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Helicopter is a system on which operates a large number of specialities from electrical domain to mechanical domain. Today, diagnosis methods are segmented by field of expertise. Each expert treats the sub-system whose he is responsible for regardless of the results of other specialities.Therefore, there is no relation between specialities. Thus, diagnosis at system level is efficient but, due to the lack of correlation between subsystems, it is incomplete. Within the framework of a helicopter, the operator of maintenance uses all the data recorded during the flight, the results of expert treatments, but also his knowledge, his experience and his capacities of observation and analysis to provide an effective global diagnosis. In order to build relation between fields of expertise and so, to obtain a diagnosis at aircraft level which could be relevant, we try to set up a concept which gets closer, at most, to human judgment but which is not well adapted to industrial environment: the normality.During our study, we established that to reach our objective and to stick at best with the concept of normality, the most relevant solution consists in building a global normal signature. This signature could be illustrated as the image of the aircraft health, qualified as normal. This paper defines the normal signature and explains, in part, the process of its building.