TSO qualification programs must meet the following requirements:Functionality. The standards of TSO C77b apply to gas turbine engines intended to provide auxiliary electrical, pneumatic, or mechanical power to support airplane systems operations. These standards do not apply to gas turbine engines intended for aircraft propulsion nor do they address the integration of the APU into the design of the airplane.
Minimum Performance Standards. The design and construction of the APU must be shown to meet the minimum performance standards (MPS) of Appendix 1 to TSO C77b.
Deviations. The Federal Aviation Administration (FAA) has provisions for using alternative or equivalent means of compliance to the criteria set forth in the MPS of TSO C77b. Applicants invoking these provisions must demonstrate that an equivalent level of safety is maintained and shall apply for a deviation in accordance with Section 21.609.