A method of controlling an outlet temperature of a heat exchanger of an aircraft includes operating an air cycle machine at a first operating speed. The air cycle machine is operatively coupled to an air cycle machine bypass valve at a first position. The air cycle machine is fluidly connected to a first heat exchanger disposed adjacent to a second heat exchanger. In response to a second heat exchanger outlet temperature less than a second heat exchanger outlet temperature threshold while the aircraft is operating at an altitude less than an altitude threshold, the method moves the air cycle bypass valve from the first position toward a second position.