An apparatus for detecting a rapid variation in an input quantity, such as the compressor delivery pressure of a gas turbine engine has a generating arrangement which provides an alternating signal whose magnitude is dependent on the value of the input quantity. The generating arrangement includes an automatic gain control and associated delay device so that the alternating signal has a substantially constant value except when there is a significant reduction in the input quantity in less than the delay time of the delay device. There is an associated detector circuit which provides an output pulse of predetermined duration in response to such a reduction in the input quantity.
