The criticality of an APU relative to flight safety in any particular aircraft installation will determine if the APU system should be considered essential, or non-essential. Airplanes that rely on APUs for provision of back-up electrical power in flight in the event of a failure of the primary power sources are usually considered essential installations. The following definitions may be helpful.

Category 1 APUs are installed where in-flight auxiliary power operation is necessary. Category 1 APUs are usually required for essential APU installations. These APUs have been shown to meet all of the test and analysis requirements of the Minimum Performance Standard (MPS) of TSO C77b, Appendix 1.

Category 2 APUs are installed where in-flight APU operation is not necessary (non-essential installations).

Both Category 1 and Category 2 APUs are acceptable for non-essential APU installations. Category 2 APUs are not required to meet all of the test and analysis requirements that Category 1 APUs are subjected to. (See TSO C77b, Appendix 1, for the detailed requirements).