what is an stc

A Supplemental Type Certificate (STC) is a civil aviation authority-approved major modification or repair to an existing type certified aircraft, engine, or propeller. It is deemed "supplemental" as it adds to the existing type certificate. In the United States, the issuance of such certificates is under the purview of the Federal Aviation Administration (FAA) . The STC approves not only the modification but also how that modification affects the original design. It is a design approval processed and issued by an FAA Aircraft Certification Office or a Designated Alteration. STCs are used to carry out aircraft type modifications that are deemed by the Airworthiness Authority concerned to be a sufficient change to the original design of an aircraft type to require a specific validation.