An experienced pilot likely became disorientated while flying through clouds before crashing into a mountain, an investigation into the deadly accident has found.
The pilot and his passenger, a married couple aged in their 70s, were killed when the single-engine aircraft went down at Pioneer Valley, near Mackay in north Queensland, in October 2023.
Their bodies were not recovered until weeks later due to the crash site's difficult, mountainous terrain.
Almost a year later, the Australian Transport Safety Bureau has released its crash investigation findings.
The couple were flying from an airstrip near Townsville to Palmyra on the morning of October 28, a trip they had completed regularly.
During the flight, the pilot rang a friend and was told about the weather at Palmyra.
"The pilot replied that they would need to go through some cloud," the bureau's transport safety director Stuart Godley said.
Shortly after commencing the descent about 8.30am, the plane began a steep left turn towards mountainous terrain.
It exceeded its designed maximum airspeed before pitching up and passing over the top of Bull Mountain.
The aircraft then went into another steep descending turn.
It crashed into the north face of Bull Mountain, destroying the plane and killing the two occupants.
The licensed pilot used visual reference and was known to rely on auto pilot when flying through clouds, the bureau said.
But due to limited evidence the bureau could not determine why the autopilot had disconnected prior to the plane entering its high rate of descent.
"The weather on the day of the flight was conducive to poor or absent visual cues and turbulence, factors which are known to contribute to spatial disorientation," the bureau's crash report said.
"The pilot was not trained or experienced in flying in low visibility.
"In these conditions, the pilot would have been required to reference the aircraft’s flight instruments to maintain control."
The pilot descended through the cloud rather than fly over the top or divert around the weather.
"The pilot very likely entered weather conditions not suitable for visual navigation, leading to spatial disorientation and collision with terrain," the report found.
Autopilot should not be relied upon when deciding to fly into unsuitable conditions, the bureau said.
It cautioned against the use of unapproved mobile devices displaying charts and data as a sole means for navigation under visual flight rules.
The plane was fitted with an approved GPS but the pilot was known to rely on an iPad app for navigation.
