The Cessna 172 was being used to assist a ground party of station hands to muster sheep on a station property approximately 46 km south-west of Onslow, WA. The manager of the station reported that the aircraft was being used to spot livestock on the ground and to muster sheep using a pilot-activated siren on the underside of the fuselage.
The aircraft was being flown along a generally east-west track, from one side of the paddock to the other, which gradually progressed towards the northern boundary. Although members of the ground party were unable to estimate the height at which the aircraft was operating, they did report that the aircraft siren was effective in moving the sheep. An experienced mustering pilot stated that a siren fitted to an aircraft would probably be quite ineffective at a height of 500 ft above ground level.
The station manager reported that the pilot appeared to be attempting to position the aircraft to cut off a mob of sheep that had broken away from the group he was following. He saw the aircraft pass approximately overhead and in a westerly direction before it commenced a left turn.
The manager looked away from the aircraft but reported that he could clearly hear its engine, which sounded normal. He immediately looked up when he heard the sound of an impact and saw that the aircraft had crashed approximately 100-200 m from where he was standing.
As a result of the accident the pilot sustained fatal injuries and the aircraft was destroyed. Damage to the aircraft was consistent with it having impacted the ground in a near vertical attitude at a low forward speed. A significant quantity of fuel was later recovered from the aircraft wreckage. There was no evidence that a mechanical defect had contributed to the accident.
The property owners had employed the pilot to fly their aircraft to assist with mustering operations. They stated that they had little knowledge of operating a light aircraft in support of their primary production activities. One of the owners was aware of a mustering type endorsement and reported that despite contacting a number of organisations and authorities, he had experienced difficulty in finding somebody to conduct mustering training for the pilot.
The pilot was issued with his private pilot licence (aeroplanes) on 4 June 1999, 6 weeks before the accident. At the time of the accident, the pilot had accumulated about 191 hours aeronautical experience, which included some helicopter training. No evidence was found to indicate that the pilot had received any formal low-flying training or that he was qualified to conduct mustering operations. Investigators were told the pilot had arranged to do training for a mustering endorsement once he had enough experience to be endorsed.
Flight and duty time limitations are specified for pilots engaged in commercial operations. However, as this mustering operation was being conducted in the private category, there was no requirement for the pilot or the aircraft operator to comply with these limitations. Consequently, the pilot was responsible for determining his daily flying activities. This was done in conjunction with the property owners, property manager and the mustering party. Investigators were told that the pilot typically commenced flying at approximately 0700 local time and continued through the day, until the last paddock had been completed.
During the course of the investigation the pilot's recent flight and duty times were reviewed to determine whether fatigue had been a factor leading to the occurrence. He had flown at least 68 hours in the 9 days since arriving at the station and had not taken a day off during this period. On the day of the accident, he had flown at least 8 hours 30 minutes. He was known to take short breaks from airborne operations about every 4 hours, during which time he would refuel the aircraft.
