Which regulation requires approval for compliance before installation of items under a Supplemental Type Certificate?

The regulation that requires approval for compliance before the installation of items under a Supplemental Type Certificate (STC) is 14 CFR Part 21. This part of the Code of Federal Regulations outlines the procedures and standards for the issuance of type certificates, production certificates, and supplemental type certificates. Part 21 specifically addresses the requirements that manufacturers must meet to ensure that their products comply with the applicable airworthiness standards.

Under this regulation, any modifications or alterations to an already certified aircraft through a Supplemental Type Certificate must be approved to ensure that they maintain compliance with safety and performance standards. This includes obtaining the necessary approvals before the installation of new equipment or changes to the aircraft, thus ensuring all aspects of the modification are thoroughly vetted and meet regulatory requirements.

The other regulations mentioned serve different functions: Part 43 deals with the maintenance, preventive maintenance, and alteration of aircraft; Part 45 provides requirements for identification and registration marking of aircraft; and Part 25 pertains to airworthiness standards for transport category airplanes. While they are all essential for aviation operations and safety, they do not specifically govern the approval of compliance regarding Supplemental Type Certificates as Part 21 does.