System for monitoring the health of a helicopter having at least one engine, said system comprising: - at least one engine sensor measuring an engine variable, at least one external conditions sensor measuring variables outside of the engine, and a device for memorising data measured by the sensors, and - a device for determining a change of state of the engine configured to collect, from the memorisation device, the data measured by the engine and external conditions sensors during at least one stable flight phase of a first flight, and implementing the following operations: a) comparison of said measured data with a reference model of the engine, b) from comparison a), determination, at each time step of the stable flight phase, of an instant offset between each measured datum and each datum estimated by the reference model of the engine, c) determination, over the whole of the stable flight phase, of an overall offset between the measured data and the data estimated by the reference model of the engine, the overall offset being calculated from a part at least of the instant offsets determined at step b), d) determination, at each time step of the stable flight phase, of an intrinsic residual corresponding to a difference between the instant offset and the overall offset, e) determination of one or several deviant portion(s) corresponding to the part(s) of the intrinsic residual not meeting a predetermined criterion, each deviant portion, when it exists, containing information relating to a non-recorded pilot action, and f) determination of a corrected residual corresponding to the instant offset from which the deviant portion(s) has been removed, the corrected residual being analysed in order to determine if it is the result of a pilot action having an incidence on the engine or if it is the result of a change of state of the engine. - at least one engine sensor measuring an engine variable, at least one external conditions sensor measuring variables outside of the engine, and a device for memorising data measured by the sensors, and - a device for determining a change of state of the engine configured to collect, from the memorisation device, the data measured by the engine and external conditions sensors during at least one stable flight phase of a first flight, and implementing the following operations: a) comparison of said measured data with a reference model of the engine, b) from comparison a), determination, at each time step of the stable flight phase, of an instant offset between each measured datum and each datum estimated by the reference model of the engine, c) determination, over the whole of the stable flight phase, of an overall offset between the measured data and the data estimated by the reference model of the engine, the overall offset being calculated from a part at least of the instant offsets determined at step b), d) determination, at each time step of the stable flight phase, of an intrinsic residual corresponding to a difference between the instant offset and the overall offset, e) determination of one or several deviant portion(s) corresponding to the part(s) of the intrinsic residual not meeting a predetermined criterion, each deviant portion, when it exists, containing information relating to a non-recorded pilot action, and f) determination of a corrected residual corresponding to the instant offset from which the deviant portion(s) has been removed, a) comparison of said measured data with a reference model of the engine, b) from comparison a), determination, at each time step of the stable flight phase, of an instant offset between each measured datum and each datum estimated by the reference model of the engine, c) determination, over the whole of the stable flight phase, of an overall offset between the measured data and the data estimated by the reference model of the engine, the overall offset being calculated from a part at least of the instant offsets determined at step b), d) determination, at each time step of the stable flight phase, of an intrinsic residual corresponding to a difference between the instant offset and the overall offset, e) determination of one or several deviant portion(s) corresponding to the part(s) of the intrinsic residual not meeting a predetermined criterion, each deviant portion, when it exists, containing information relating to a non-recorded pilot action, and f) determination of a corrected residual corresponding to the instant offset from which the deviant portion(s) has been removed,