This paper deals with the modelling and simulatation of an aircraft Longitudinal control system including a hydromechanical booster. A bond graph model of the booster is developed. the equation's describing the booster are deduced from the bond graph taking into consideration the basic nonlinearities. The motion of aircraft is represented by a transfer function describing its behavior during the short period longitudinal oscillations. The validity of the deduced model the booster is studied by the comparison of calculated transient response of booster with corresponding experimental results. The effect of some constructional and operational parameters on the dynamic behaviour of the hydraulic  booster and aircraft is studied by the exploitation of the deduced model.