AUXILIARY POWER UNIT OF AN AIRCRAFT, which comprises a power module supplied with fuel, a gearbox, an electric generator, a compressor, and a hydraulic pump in order to provide a functioning mode for production of power selected from between electric, pneumatic, hydraulic and combination of said power sources; wherein the auxiliary power unit also includes: an electric motor fed by an electric power supply and integrated into the electric generator, wherein both the electric motor and the electric generator are connected to the power module via the gearbox and a main clutch; in such a way that both the compressor and the hydraulic pump are also connected to the power module via a secondary clutch of the gearbox and via the main clutch, and a control module fitted with means of establishment of a functioning mode selected from between: a mode for simultaneous generation of pneumatic, electric and hydraulic power with the compressor, the hydraulic pump, and the electric generator acting by means of the power module, a mode for simultaneous generation of pneumatic power and electric power with the compressor and the electric generator acting by means of the power module, a mode for simultaneous generation of hydraulic power and electric power with the hydraulic pump and the electric generator acting by means of the power module, a mode for the production solely of electric power with the electric generator acting by means of the power module, a mode for the production solely of power selected from between pneumatic and hydraulic with the compressor or the hydraulic pump being turned by the electric motor; and a start-up mode for the power module by means of the electric motor and with the compressor and the hydraulic pump being kept decoupled. an electric motor fed by an electric power supply and integrated into the electric generator, wherein both the electric motor and the electric generator are connected to the power module via the gearbox and a main clutch; in such a way that both the compressor and the hydraulic pump are also connected to the power module via a secondary clutch of the gearbox and via the main clutch, and a control module fitted with means of establishment of a functioning mode selected from between: a mode for simultaneous generation of pneumatic, electric and hydraulic power with the compressor, the hydraulic pump, and the electric generator acting by means of the power module, a mode for simultaneous generation of pneumatic power and electric power with the compressor and the electric generator acting by means of the power module, a mode for simultaneous generation of hydraulic power and electric power with the hydraulic pump and the electric generator acting by means of the power module, a mode for the production solely of electric power with the electric generator acting by means of the power module, a mode for the production solely of power selected from between pneumatic and hydraulic with the compressor or the hydraulic pump being turned by the electric motor; and a start-up mode for the power module by means of the electric motor and with the compressor and the hydraulic pump being kept decoupled. a mode for simultaneous generation of pneumatic, electric and hydraulic power with the compressor, the hydraulic pump, and the electric generator acting by means of the power module, a mode for simultaneous generation of pneumatic power and electric power with the compressor and the electric generator acting by means of the power module, a mode for simultaneous generation of hydraulic power and electric power with the hydraulic pump and the electric generator acting by means of the power module, a mode for the production solely of electric power with the electric generator acting by means of the power module, a mode for the production solely of power selected from between pneumatic and hydraulic with the compressor or the hydraulic pump being turned by the electric motor; and a start-up mode for the power module by means of the electric motor and with the compressor and the hydraulic pump being kept decoupled.