What are the methods of approval for retrofit shoulder harness installations?
1. Supplemental Type Certificate (STC). An STC is the most desirable and most rigorous approval. The STC offers the highest assurance that all of the airworthiness regulations have been met. The STC approvals are issued by the FAA Aircraft Certification Offices (ACO’s). STC approvals are usually obtained by a shoulder harness installation kit supplier for multiple airplane installations in an airplane model or model series. AC’s 21-34 and 23-4 (References 1 and 2) provide guidance and acceptable means of compliance for shoulder harness and seat belt installations. AC 23-4 specifically addresses part 23 installations. These AC’s are also applicable to installations in airplanes having a certification basis of predecessor regulations (for example, CAR 3). An applicant for an STC may use a salvaged airplane fuselage to substantiate the strength of the fuselage and the shoulder harness attachment fittings by structural tests, since the shoulder harness attachment structural test may damage
