An auxiliary power system for an airplane includes at least one fuel cell unit each with at least one fuel cell, a voltage output, a fuel intake and an outlet for reaction products, a fuel tank that is couplable with the fuel intake of the fuel cell unit, at least one compressor unit with an air intake and air outlet and an electric motor, which is couplable with a voltage output of the at least one fuel cell unit and, by way of a shaft, with the at least one compressor unit. At least the at least one fuel cell unit, the compressor unit and the electric motor are interconnected to yield a coherent unit, which continuously provides electrical power and pressurized air.