High–performance aircraft configurations, characterized by a small span and swept wings, have rolling moments of inertia that are significantly smaller than the pitching or yawing moments of inertia. As a result, nonlinear coupling during high–roll–rate manoeuvres produces significant yawing and pitching moments. For certain critical flight conditions, inertial coupling causes jump phenomena called roll–coupling instabilities. These jump phenomena typically occur as a result of turning–point bifurcations of the aircraft steady states. Analysis of the moment balances along the steady solution branches provides physical insight into the causes of these instabilities and potential means of eliminating them. Analysis performed by using the full eight–degree–of–freedom equations of motion shows that the critical control–surface deflections are essentially the same as for the fifth– and sixth–order equations of motion. Solving the full eight–degree–of–freedom equations allows one to determine the orientation of the aircraft before and after the instability. For the aircraft model studied here, roll–coupling instabilities result in a change in sign of the angle of attack of the aircraft. The equilibrium state of the aircraft changes from a spiral dive, with the bottom of the aircraft closest to the axis of the spiral, to a spiral dive where the top of the aircraft is nearest the axis of the spiral, or vice versa depending on the trim angle of attack from which the manoeuvre was initiated. Pitching moment balance is shown to be central to the instability.