A method of controlling an aircraft gas turbine engine cooling system. The cooling system includes a heat exchanger having a first fluid path through which fan air flows, and a second fluid path through which relatively hot compressor air flows. The cooling system includes a valve configurable between an open position corresponding to a first operating mode, where fan air flows through the first path cooling the compressor air in the second path to a lower temperature, and a closed position corresponding to a second operating mode, where fan air may flow through the first path at a reduced rate, thus the compressor air in the second path could be cooled to a lesser extent. The valve is operated in first mode when the aircraft is in a descent mode, or when turbine entry temperature is above a predetermined amount. Otherwise, the valve is operated in second mode.