A non-aircraft-propelling auxiliary gas turbine engine installable in an aircraft having a cabin adapted to be pressurized. The auxiliary gas turbine engine includes an auxiliary-gas-turbine-engine compressor having an inlet, wherein the inlet is adapted to receive pressurized air from the cabin. A method for operating a non-aircraft-propelling auxiliary gas turbine engine of an aircraft includes providing pressurized air from the cabin of the aircraft to an inlet of a compressor of the auxiliary gas turbine engine. The method includes providing compressed air from the compressor to a combustor of the auxiliary gas turbine engine and includes providing combustion gases from the combustor to a turbine of the auxiliary gas turbine engine, wherein the turbine is mechanically coupled to the compressor.