An aircraft environmental control system (4) for controlling a temperature of a fluid, the control system (4) comprising: a temperature sensor (30) configured to measure a temperature of the fluid and to generate a signal, the first signal being indicative of the measured temperature; a control signal generator (52) configured to, dependent upon the first signal, generate a control signal for controlling the temperature of the fluid; and one or more processors (64) configured to, responsive to determining that the measured temperature oscillates about a central value, decrease a gain of the control signal generator (52).