A Supplemental Type Certificate (STC) is a change to the original aircraft type certificate (TC) issued when an applicant has received FAA approval to modify an aeronautical product from its original design. The STC, which incorporates by reference the related TC, approves not only the modification but also how that modification affects the original design. A supplemental type certificate (STC) is a design approval processed and issued by an FAA ACO or a Designated Alteration Station for a modification to an aircraft in service, or by an FAA ACO for replacement or modification parts for use on type-certified products. A STC is the approved data required by the Aviation Authorities to perform a Major Alteration on an aircraft. To approve for return to service an aircraft modified by a STC, a form 337 must be completed. The STC is used as FAA approved data. The 337 must be signed off by an authorized person.Item 6 (conformity statement) must be signed by the person performing the alteration(A&P or Repair Station). Item 7 (approval for return to service) must be signed by the person performing the inspection, IA(Inspection Authorization) or Repair Station. A logbook entry is also required. STC examples: https://technologies.blackhawk.aero/stcs/
A Piper PA-42-1000 with 5-blade MTV-27 Propellers. Modified by STC SA03525NY.