Supplemental type certificate
A supplemental type certificate (STC) is an FAA 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'.
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 will vary 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 FAA or FAA approved person or organization.
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):
- A data approval issued by a Designated Engineering Representative which is usually issued on a FAA Form 8110-3
- A data approval issued by an Organization Designation Authorization Holder using ODA Procedures
- A Field Repair which may be issued on a FAA Form 337, or
- A Supplemental (or Supplementary) Type Certificate (STC).
An STC has the following advantages over other forms of modification or repair approval:
- It may be developed and owned by a person other than the aircraft or equipment manufacturer,
- It is essentially unlimited in the scope of the modification.
- It may cover a single aircraft (serial number) or many types of aircraft.
- It may be bought and sold
- It has greater recognition and acceptance in foreign countries.
Due to these advantages many minor modifications are also approved under STCs for increased salability. Some National Airworthiness Authorities also issue Limited or LSTCs that are only applicable to a single 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.
The United States regulations for STCs are found at 14 C.F.R. § 21.111
The European Community regulations for STCs are found at COMMISSION REGULATION (EC) No 1702/2003 (24 September 2003), § 21.A111 et seq.
In 2010, at the US/Europe International Aviation Safety Conference, Eurocopter introduced a discussion item concerning potential risks and incompatibilities associated with STCs. In response, the Modification and Replacement Parts Association (MARPA) highlighted the success that the Parts Manufacturer Approval (PMA) industry has experienced in implementing Continued Operational Safety (COS) programs as a method for predicting and proactively addressing 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.