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The circumstances of this occurrence are consistent with the aircraft being flown at the limit of its performance capabilities, in the prevailing weather conditions. In addition, the reported medical condition of the pilot and the stress associated with operating an aircraft in such weather conditions, requires balanced consideration of the possibility of pilot incapacitation being a factor in the accident.

The Area 21 forecast indicated that the first part of the flight could be conducted clear of cloud during the climb and cruise, with the cloud tops forecast to extend to 8,000 ft in the area through which the aircraft was flying. The recorded radar data indicates that the aircraft was not significantly affected by airframe ice on initially reaching the planned cruising altitude of 10,000 ft.

However, the aircraft did appear to encounter icing conditions as it approached Cooma. The progressive reduction in aircraft groundspeed and the minor altitude variations from the aircraft transponder are consistent with the aircraft operating in convective cloud and accumulating airframe ice. A short time later, the aircraft was observed on radar to conduct a descending orbit and the pilot indicated that he was diverting to Cooma for a landing.

It is likely that during the descent, the aircraft broke clear of cloud and the pilot considered that he was able to continue towards his planned destination. The pilot was familiar with the route sector being flown and would have been aware of the height of terrain in the vicinity. It is unlikely that the pilot would operate the aircraft in cloud, below the lowest safe altitude and continue to fly towards rising terrain.

The apparent improvement in the stability of the radar recorded descent profile also suggests that the pilot had established visual reference during the latter stages of the initial descent from 10,000 ft. The radar recorded data, particularly the groundspeed that the aircraft achieved after levelling off, supports the pilot's report of having "unloaded" the airframe ice. It is likely that the aircraft was no longer operating in cloud and was not significantly affected by airframe ice at this time. The pilot had also commented about being able to get over the cloud at Kosciusko which further suggests the aircraft was established clear of cloud at this time.

Approximately eleven minutes before the accident the pilot reported that he had commenced climbing to 10,000 ft. The aircraft subsequently reappeared on radar and was observed to take up a track that would pass directly overhead Mt Jagungal, in conditions that were conducive to the formation of mountain waves and the forecast probability of occasional severe turbulence. The reported weather conditions at the time of the accident suggest that Mt Jagungal was probably covered by cloud and the pilot may have been unaware of his proximity to the mountain peak.

Based on the reported wind direction and strength, the radar-recorded low groundspeed suggests that the aircraft was climbing at a lower than normal airspeed. This would have provided the pilot with a reduced safety margin above the stalling speed. The apparent reduction in climb performance as the aircraft approached 9,000 ft can be attributed to the aircraft flying into the descending air associated with mountain wave activity. Had the pilot elected to level out at this altitude, it would be reasonable to expect that a measurable increase in groundspeed would be associated with the setting of a flight attitude for straight and level flight.

The witness sighting of the aircraft a short time before the accident indicates that at this point, the aircraft was established clear of cloud, but with a broken layer of cloud below. It was not possible to determine if the aircraft was significantly affected by airframe ice at the time of the accident. The presence of any ice on the airframe would have further increased the aircraft's stalling speed and further reduced the margin for any airspeed fluctuations due to turbulence.

It is possible that while attempting to continue climb to 10,000 ft the aircraft encountered moderate to severe turbulence. The unpredictable fluctuation in airspeed could have resulted in an inadvertent stall. The aircraft departed controlled flight immediately prior to the accident and impacted the ground at high speed in a near vertical attitude, consistent with an uncontrolled spiral dive. The reason for the loss of control could not positively be established.

Furthermore, the possibility of pilot incapacitation cannot be excluded as a contributing factor in the occurrence. The reported operation of the aircraft engine to the point of impact, together with the uncontrolled nature of the descent, indicates that there had been no effective response initiated to counter the rapid descent of the aircraft.

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