an apu compressor 60 an apu combustor 62 an apu turbine 64 a reduction gearbox 66 and an electrical machine 68 the discharge of compressor 60 is in fluid communication with combustor 62 the discharge of combustor 62 is in fluid communication with turbine 64 turbine 64 is coupled to compressor 60 and operative to drive compressor 60 in one form reduction gearbox 66 is coupled to and driven by turbine 64 in other embodiments reduction gearbox 66 may be coupled to compressor 60 and driven by turbine 64 via compressor 60 or a shaft extending from turbine 64 in one form reduction gearbox 66 is considered a part of apu 22 in other embodiments reduction gearbox 66 may be considered a separate component that is powered by apu 22 in one form electrical machine 68 is coupled to compressor 60 in other embodiments other mechanical arrangements may be employed for example electrical machine 68 may be coupled directly to turbine 64 electrical machine 68 is operative to convert mechanical power to electrical power in some embodiments electrical machine 68 may be also configured to convert electrical power to mechanical power e g for starting apu 22 in some embodiments a power conditioner 70 is electrically coupled to electrical machine 68 and operative to condition the power output of electrical machine 68 e g for use in supplying electrical power to one or more systems of aircraft 10 during aircraft 10 ground operations and or.