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). (Category 1 and Category 2 APU, 2005). (See Appendix a for another example of an APU)

History (may need something different for this part - raw research) significant early demonstration was of the first liquid-fueled, fuel cell APU on...
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