Abstract: This paper presents a sufficiently accurate,on-line and realizable ronlinear observer,which takes the coupling between longitudinal and lateral motion into accountand is provided with proper feedbacks for all three acceleration components.Such anobserver can be used for estimating simultaneously the angle of attack and ship angle from accurately measured flight test data under either steaoy or nonsteady manoeuver conditions.As we know,the common sensor(wind vane)is not able to measureaccurately both the angle of attack and the side slip angle.In order to make it on-lineapplicable to a flight control/navigation system,the authors employ a simplifiedsecond order observer in this paper.It is proved by on-line estimated results from flight test data that the observerhas not only very good estimation qualities in different flight monceuvers,meeting there quired accuracy of 0.2°in the flight control/navigation system,but also very strongro bustness against stochastic atmospheric turbulence and drastic changes of flight conditions.It indicates that this observer can be applied to actual system.