Method of controlling a cooling system

A method of controlling an aircraft gas turbine engine cooling system (20). The cooling system comprises a heat exchanger (22) having a first fluid path (24) through which fan air can flow, and a second fluid path (26) through which relatively hot compressor air can flow. The cooling system (20) further comprises a valve (30) configurable between an open position corresponding to a first operating mode, in which fan air flows through the first path (24) to cool the compressor air in the second path (26) to a lower temperature, and a closed position corresponding to a second operating mode, in which fan air flows through the first path (24) at a reduced rate, or not at all, such that the compressor air in the second path (26) is cooled to a lesser extent, or not at all. The valve (30) is operated in the first mode when the aircraft determined to be in a descent mode, or when turbine entry temperature (T30) of the gas turbine engine is above a predetermined amount. Otherwise, the valve (30) is operated in the second mode.