
Derivation
‘Supplemental’ comes from Latin supplementum, meaning ‘something added to complete or extend.’ An STC supplements the original Type Certificate — it doesn’t replace it, it adds an approved change on top of it.
Why Pilots Care
Confirms that modifications are legal and airworthy; an unapproved change can make the aircraft uninsurable or unsafe to fly.
Intuition Check
Do not think of an STC as the new part itself. The STC is the FAA approval for the change, and the installed part or system must match that approval.
Example Sentence 1
The aircraft’s new GPS unit was installed under an STC, so the modification is fully approved and documented.
Example Sentence 2
Reviewing the STCs helped the buyer understand every approved change on the used aircraft.