An aircraft comprises a landing gear assembly on a part of which is mounted a motor for driving a wheel of the landing gear assembly, for example to enable the aircraft to perform taxiing manoeuvres. The motor is a liquid-cooled motor and the liquid that is used to cool the motor is hydraulic fluid, preferably hydraulic fluid taken from a hydraulic system that already exists on the aircraft to supply other consumers of hydraulic fluid pressure on the aircraft. The hydraulic fluid thus has other utility (i.e. other than cooling the motor) on the aircraft and therefore has a significant volume (and therefore a significant heat capacity). There may be a need to convert from high pressure hydraulic fluid to a lower pressure supply.