METHOD OF MONITORING A DEGRADATION OF A DEVICE ON BOARD AN AIRCRAFT INCLUDING THE DETERMINATION OF A COUNTING THRESHOLD

A method of monitoring a degradation of a device on board an aircraft, implemented by a computer, the degree of the degradation of the onboard device being defined by an abnormality score formed by the counting of occurrences of anomalies noted by a control system of the device, the method of monitoring comprising a step of comparing an abnormality score obtained for an observation sequence of given length (n, t) with a decision threshold (ks) and a step of emitting an alarm in case of attainment or overshoot of the decision threshold (ks), the decision threshold (ks) being determined automatically for a given alarm probability Pa, corresponding to the probability that an alarm is emitted in the course of the monitoring method although the onboard device is healthy, by means of the following steps: a step of obtaining an abnormality score (r) on at least one reference sequence corresponding to flights of the aircraft without degradation and of length (m, tc) equal to a plurality of lengths (n, t) of observation sequences; a step of fitting a discrete probability law making it possible to retrieve the abnormality score (r) obtained on the reference sequence; a step of calculating the decision threshold (ks) such that by applying the discrete probability law fitted during the previous step to an observation sequence having the given length (n), the probability that an anomaly score greater than or equal to the decision threshold (ks) will occur is less than the given alarm probability Pa or an elementary probability dependent on Pa in the case where a confirmation strategy is used.