The pilot was positioning the helicopter before carrying out a medical evacuation from a ship. Prior to DEPARTURE he had arranged to land on the school oval at Lancelin, to refuel, and to have two vehicles positioned at the oval to provide lighting for the landing. During the subsequent night approach to the oval the helicopter collided with sand dunes. Witnesses reported that during the approach the aircraft descended to a low altitude and disappeared from sight behind the sand dunes before impact. An inspection of the wreckage revealed that apart from the altimeter no other faults were found with the aircraft that could have contributed to the occurrence. Examination of the altimeter found that it was outside operating limits and during operation in the aircraft was likely to have been erratic in its indications. No such erratic indications were reported by the pilot or his passenger, who was also monitoring the altimeter. The weather at the time of the occurrence was reported as overcast with light drizzle and a light wind. The visibility was 20 kilometres although the night was dark and the only source of light in the area was from the town and the headlights of the two vehicles being used to light the landing area. These conditions are conducive to the pilot suffering from visual illusions and it is possible that these illusions caused the pilot to misjudge the altitude of the helicopter during the approach.
