A supplemental type certificate (STC) refers to a certificate issued by an aviation authority approving a product modification. An STC contains the product design change and states how the modification affects the existing type design.
The administrator of the Federal Aviation Administration issues a type certificate designated as a supplemental type certificate for a change to an aircraft, aircraft engine, propeller, or appliance. A supplemental type certificate consists of the change to the aircraft, aircraft engine, propeller, or appliance with respect to the previously issued type certificate for the aircraft, aircraft engine, propeller, or appliance. [49 USCS § 44704].
The type certificate authorizes the manufacturer to build the model specified. [Butler v. Bell Helicopter Textron, Inc., 109 Cal. App. 4th 1073, 1082 (Cal. Ct. App. 2003)].