A coolant system

A coolant system of a generator 208 driven by an aircraft prime mover comprises a fluid circuit 201 that contains a fluid such as oil that passes via a cooled component 220 of the generator. Fluid circuit 201 comprises a first fluid flow source 221 to deliver a first fluid flow to a cooling path 202; a second fluid flow source 222 to generate a second fluid flow; and a valve arrangement 212 configured to selectively direct at least a proportion of the second fluid flow away from the cooling path in dependence on a measured operational parameter of the generator. The first flow may supply fluid continuously to the path combined with fluid from the second path via non-return valve 236 when dump valve 234 is closed but when valve 234 is open the second path may divert fluid to the reservoir 231. The measured parameter may be the fluid pressure in the fluid circuit measured at a point 214 downstream of a cooler in the path. First and second fluid sources 221,222 may be driven via the generator and comprised in a single pump 206 or in first and second respective pumps.