Definition
An FAA-issued certificate that approves a major modification or alteration to an aircraft, engine, or propeller after its original type certificate has been granted. The STC documents that the change has been engineered, tested, and approved by the FAA, and it identifies the specific product (by make and model) the modification can be installed on. Common STC modifications include alternative fuel approvals, engine upgrades, avionics installations, and airframe changes.
Plain English
An official FAA approval that says a specific change to an aircraft is safe and legal. The original aircraft was approved one way; the STC approves it being modified in a particular way.
Context Anchor
Seen when checking whether a fuel type, fuel system change, engine change, or other aircraft modification is legally approved for that exact aircraft.
Derivation
Supplemental means 'added on top of something already there.' A type certificate is the original FAA approval of the aircraft design. So a Supplemental Type Certificate is an extra approval added on top of the original — it doesn't replace the type certificate, it supplements it for a specific change.
Why Pilots Care
It makes many useful modifications legal to install while keeping the aircraft airworthy and insurable.
Intuition Check
An STC is not a pilot certificate, and it is not a general permission slip for all airplanes. It is approval for a specific change on specific aircraft, engine, or propeller models.
Example Sentence 1
The owner obtained an STC allowing the engine to run on unleaded avgas instead of 100LL.
Example Sentence 2
The mechanic confirmed the STC covered both the engine and propeller combination on this model.