The Focke Achgelis Fa-330 Bachstelze (Wagtail) specially developed for use aboard U-boats - in effect replacing the mast as an observation post - was a simple autogyro weighing little more than 75 kg. The 'pilot' or 'observer' sat under a free-turning three-blade rotor mounted on a vertical pylon attached to a simple framework and controlled a rudder and horizontal stabiliser mounted on a tubular boom structure. Connected to the U-boat by cable and winch the 'rotor kite' was designed to be towed behind a surfaced U-boat, the observer communicating by telephone. A German Air Ministry conference report referred to the engine-less contraption as a "witch's broom"..
About 200 Bachstelze were built although only a handful were used operationally and then only in the Southern Atlantic and the Indian Ocean where aircraft patrols were infrequent. In an emergency the pilot could jettison the rotor and deploy his parachute although at least one successful pilot recovery was recorded following autorotation after a towing cable snapped. According to Coates in "Helicopters of the Third Reich" there was only one recorded sinking by a U-boat following a sighting from a Bachstelze when U-177 sank a Greek coal freighter south of Madagascar in July 1943.
