A pilot flying low over the ocean in foggy conditions crashed his plane into the sea because he did not know which way was up, killing all four people on board, a report has found.
Pilot Geoff Milne, his wife Mathilde, their son and his girlfriend died when their twin-engine Cessna 337 Skymaster crashed into the sea at Cape Liptrap, 160km south-east of Melbourne, on November 17, 2007.
As well as the weather conditions, the Australian Transport Safety Bureau (ATSB) report, released on Tuesday, said Mr Milne's lack of experience in a Cessna 337 Skymaster contributed to the crash.
The family had left Melbourne's Moorabbin Airport at lunchtime and planned to fly to Merimbula in NSW to stay with friends for the night.
When they failed to arrive, the alarm was raised and a massive three-day air, land and sea search was conducted covering 5,000 square nautical miles and involving 23 aircraft.
Three bodies were recovered, but Mr Milne's was never found.
The ATSB report quoted witnesses as saying the plane was flying "lower and louder" than most aircraft flew over the area.
In the last sighting of the aircraft, it was flying close to the water line before making a sharp right turn and disappearing from sight into the fog.
"Wreckage from the aircraft was recovered on the nearby beach and ocean, so it is likely that the aircraft impacted water soon after it disappeared from sight," investigators found.
"By turning away from the land in foggy conditions, the pilot would have encountered a featureless, grey environment with no visible horizon, making it extremely difficult for him to judge the aircraft's altitude and/or height."
The pilot would then "easily" become spatially disorientated - not knowing which way was up - and been unaware of the aircraft's descent and impending collision with water until it was too late.
Although Mr Milne had been issued with a pilot's licence in 1963, there was no record showing he had any instrument flying training and he had minimal experience flying Cessna C337s.
An instructor who flew with the pilot two months before the crash said he was "rusty" but "quite competent" and "very confident".
He was only qualified to fly in good weather conditions and he had failed to obtain weather forecasts for the flight plan before taking off.
Bent, twisted and torn wreckage was found on beaches and 200 metres out to sea, with no evidence of any pre-existing defects.
Major components such as the engines and propellers were never found.