Abstract: A head-up display system comprising an optical combiner 5 for positioning in front of a pilot's head, in-line with the pilot's normal line of sight and in a plane substantially transverse to said line of sight. Sensor means, comprising a pair of optical sensors 9,10, is arranged to detect motion of the pilot's head towards the combiner 5 and to cause a trigger signal to be generated, prior to impact between the pilot's head and the combiner, if the velocity of said motion exceeds a threshold velocity. Retraction means 7 are coupled to the sensor means for receiving therefrom said trigger signal and are arranged, in response to receipt of said trigger signal, to retract the combiner 5 out of the trajectory of the pilot's head prior to impact between the head and the combiner.