An aircraft environmental control system (4) for controlling a temperature of a fluid, the control system comprising: a temperature sensor (30) configured to measure a temperature of the fluid and generate a signal indicative of that temperature; a control signal generator (52) configured to, using control laws (78) and the first signal, generate a control signal for controlling the temperature of the fluid; a memory (56) for storing performance data; a transmitter (60) configured to transmit, from the aircraft environmental control system (4), the performance data; a receiver (60) configured to receive, responsive to transmitting the performance data, update information; and an update module (58) configured to, using the update information, update the control laws (78).