1. A method for establishing an aid in diagnosis of a complex system of an aircraft comprising a plurality of sub-systems, at least one sub-system of the plurality of sub-systems comprising circuitry configured to monitor and provide notifications regarding at least one detected event, the method using: a failure condition graph at least partially modeling the complex system, the failure condition graph comprising a plurality of peaks, each peak being connected by a logic implication relationship to at least one other peak of the plurality of peaks, the plurality of peaks comprising: one peak representing a failure condition event; and one peak representing only at least one element of the complex system, the at least one element being able to break down, and the method comprising: receiving at least one message of notification of occurrence of the at least one detected event; creating a set of failure events, each said failure event of the set of failure events being associated with a peak of the failure condition graph linked to the at least one received notification message; for each said failure event of the set of failure events, constructing, from the failure condition graph, at least one logic expression leading to the failure event, the at least one logic expression being based on elements of the complex system; creating at least one group of failure events of the set of failure events, the at least one element being common to two logic expressions linked to two separate failure events of the at least one group, the logic expressions associated with the failure events of the at least one group representing a diagnosis relating to the at least one detected event; and determining minimal vertexes of the logic expressions of the failure events of the at least one group of failure events, the minimal vertexes forming minimal diagnoses of the diagnosis relating to the at least one detected event.