Definition
An FAA-issued certificate that approves a major modification or alteration to an aircraft, engine, or propeller that already holds a Type Certificate. The Supplemental Type Certificate (STC) authorizes the changed design and identifies how the original type-certificated product has been altered, while leaving the original Type Certificate intact.
Plain English
Official FAA permission to modify an already-approved aircraft design in a specific, tested way. The STC says the modification has been engineered, tested, and approved as safe to use.
Context Anchor
Pilots usually encounter Supplemental Type Certificates in aircraft records, especially when an airplane has added equipment, a changed engine, a different propeller, or another approved modification.
Derivation
Supplemental comes from Latin supplementum, meaning something added to complete or extend. The STC supplements the original Type Certificate rather than replacing it — the base aircraft is still approved under its original certificate, with the STC adding approval for the specific change.
Why Pilots Care
Allows legal installation of equipment such as avionics or engines while keeping the aircraft airworthy and insurable.
Intuition Check
Do not read supplemental as merely optional or informal here. A Supplemental Type Certificate is an official approval for a design change, not just extra paperwork.
Example Sentence 1
The owner installed a more powerful engine under an STC that had been issued for that airframe.
Example Sentence 2
During the pre-purchase inspection the mechanic verified that every modification was supported by a valid Supplemental Type Certificate.