This paper presents an application of decoupling theory for designing a controller with output feedback in rapid, non-linear aeroplane manoeuvres. For simplicity in control law derivation, only momenta caused by aileron, rudder and elevator are considered, and control forces are neglected. For a class of non-linear systems, a method for designing an observer is presented. Using this approach, angle of attack and sideslip are constructed. An approximate observer is obtained by neglecting the derivatives of roll rate and aileron deflection. The non-linear controller is synthesized using estimated angle of attack and sideslip obtained from the approximate observer. Simulated responses of the closed-loop system, including the control forces, show that the neglected forces have no significant effects and the large, simultaneous lateral and longitudinal manoeuvres can be precisely performed. The control law shows only a small effect of wind gust.