1. A method for automatic fault detection with data representing a fault in an on-board system, the method comprising: determining, with a processor, at least two physical variables represented by said data, said at least two physical variables including at least one time-dependent variable representative of a persistence of an event in time, and at least one instantaneous variable; representing, with the processor, said data by points of a space whose coordinates are values of said at least two physical variables; and detecting, with the processor, a fault by employing a frontier defining a subspace of said space in which representation of a datum is associated with the detection of a fault.