What happened
During a solo navigation flight, the aircraft entered a period of increasing descent and turning that culminated in a collision with terrain. Prior to the spiral, the aircraft had maintained steady altitude and groundspeed, showing no signs of engine or control failure. The flight path included two controlled turns after passing Khancoban, which appeared to be deliberate maneuvers to navigate around cloud or precipitation or to seek alternative landing sites. However, the aircraft's rate of descent and turn increased steadily until impact, with the aircraft exceeding its maximum allowable airspeed just before the collision.
The investigation
The investigation examined weather forecasts and local observations near Khancoban. Forecasts indicated broken cloud between 3,000 ft and above 10,000 ft AMSL, along with drizzle and overcast conditions. While specific weather at the impact site could not be precisely determined due to a lack of local stations, satellite imagery and witness accounts confirmed cloud presence. The investigation also reviewed the pilot's training and medical status. The pilot held a valid basic class 2 medical certificate and had no known medical issues that would impact flight ability. However, training records revealed the pilot had only one hour of instrument flight training and had not yet completed the syllabus required for a navigation endorsement. Consequently, the pilot was not authorized to fly the 189 NM distance between Wangaratta and Moruya.
Findings
- The pilot likely encountered instrument meteorological conditions (IMC), which significantly increased the risk of spatial disorientation.
- The aircraft's flight pattern was consistent with the somatogyral illusion, often referred to as a 'graveyard spiral'.
- The pilot's limited experience in interpreting complex weather forecasts may have led to an underestimation of the difficulty of the flight conditions.
- The pilot was operating without the necessary training or authorization for the specific cross-country distance being flown.
- While aircraft icing or control issues were considered, the primary cause was identified as the pilot's encounter with low-visibility conditions.