Abstract: In recent years,modern estimation theory—such as Kalman-Bucy-filter or Luenberger-observer^1-5—has been applied to the estimation of aircraft states in order toprovide analytical Redundancy of flight control system.Linear estimation theory isquite successful in steady flight,but it fails in the case of intense nonlinear time-varying nonsteady manoeuvcr under stochastic atmospheric turbulence⁷.This paper introduces a few nonlinear observers with different structures,which take into account the coupling between longitudinal and lateral motions.Such observers can be applied to simultaneous estimation of the angle of attack and side slip angle fromaccurately measured flight test data either in the steady or in the nonsteady manoeuver conditions.As we know,the common sensor (wind vane) is not able to measure accurately both the angle of attack and the side slip angle.The off-line estimated results from flight test data prove that the observers withthe acceleration feedbacks have very good estimation qualitie in different flight manoeuvers,meeting the required accuracy of 0.2°in flight control system⁶.It is spectacular that these observers possess unexpectedly very strong robustness against stochastic atmospheric turbulence and against drastic changes of flight conditions.