Propulsion system for an aircraft

A propulsion system for an aircraft, comprising a gas turbine engine (20) having a high pressure spool (24) and a low pressure spool (26), wherein the low pressure spool (24) is operative to drive a propulsor (28); and an auxiliary power unit (APU) (22) mechanically coupled to both the high pressure spool (26) and the low pressure spool (24), wherein the APU (22) is operative to supply rotational power to both the high pressure spool (26) and the low pressure spool (24).