The invention relates to a device for system diagnosis of an aircraft. The device includes: means for the monitoring of equipment of the system, the means for emitting messages of observations on the basis of effects produced by equipment; means for the determination of a set of observations on the basis of the messages of observations arising from the monitoring of a log of the messages of observations and of a model representing a current state of the system; means for the determination of indictments on the basis of observations of the set of observations and of a behavioral model of the system, the indictments being logical relations between operating modes of equipment having produced effects; and means for the determination of maintenance operations on the basis of the indictments.