The phenomenon of aerodynamic coupling between the longitudinal and lateral motion at high incidence is well known. In recent years, there have been a number of attempts to analyse the behaviour of aircraft allowing for this coupling. One of the more practical methods of approach is that of Kalviste, in which the aircraft rotary motion alone is considered, the aircraft having a certain angle of sideslip in the steady state. However, that method is essentially based on linearised equations. In this note, we show how Kalviste's analysis can be modified to include certain non-linear aerodynamic effects. In particular, some unstable modes only become apparent on introducing the non-linear terms.