Accessory drive system for a gas turbine engine

An accessory drive system for a gas turbine engine having a main electrical power supply, comprising:

• a gearbox driving a plurality of accessories including at least a cabin air compressor to provide cabin air and an engine starter to start the gas turbine engine;
• an electrical power supply separate from the main electrical power supply of the gas turbine engine and electrically coupled to the accessories;
• and
• a clutch selectively engaging an output of the gas turbine engine to mechanically couple the output to the gearbox, and operating between an electrical drive state wherein the clutch disengages from the output and only the electrical power supply provides electrical power to at least one of the cabin air compressor and the engine starter, and a mechanical drive state wherein the clutch engages the output and the gearbox drives at least one of the accessories.