Aircraft auxiliary power unit

An auxiliary power unit for an aircraft comprising;

• a gas turbine engine comprising an engine core comprising a core compressor and core turbine coupled by a core shaft;
• a load spool comprising a load compressor, a load turbine and a permanent magnet electric machine, each being coupled by a load shaft;
• and
• circuitry, wherein:
• the load shaft and core shaft are configured to rotate independently of one another, and
• the electric machine includes a power electronics unit coupled to the electric machine such that the circuitry is configured to cause the gas turbine engine to operate:
• (i) with the gas turbine engine running and with compressed air from the engine core not being prevented from entering the load turbine, and
• (ii) with the gas turbine engine not running, the electric machine being configured to operate as a motor configured to drive the load compressor to provide compressed air to the aircraft, and uncompressed air being prevented from entering the load turbine from the engine core by way of closure of a valve.