A Supplemental Type Certificate (STC) is an FAA or EASA-approved major modification or repair to an existing type certified aircraft, engine, or propeller. As it adds to the existing type certificate, it is deemed 'supplemental'. As its name suggests, an STC is a certificate. It defines the product design change, states how the modification affects the existing type design, and lists the serial number effectively. It also identifies the certification basis, listing specific regulatory compliance for the design change.
An STC is a supplemental type design approval for a major alteration specific to a make and model or even a specific serial number as a one “off” STC
The STC Holder – STCH remains ultimately responsible for the certification of the modification. An STC is a design approval, however it doesn’t allow the holder to produce anything. (typically created by Part 21 /FAR 21 Subpart J Organisation Design Approval Holder (DAH) Design Organisation Approval (DOA)).
Note - The STC, which incorporates by reference the related Type Certificate (TC), approves not only the modification but how the modification affects the original design.
The application must be made in the form and manner prescribed by the FAA or EASA.
The STC and its related information - all drawings, data, and specifications - are the property of the STC holder. However to also consider that the Continuing Airworthiness Management Organisation (CAMO) remains responsible for Continuing Airworthiness Compliance including STC Conformity. Therefore the CAMO should always ensure that you have the necessary access to the full design information package. (such data forms part of the back-to-birth continuing airworthiness records)
EASA STC Related Manufacturing.
To manufacture replacement or modification parts, you have to have to hold production approval. (typically produced by Part 21 /FAR 21 Subpart G Production organisation Approval Holder (PAH) Production Organisation Approval (DOA)).
STCs provide an opportunity to enable DOA/PAH to offer performance and or maintenance-related changes as well as customized modifications with airworthiness considerations.
Examples of STC’s include the following
Re-engining an existing airframe, installation of advanced avionics, pax -to-freighter conversions.
STC Potential Concerns
Due to their inherent criticality, all changes to the design of an aircraft are required to be approved. In particular to assess the STC for Continuing Airworthiness implications
Under the U.S. FAA's regulatory system, there are several methods to obtain approval for a major modification or repair to a type of certified aircraft. For example, a data approval issued by a Designated Engineering Representative which is usually issued on FAA Form 8110-3 equivalent to 8100-9 is one example ( Note that there may be EASA Part 21 issues with this statement)
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