Abstract: A pyrometer which senses blade temperature in the engine as also used to detect the onset of an engine surge. The output of the pyrometer 2 (FIG. 1) passes to a level detector unit, comprising a differential amplifier and a constant reference voltage, and which produces an output when the pyrometer signal exceeds the reference voltage. A duration detector comprising a monostable 12, inverter 14, delay line 18 and AND gate 16 determines the duration of the excess level signal and if the duration exceeds a given time, set by the monostable, an output from the duration detector actuates a fuel chopper to minimize the effect of the surge.