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Analysis

Summary

The absence of witnesses to the accident, and of an emergency radio broadcast from the pilot meant that there was no information available to the investigation about the pilot's situation immediately prior to the accident. However, the low-level manoeuvring carried out by the pilot overhead the sheltering sheep, and interaction via radio with the driver of the four wheel drive vehicle was consistent with the pilot attempting to disturb the sheep from their position.

The steepness of the angle of bank and the nose-down pitch attitude at the aircraft's point of ground impact indicated that the aircraft was in a steep left turn at that time. Those indications and the minimal forward movement of the aircraft after ground contact were consistent with the aircraft having stalled and slipped out of the turn. The lack of aircraft rotation at impact indicated that there had been insufficient time for the stall to develop into a spin, consistent with it occurring at low level. It was likely that there was insufficient time for the pilot to recover before impacting the ground. Given the variety of opinion relating to the use of wing flap during mustering operations, the investigation was unable to determine the degree of influence that the lack of flap had on the development of the accident.

The pilot's probable focus on the sheltering sheep, together with the need to operate the UHF radio may have distracted the pilot from the primary task of flying the aircraft. In addition, any sensory illusion as a result of the pilot moving his head during the low-level manoeuvring, or inadvertent movement of the flight controls could have resulted in an unintentional increase in the aircraft's angle of bank. In either case, it was likely that the pilot was initially unaware that the aircraft was in such a steep turn, or that the airspeed was insufficient for the angle of bank. The stall warning probably sounded before the stall, but given warnings were often activated during aerial mustering, it may not have had a significant effect on the pilot's awareness of the impending stall.

A possible influence on the development of the stall was a decrease in available engine power. However, the damage to the propeller indicated that the engine was developing power at ground impact, there were no identified engine defects, there was adequate fuel on board and the pilot had flown for about 1.5 hours without any apparent performance degradation prior to the accident. That evidence indicated that the engine was capable of performing normally. Notwithstanding that evidence, the use of a mix of aviation gasoline and unleaded petrol, and the estimated dewpoint depression at about the time of the accident, meant that the investigation could not discount the possibility of the formation of carburettor icing. The result in that case was the possible loss of some engine power.

During the pilot's 18 years experience operating the Cessna 206, he would have become accustomed to the relatively large amount of engine power available during aerial mustering manoeuvres in that aircraft. The investigation considered whether the pilot might have unwittingly expected the same performance from the Cessna 150. However, the pilot had operated the Cessna 150 on aerial mustering operations for over 15 months, and it was concluded that the pilot would most probably have been aware of, and adjusted to that performance difference between the aircraft types during that time.

Although the pilot's flight reviews included the practice of stall recovery in turns, and the pilot had been mustering for 18 years, the lack of an aerial stock mustering permission meant that the pilot had not completed formalised training in all of the competencies inherent in the award of that permission. As a result, there was the potential that the pilot may have acquired and, over time, reinforced perhaps inappropriate responses to some of the risks inherent in the mustering environment. The completion by the pilot of the aeronautical experience requirements of Civil Aviation Order 29.10 would have provided some assurance that he had acquired the appropriate knowledge and skills necessary to manage the risks inherent in the low-level, low-speed, and high workload mustering environment.

 
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