failures the raw data from the system identification module 42 is separated into two data streams namely raw values of the steady state gain k hereinafter u2018 k_raw u2019 and the time constant t hereinafter u2018 t_raw u2019 a signal corresponding to k_raw is passed through a low pass filter at step 402 which removes insignificant high frequency variations in the signal noise for example so as to provide an initial filtered value of steady state gain k hereinafter u2018 k_filtered u2019 during the first few minutes of engine operation the temperature of the fuel will rise to a working fuel temperature which will reduce the viscosity of the fuel this will have the effect of increasing the level of leakage in the system which will have a corresponding effect of reducing the steady state gain value k in order to compensate for the change in k due to fuel temperature changes k_filtered is input into a temperature compensation unit 404 once the effects of increased fuel temperature have been removed the k_filtered signal is passed through a high pass filter 406 having a filter time constant in the order of minutes to provide a second filtered value of k hereinafter u2018 k_high_filtered u2019 in turn k_high_filtered is input into a rate of change calculation unit 408 which calculates the derivative of k_high_filtered with respect to time hereinafter u2018 k u2032 _high filtered u2019 finally the rate of change calculation unit 408 outputs the k u2032 _high_filtered signal to a knowledge module 410 the knowledge module 410 monitors the received value of k u2032 _high_filtered and compares it with predetermined values of u2018 k u2032 _high_filtered u2019 that are indicative of a burst fuel supply pipe and which are stored by the knowledge module 410 for example if a.