Abstract: Engine (10) shaft power provides a first input (16) for a differential gear box (18). This input (16) is additive in a gear box (18) to a second input (30) from an electric motor (32). The output (22) of the gear box (18) powers a rotor (24) of an auxiliary compressor (12) which produces compressed air for providing needed pneumatic services aboard the aircraft. Compressor rotor speed is sensed (sensor 40) and used for controlling operating current (via controller 42 and frequency converter 44) to the electric motor (32). Motor (32) is an electronically commutated brushless electric motor having a high torque output which is substantially linearly proportional to the input current.