Mast bumping - likely brought on by turbulence and flight speed - is being blamed for a helicopter crash that killed its pilot.
Police-led search specialists on Lake Wanaka during the operation to recover the body of crashed pilot Matt Wallis in 2008. Photo: Supplied / NZ Police
The Transport Accident Investigation Commission released its final report into the crash over that killed pilot Matthew Wallis in 2018, in Lake Wanaka.
He was flying a Robinson R-44 - a helicopter that continues to be on the commission's 'watch-list' due to multiple deaths and ongoing risks.
Wallis was an experienced pilot - the son of the aviator and founder of Alpine Helicopters, Sir Tim Wallis.
The 39-year-old had flown the Wanaka to Upper Estuary Burn Valley route hundreds of times before but his flight that afternoon, on 21 July 2018, would be his last.
Wallis' Robinson R-44 helicopter broke apart mid-flight and hurtled down into Lake Wanaka.
Matthew Wallis was flying a Robinson R-44 helicopter when the accident happened. Photo: Supplied
The Transport Accident Investigation Commission has found that turbulence and flight speed likely contributed to what is called 'mast-bumping'.
Nineteen others have died in the past 30 years from Robinson mast-bumping accidents.
Mast bumping occurs when a two-bladed teetering rotor head see-saws so much that it breaks its own drive shaft.
Chief Commissioner Jane Meares said in this case, the blade sliced through the cabin.
"Examination of the accident site and the wreckage showed that the helicopter very likely broke up in flight," she said.
The spread of pieces of perspex on the water and on land indicated the canopy was damaged in flight, Meares added.
Turbulence is common in New Zealand and has been a factor in five out of six Robinson helicopter crashes involving mast-bumping, in the 10 years prior to this accident.
It likely contributed to this crash, Meares said.
"Evidence shows that local pilots knew strong localised turbulence was likely over Stevenson's Arm, in north-westerly conditions, when the wind funnels through the saddle at the top of the arm and swirls around in the lea of the hills."
There was a north-westerly wind and a front moving through region at the time of the accident, making it more likely there was turbulence.
Speed could have also been a factor.
"The helicopter was relatively light and weather conditions were less than ideal in a mountainous area known for localised turbulence, so flying close to the maximum permitted speed increased the risk of an adverse outcome."
Robinson Helicopters urges pilots to reduce their speed in 'significant turbulence' but up until this year, the definition of 'significant' wasn't clear in the pilot handbook.
Investigating the causes of Robinson crashes can be difficult due to a lack of information, Meares said.
"The lack of reliable data on the initiating cause, or causes of mast-bumping occurrences, continues to limit the effectiveness of safety investigations."
The commission did not make any new recommendations, instead reiterating ones it made to Robinson following previous crashes - to upgrade its data gathering tools and study why mast-bumping in turbulence happens.
Both of these recommendations are being implemented, she said.
There is no call to ban the helicopter, despite its record for flight-related deaths in New Zealand.
New Zealand's low-G mast-bumping accident rate is nine times higher than in the US.
Instead, Meares urged pilots to be cautious when operating these helicopters.
"There is no reason why these helicopters shouldn't be safe if they are flown in appropriate conditions and appropriately maintained."
The Wallis family faced another tragedy just three months after Matthew's death, when his brother Nick and two others died in a second Robinson helicopter crash.
Interim reports show it is likely a door opened during flight, causing a pair of over-trousers to blow out and become entangled in the rear blades.
That accident is still under investigation.
In a statement, the Wallis family said they are relieved this report is complete.
Family spokesperson Jonathan Wallis - brother to Matthew and Nick - said the family accepts the findings that a lack of reliable evidence on why the mast-bumping happens continues to hinder safety investigations.
"Our focus remains on moving forward as a family and in particular supporting Matthew's young son Arthur," Jonathan Wallis said.
Robinson Helicopters has been contacted for comment.