Abstract: An aircraft operating in a gravitational field and having conventional sensors for measuring true air speed (sensors 10, 11), angles of incidence and yaw (sensors 13, 12), rotation about x, y and z axes and acceleration therealong (sensors 14, 15, 16) is provided with means for calculating the inertial component of the acceleration from data concerning the true air speed, heading and rotation of the aircraft obtained from the sensors, and means for comparing the inertial component with the total acceleration sensed, thereby to deduce the orientation of the gravitational component and hence obtain an estimate of the pitch and bank angles of the aircraft.