Abstract: An improved air conditioning system is provided for an aircraft, particularly with respect to providing an enhanced supply of fresh air to the aircraft cabin or cockpit. The system includes a dual inlet turbine driven at least in part by pressurized bleed air from an aircraft engine, wherein the turbine expands and cools the air preparatory to supply thereof as conditioned air to the aircraft cabin. During relatively low altitude operation, the bleed air flow is supplemented by compressed fresh air discharged from a turbine-driven compressor and supplied to the dual inlet turbine. At relatively higher altitude operation, this fresh air flow is bypassed around the turbine for addition to the bleed air flow at a downstream side of the turbine, and the entire nozzle area of the turbine is opened to the bleed air flow.