The Avro York was on a positioning flight from London-Stansted to Lyneham. Approaching Lyneham, the crew started a Ground Controlled Approach (GCA) to runway 07, intercepting the glide path at 2000 feet. At a distance of 400 m short of the runway, the aircraft suddenly attained a high rate of descent. The aircraft struck some treetops and crashed onto a ridge. PROBABLE CAUSE: “The accident occurred as a result of the captain’s allowing the aircraft to descend below critical height during a Ground Controlled Approach without having visual reference to the ground. The contributory causes were: 1) The captain was not warned by the talk-down controller that the aircraft was approaching break-off altitude. 2) The visibility at the threshold of runway 07 was less than airfield visibility which had been reported to the captain. 3) The aircraft was effected by a downdraft which caused it to sink below the glide path. The glide path at Lyneham allows for less obstacle clearance than is normal with a civil installation.”