The pilot of a seaplane that crashed into an Australian river, killing all on board, had been left confused and disorientated by leaking exhaust fumes, investigators have confirmed.
The Canadian pilot and five members of a British family died in the crash north of Sydney in December 2017.
All were found to have higher than normal levels of carbon monoxide in their blood, a final report has found.
It recommended the mandatory fitting of gas detectors in all such planes.
British businessman Richard Cousins, 58, died alongside his 48-year-old fiancée, magazine editor Emma Bowden, her 11-year-old daughter Heather and his sons, Edward, 23, and William, 25, and pilot Gareth Morgan, 44. Mr Cousins was the chief executive of catering giant Compass.
The family had been on a sightseeing flight in the de Havilland DHC-2 Beaver plane when it nose-dived into the Hawkesbury River at Jerusalem Bay, about 50km (30 miles) from the city centre.
The final report by the Australian Transport Safety Bureau (ATSB) confirmed the findings of an interim report published in 2020.
It said pre-existing cracks in the exhaust collector ring were believed to have released exhaust gas into the engine bay. Holes left by missing bolts in a firewall then allowed the fumes to enter the cabin.
“As a result, the pilot would have almost certainly experienced effects such as confusion, visual disturbance and disorientation,” the report said.
“Consequently, it was likely that this significantly degraded the pilot’s ability to safely operate the aircraft.”
The ATSB recommended the Civil Aviation Safety Authority consider mandating the fitting of carbon monoxide detectors in piston-engine aircraft that carry passengers.
It previously issued safety advisory notices to owners and operators of such aircraft that they install detectors “with an active warning” to pilots”.
Operators and maintainers of planes were also advised to carry out detailed inspections of exhaust systems and firewalls.