Method for determining an efficiency fault (R11-R15) of at least one module (11-15) of a turboshaft engine (T) of an aircraft (A), the turboshaft engine (T) comprising at least one compressor module , at least one combustion chamber module and at least one turbine module , each module having an efficiency (R11-R15) that is specific to it, the method of determining comprising: - A step of determining (E1) a performance mapping of the turboshaft engine (T), hereinafter "estimated real mapping" (CARE), the estimated real mapping (CARE) defining a plurality of mathematical relationships (Fg) between parameters (Pj) of the turboshaft engine (T), each mathematical relationship (Fg) being determined from the measurements of parameters (Pj) of the turboshaft engine (T) acquired during a given flight (VOL) of the aircraft (A), - A step of determining (E2) real indicators (IRE) from the real mapping (CARE), said real indicators (IRE) corresponding to operating singularities of the predetermined turboshaft engine (T), - A step of determining (E3) a plurality of performance mappings from a simulation of a theoretical model (MOTH) of the turboshaft engine (T) for different efficiency configurations (CR), hereinafter "simulated mappings" (CARSx), the efficiency configuration (CR) determining the efficiency (R11-R15) of each module (11-15), - A step of determining (E4) simulated indicators (lSx) for each simulated mapping (CARSx), said simulated indicators (ISx) corresponding to operating singularities of the predetermined turboshaft engine (T), - A training step (E5) of a mathematical model (CLASS) by linking simulated indicators (lSx) to efficiency configurations (CR) and - An application step (E6) of said mathematical model (CLASS) to the real indicators (IRE) in order to deduce a real efficiency configuration (CR). - A step of determining (E1) a performance mapping of the turboshaft engine (T), hereinafter "estimated real mapping" (CARE), the estimated real mapping (CARE) defining a plurality of mathematical relationships (Fg) between parameters (Pj) of the turboshaft engine (T), each mathematical relationship (Fg) being determined from the measurements of parameters (Pj) of the turboshaft engine (T) acquired during a given flight (VOL) of the aircraft (A), - A step of determining (E2) real indicators (IRE) from the real mapping (CARE), said real indicators (IRE) corresponding to operating singularities of the predetermined turboshaft engine (T), - A step of determining (E3) a plurality of performance mappings from a simulation of a theoretical model (MOTH) of the turboshaft engine (T) for different efficiency configurations (CR), hereinafter "simulated mappings" (CARSx), the efficiency configuration (CR) determining the efficiency (R11-R15) of each module (11-15), - A step of determining (E4) simulated indicators (lSx) for each simulated mapping (CARSx), said simulated indicators (ISx) corresponding to operating singularities of the predetermined turboshaft engine (T), - A training step (E5) of a mathematical model (CLASS) by linking simulated indicators (lSx) to efficiency configurations (CR) and - An application step (E6) of said mathematical model (CLASS) to the real indicators (IRE) in order to deduce a real efficiency configuration (CR). <EOS>