A Socata light aircraft's high rate of descent before impacting steep mountainous terrain was consistent with the pilot becoming spatially disoriented after flying into cloud, an ATSB investigation report details.
The five-seat, single-engine Socata TB-20 was conducting a private flight from Montpelier, near Townsville, to Palmyra, near Mackay, on 28 October 2023, with a pilot and passenger (who was also a licenced pilot, but was not current) on board.
During the flight, the pilot contacted a friend, also a licenced pilot, who was on the ground at Palmyra, to ask about the weather at the destination.
"After the friend advised of the prevailing conditions including cloud at Palmyra and blue sky to the south, the pilot replied that they would need to go through some cloud," Director Transport Safety Dr Stuart Godley said.
Recorded data of the flight from the OzRunways electronic flight bag app being used by the pilot showed around 30 NM from the destination, shortly after commencing descent, the aircraft began a steep descending turn to the left towards mountainous terrain.
During this descent, the aircraft exceeded the airframe's designed maximum airspeed before pitching up and passing over the top of Bull Mountain. The aircraft then entered a second steep descending turn, this time to the right, before the recorded flight path data ceased.
The aircraft collided with terrain in dense forest on the north-east face of Bull Mountain at about 1,900 ft above sea level.
"The instability of the flight path with excessive rates of descent and climb are markers of spatial disorientation where pilots are aware of a departure from controlled flight and attempt to correct the unusual attitude," Dr Godley said.
"It is very likely the pilot had entered weather conditions not suitable for visual navigation, leading to spatial disorientation and the descent into mountainous terrain."
The aircraft was destroyed in the accident, and both occupants were fatally injured.
The report notes the pilot, who was not qualified for flight in instrument meteorological conditions, had regularly flown between Montpelier and Palmyra, and would, according to peers, generally fly the cruise portion of the flight on autopilot and use the autopilot if they ever had to fly through cloud.
"For a non-instrument rated pilot, even with basic attitude instrument flying proficiency, maintaining control of an aircraft in IMC by reference to the primary flight instruments alone entails a very high workload that can result in narrowing of attention and loss of situational awareness," Dr Godley said.
"While autopilot can be used to reduce workload, it is not infallible and should not be relied upon or used by VFR pilots as a risk mitigator to decide to fly into unsuitable conditions."
Due to the limited evidence available to the ATSB, the investigation was unable to determine why the autopilot disconnected, either due to pilot action or technical failure, prior to the aircraft entering its high rate of descent.
"Appropriate pre-flight preparation and planning is the key risk control for visual flight rules pilots to avoid entering IMC," said Dr Godley.
"While forecasts will assist in selecting the route to be flown, pilots should plan an alternate or be prepared to make necessary deviations from the planned route, should actual weather conditions indicate the possibility of not being able to remain visual."
The investigation report also cautions against the use of unapproved mobile devices displaying charts and data as the sole means of navigation when operating under the VFR.
Tablet computers running electronic flightbag apps such as OzRunways are widely used and a useful supplement to aircraft navigation, but are not approved for use as a primary means of navigation. The accident aircraft was fitted with an approved GPS for navigation. However, the pilot was not known to program flight plans into the unit, instead relying on OzRunways running on an iPad for navigation.
"Pilots should use navigation equipment approved for aviation and maintain skills in navigating by reference to approved charts," Dr Godley noted.
The ATSB publication, Accidents involving Visual Flight Rules pilots in Instrument Meteorological Conditions includes a selection of weather-related general aviation accidents and incidents that show weather alone is never the only factor affecting pilot decisions that result in inadvertent IMC encounters.
"These investigations consistently highlight that conducting thorough pre-flight planning is the best defence against flying into deteriorating weather."
You can find here the final report: VFR into IMC, loss of control and collision with terrain involving SOCATA-Groupe Aerospatiale TB-20, VH-JTY, 65 km west of Mackay Airport, Queensland, on 28 October 2023
