Pre-flight Planning
The flight was planned to track from the departure aircraft landing area (ALA) direct to Charters Towers and then Atherton, a distance of about 425 NM. The flight north of Charters Towers was flight planned at 2,500 ft above mean sea level (AMSL); however, terrain elevations on the planned route north of Charters Towers were consistently higher than 2,500 ft AMSL. The pilot had obtained a weather forecast for Mareeba Airport (close to their intended destination), which indicated a cloud height of 2,000 ft above the airport elevation (1,564 ft), conditions that the pilot considered suitable for visual flight rules (VFR) flight. An updated available graphical area forecast (GAF), issued about 4 hours prior to departure, indicated cloud heights were forecast to be about 2,000 ft AMSL at the time the aircraft had planned to be flying over areas of high terrain. While the pilot was aware of encroaching weather and accelerated their planned flight to a day earlier to avoid the weather, the pilot’s pre-flight planning in respect to planned altitude north of Charters Towers and the weather conditions at the time of the flight was inadequate. Without the required aviation forecast, or appreciation of weather conditions en route, the pilot departed for their destination without the knowledge of expected cloud en route that was lower than terrain elevation and would likely have prevented visual flight.
Continued Flight at Low Level
After assessing in flight that conditions were unsuitable for continued flight direct to Atherton due to the low cloud height, the pilot planned for an alternate airport of Mareeba, visually tracking west to avoid higher terrain. About 35 minutes after the diversion, the pilot intercepted and began to track north following Kennedy Developmental Road towards rising terrain. The pilot described having visibility of greater than 10 km and a good horizon while tracking north following the road. As the terrain elevation increased about 900 ft during the few minutes of northerly flight along the road, the pilot was unable to maintain the minimum terrain clearance of 500 ft above ground level or the minimum 5 km visibility before entering instrument meteorological conditions (IMC). The pilot stated that they had turned back several times on previous flights due to marginal weather conditions. The pilot and passenger were travelling on a private flight; it was unlikely that there was time pressure to arrive at the intended destination. Consistent with other occurrences of visual flight rules (VFR) into IMC, the aircraft entered IMC conditions within the last 20% of the flight after continuing flight below the minimum required altitude. Although the pilot recalled initially having good visibility, they continued flight towards the destination below a safe altitude, this indicated a desire to ‘press on’ to the destination and increased the risk of unintended entry into IMC and collision with terrain.
Spatial Disorientation
The pilot described being surprised how quickly they entered a ‘white-out’ that appeared in front of the aircraft. Likely as a result of attempting to avoid entering the cloud and losing visual reference, they instinctively reduced power and commenced a left turn. During the turn the aircraft entered cloud and the pilot described becoming ‘totally disorientated’ shortly thereafter. Data showed that the aircraft altitude began to fluctuate with several changes of up to 500 ft vertically in about a 60-second period. While in cloud the aircraft came close to impacting terrain on more than one occasion. The instability of the flight path with numerous rates of climb and descent are commonly observed in spatial disorientation occurrences where pilots perceive a departure from stable flight and attempt to correct the unusual flight sensations without visual reference. Unable to reference the aircraft’s visual position or orientation to terrain after entering cloud, the pilot conducted a steep left turn and then engaged the autopilot with the intent to stabilise the aircraft.
Autopilot Engagement and Aircraft Stall
The engagement of the autopilot levelled the aircraft’s wings and held a constant heading. However, the aircraft became established in a climb due to the aircraft attitude when the autopilot was engaged, capturing a high rate of climb. The pilot used the heading bug on the horizontal situation indicator to reverse their track 180° to try to fly out of cloud. The aircraft airspeed was likely less than the recorded ground speed of 54 kt due to a tailwind and therefore most likely below the aircraft’s published stall speed. The pilot’s decision to engage the autopilot stabilised the aircraft’s heading, however without adequate power, the autopilot maintained the captured rate of climb while the airspeed reduced. As the aircraft commenced the pilot‑commanded left turn, the increased angle of bank and slow speed likely resulted in the aircraft stalling and entering a rapid descent.