Supplemental type certificate

A supplemental type certificate (STC) is a national aviation authority-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". In the United States issuance of such certificates is under the purview of the Federal Aviation Administration (FAA).

Due to their inherent criticality, all changes to the design of an aircraft are required to be approved in some form. How this is achieved varies from country to country. Generally, modifications and repairs deemed to be "minor", such as the installation of a radio or the repair to upholstery, may be approved by the aircraft mechanic or maintenance engineer. However, larger changes, such as changing the type of engine or increasing the maximum weight, are deemed "major" and require formal approval from the applicable authority.

Under the U.S. FAA's regulatory system, there are various methods to obtain approval for a major modification or repair to a type-certified aircraft (the following list is not exhaustive):

An STC has the following advantages over other forms of modification or repair approval:

Due to these advantages, many minor modifications are also approved under STCs for increased salability. Some national aviation authorities also issue Limited or LSTCs that are only applicable to a single aircraft or small number of specific serial numbers.

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 serial number effectivity. It also identifies the certification basis, listing specific regulatory compliance for the design change. Information contained in the certification basis is helpful for those applicants proposing subsequent product modifications and evaluating certification basis compatibility with other STC modifications.An application for STC shall be made in a form CA-33.

The United States regulations for STCs are found at 14 CFR 21.111

The European Union regulations for STCs are found Commission Regulation (EU) No 748/2012 of 3 August 2012 as amended, Part-21, Subpart E et seq.

In 2010 at the US/Europe International Aviation Safety Conference, Eurocopter discussed concerns about potential risks with, and incompatibilities in, STCs. In response the Modification and Replacement Parts Association (MARPA) highlighted the success that the Parts Manufacturer Approval (PMA) industry had experienced in implementing Continued Operational Safety (COS) programs for the prediction and proactive addressing of hazards. Jason Dickstein, President of the Modification and Replacement Parts Association, suggested that STC Holders could use the same techniques to better manage the safety of their STCs.

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