The pilot had been operating the aircraft on gypsum spreading operations in mountainous terrain during the morning. The task involved flying along a valley, crossing a ridge at the head of the valley and spreading the gypsum on the western slopes of the ridge. Shortly after the aircraft departed on the accident flight, smoke was observed rising from the valley. It was discovered that the aircraft had impacted the side of the valley with a high rate of descent and very low forward speed. The hopper load had not been jettisoned prior to impact and the hopper doors were closed. The aircraft was destroyed by impact forces and the subsequent fire. The investigation did not reveal the presence of any pre-existing mechanical defect which might have contributed to the accident. The engine was operating at high power at the time of impact. Because of the damage to the aircraft, the hopper and fuel loads were not able to be precisely determined. Evidence suggested that the aircraft was probably heavier than the maximum all up weight approved for agricultural operations. On previous flights the aircraft had been observed to orbit over the valley in order to gain sufficient height to safely clear the ridge. It is considered possible that on this occasion the pilot commenced the orbit from a position and height which did not allow sufficient room for the manoeuvre to be completed at moderate bank angles. It was evident that during the orbit the aircraft stalled and entered an incipient spin. Insufficient height remained to permit recovery before impact with the ground. The weather conditions at the time were conducive to the formation of downdrafts, but whether this phenomenon had any bearing on the loss of control leading to the accident was not established. In addition, the reason that the hopper load was not jettisoned could not be determined.
