Soaring and manoeuvring flight of a steppe eagle Aquila nipalensis

We used an onboard inertial measurement unit, together with onboard and ground-based video cameras, to record the movements of the body, wings and tail of a steppe eagle Aquila nipalensis during wide-ranging flight. The eagle's flight consisted of a more or less continuous sequence of banked turns, interrupted by occasional wing tucks and roll-over manoeuvres, and ultimately terminated by a wing-over manoeuvre leading in to a diving landing approach. The flight configuration of the bird, and its pattern of movement during angular perturbations, together suggest that the eagle is inherently stable in pitch and yaw, and perhaps also in roll. The control inputs used to generate roll moments during banked turns were too subtle to be detected. Control of yaw and pitch during banked turns involved a consistent pattern of tail movement, wherein the tail was spread and depressed immediately before the turn, and then overbanked with respect to the bird during the latter part of the turn. Differential adjustment of wing posture is probably also involved in the control of banked turns, but it was only consistently apparent during more extreme roll manoeuvres. For example, roll-over and wing-over manoeuvres were both accomplished by differential changes in the angle of incidence and spread of the wings. In general, however, the bird appeared to maintain positive loading on its wings at all times, except during extreme flight manoeuvres.