A pilot probably flew into cloud cover and felt internal pressure to push on before crashing a light plane west of Brisbane, killing everyone on board.
The four-seat Cessna crashed on August 29, 2022 in dense bushland near Fernvale en route to Archerfield Airport, in southern Brisbane, from Dalby on the Darling Downs.
Pilot Gary Liehm, farmer and agriculture industry leader Tom Strachan and his 20-year-old son Noah all died in the crash.
No mayday calls were reported and it was not until four hours after the flight went missing that the alarm was raised.
An Australian Transport Safety Bureau (ATSB) investigation found the aircraft left Dalby and flew toward forecast weather unsuitable for visual flight, despite Mr Liehm only being qualified for flight in visual conditions.
After crossing a section of the Great Dividing Range below cloud, and with minimal terrain separation, Mr Liehm continued the flight in similar conditions toward the Lake Manchester route, adjacent to the D'Aguilar Range.
ATSB Chief Commissioner Angus Mitchell said the plane very likely entered cloud while manoeuvring in the area before crashing into the terrain.
"The pilot was probably influenced by plan continuation bias - an internal pressure or desire to get to the destination - to continue the flight, which probably became stronger as they got closer to Archerfield," he said on Thursday.
The bureau was unable to determine the reasons why the pilot continued the flight at cruise speed and low level into unsuitable weather around known high terrain.
Any direct or perceived organisational pressure on the pilot to continue the flight was considered unlikely, it said.
While not determined as a contributing factor in the deadly crash, the operator's hazard and risk register did not identify inadvertent entry into non-visual conditions as a hazard.
Commissioner Mitchell said it reduced the operator's ability to effectively manage the risk.
The operator has since removed aeroplane operations from its air operator's certificate and plans to implement two risk controls for its helicopter operations by February 2024.
Commissioner Mitchell used the tragedy to remind all visual pilots to be mindful of the subtle pressures of plan continuation bias this holiday period.
"Be prepared to amend and delay plans to fly due to poor or deteriorating weather and environmental conditions, and not to push on," he said.
