Definition
An FAA-issued certificate that approves a major modification or alteration to an aircraft, engine, or propeller that changes the original type design. The STC documents that the modification meets airworthiness standards and authorizes the modified design as a legal addition to the original Type Certificate.
Plain English
Official FAA permission to make a significant change to an aircraft beyond what the manufacturer originally certified. It proves the change has been tested, approved, and is legal to use.
Context Anchor
Seen in aircraft records, operating handbook supplements, maintenance paperwork, and fuel approvals, especially when an airplane has been modified from its original factory setup.
Derivation
‘Supplemental’ comes from the Latin supplementum, meaning ‘something added to fill a gap.’ The 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 a modification such as an alternate fuel tank or engine is safe, legal to install, and maintains airworthiness.
Intuition Check
Do not read supplemental as meaning informal, optional, or just extra paperwork. In this context, it means a formal FAA approval that legally adds a specific change to the aircraft’s approved design.
Example Sentence 1
The owner installed an auxiliary fuel tank under an STC, which permitted longer cross-country flights without modifying the original type design illegally.
Example Sentence 2
Many owners add wingtip tanks under an STC to increase range without voiding the aircraft's airworthiness certificate.