A method of simulating an operating state of an aircraft, the method comprising: operating the aircraft via a control system, the control system being arranged to receive aircraft operating commands from a person controlling the aircraft and to create aircraft operating command signals, selectively operating the aircraft in a normal state of operation of an actual aircraft having an actual aircraft load, selectively operating the aircraft in a simulated state of operation, the simulated state of operation being different from the normal state of operation of an actual aircraft, wherein in a simulated state of operation the aircraft operating command signals are transformed so as to simulate operation of a simulated aircraft carrying a simulated aircraft load that is different from an actual aircraft load, and wherein a load database contains data for all aircraft loads, the current configuration for the aircraft and how the loads affect the aircraft, an aircraft model predicts the motion of the aircraft based on loads and operating commands, a calculation unit converts the calculated motion of the simulated aircraft into aircraft command signals so that the aircraft follows the motion of the simulated aircraft, the information about the actual aircraft motion is obtained from motion measurements which measure the aircraft response, the motion data for the actual aircraft from the motion measurements is, during the training mission, continuously transferred to a calculation unit, the data of the motion of the simulated aircraft and data for the actual flight condition are used for calculating the vehicle command signals, the actual aircraft motion commands are transmitted in a way that corresponds to the state simulated to the ordinary flight control functions, where they are used for affecting the aircraft control surfaces, and the actual engine commands are transmitted in a way that corresponds to the state simulated to the ordinary engine control where they are used for controlling the engine actuators. operating the aircraft via a control system, the control system being arranged to receive aircraft operating commands from a person controlling the aircraft and to create aircraft operating command signals, selectively operating the aircraft in a normal state of operation of an actual aircraft having an actual aircraft load, selectively operating the aircraft in a simulated state of operation, the simulated state of operation being different from the normal state of operation of an actual aircraft, wherein in a simulated state of operation the aircraft operating command signals are transformed so as to simulate operation of a simulated aircraft carrying a simulated aircraft load that is different from an actual aircraft load, and wherein a load database contains data for all aircraft loads, the current configuration for the aircraft and how the loads affect the aircraft, an aircraft model predicts the motion of the aircraft based on loads and operating commands, a calculation unit converts the calculated motion of the simulated aircraft into aircraft command signals so that the aircraft follows the motion of the simulated aircraft, the information about the actual aircraft motion is obtained from motion measurements which measure the aircraft response, the motion data for the actual aircraft from the motion measurements is, during the training mission, continuously transferred to a calculation unit, the data of the motion of the simulated aircraft and data for the actual flight condition are used for calculating the vehicle command signals, the actual aircraft motion commands are transmitted in a way that corresponds to the state simulated to the ordinary flight control functions, where they are used for affecting the aircraft control surfaces, and the actual engine commands are transmitted in a way that corresponds to the state simulated to the ordinary engine control where they are used for controlling the engine actuators.