A device for monitoring over time a state of at least one propulsion engine of at least one aircraft, wherein the device comprises a module for acquiring data (DV i) of flights (V i) of the at least one propulsion engine of the at least one aircraft, comprising, in association for each flight (V i), values (E1 i) of input variables of the engine during the flight (V i), values (ENV1 i) of environment variables of the engine during the flight (V i) and values (S1 i) of output variables of the engine during the flight (V i), a learning module (APPR) configured to compute by learning from the data (DV i) of each flight (V i) an individual flight model (f i) for the flight (V i), linking the input variables of the engine, the environment variables of the engine and the output variables of the engine, and characterized by comprising a using module for using the individual flight (V i) models (fi), configured to compute, for each flight (V i): estimates of the values (S2 i =f i (E R, ENV R)) of the output variables of the engine, which are obtained by applying the individual flight (V i) model (f i) to reference values (E R) of the input variables of the engine and to reference values (ENV R) of the environment variables of the engine, and an error (ε i) associated with said estimates of the values (S2 i =f i (E R, ENV R)) of the output variables of the engine, which have been generated by the individual flight (V i) model (f i) and which have been obtained by applying the individual flight (V i) model (f i) to the reference values (E R) of the input variables of the engine and to the reference values (ENV R) of the environment variables of the engine, wherein the reference values (E R, ENV R, S R) belong to a prescribed set of reference data, which are identical for the plurality (i) of individual flight (V i) models (f i). a module for acquiring data (DV i) of flights (V i) of the at least one propulsion engine of the at least one aircraft, comprising, in association for each flight (V i), values (E1 i) of input variables of the engine during the flight (V i), values (ENV1 i) of environment variables of the engine during the flight (V i) and values (S1 i) of output variables of the engine during the flight (V i), a learning module (APPR) configured to compute by learning from the data (DV i) of each flight (V i) an individual flight model (f i) for the flight (V i), linking the input variables of the engine, the environment variables of the engine and the output variables of the engine, and characterized by comprising a using module for using the individual flight (V i) models (fi), configured to compute, for each flight (V i): estimates of the values (S2 i =f i (E R, ENV R)) of the output variables of the engine, which are obtained by applying the individual flight (V i) model (f i) to reference values (E R) of the input variables of the engine and to reference values (ENV R) of the environment variables of the engine, and an error (ε i) associated with said estimates of the values (S2 i =f i (E R, ENV R)) of the output variables of the engine, which have been generated by the individual flight (V i) model (f i) and which have been obtained by applying the individual flight (V i) model (f i) to the reference values (E R) of the input variables of the engine and to the reference values (ENV R) of the environment variables of the engine, wherein the reference values (E R, ENV R, S R) belong to a prescribed set of reference data, which are identical for the plurality (i) of individual flight (V i) models (f i). estimates of the values (S2 i =f i (E R, ENV R)) of the output variables of the engine, which are obtained by applying the individual flight (V i) model (f i) to reference values (E R) of the input variables of the engine and to reference values (ENV R) of the environment variables of the engine, and an error (ε i) associated with said estimates of the values (S2 i =f i (E R, ENV R)) of the output variables of the engine, which have been generated by the individual flight (V i) model (f i) and which have been obtained by applying the individual flight (V i) model (f i) to the reference values (E R) of the input variables of the engine and to the reference values (ENV R) of the environment variables of the engine, wherein the reference values (E R, ENV R, S R) belong to a prescribed set of reference data, which are identical for the plurality (i) of individual flight (V i) models (f i).