1. Computer method for establishing an aid in diagnosis of a complex system of an aircraft ( 700 , 800 ) comprising a plurality of sub-systems ( 705 , 805 ), at least one sub-system of the said plurality of sub-systems comprising means for monitoring and notification of at least one detected event, this method being characterized in that: it uses a graph of feared events ( 200 , 300 ) at least partially modeling the said complex system, the said graph of feared events comprising a plurality of peaks ( 205 - 250 ), each peak of the said plurality of peaks being connected by a logic implication relationship to at least one other peak of the said plurality of peaks, the said plurality of peaks comprising at least, one peak representing a feared event ( 205 , 210 ); and, one peak representing at least one element ( 245 , 250 ) of the said complex system, the said at least one element being able to break down; it comprises the following steps, receiving ( 500 ) at least one message of notification of occurrence of the said at least one detected event; creating ( 510 ) a set of feared events, each feared event of the said set of feared events being associated with a peak of the said graph of feared events linked to the said at least one received notification message; for each feared event of the said set of feared events, constructing ( 515 ), from the said graph of feared events, at least one logic expression leading to the said feared event, the said at least one logic expression being based on elements of the said complex system; creating ( 520 ) at least one group of feared events of the said set of feared events, at least one element being common to two logic expressions linked to two separate feared events of the said at least one group, the logic expressions associated with the said feared events of the said at least one group representing a diagnosis relating to the said at least one detected event.