A tragic helicopter crash over Lake Wanaka that killed pilot Matt Wallis - three months before his brother Nick Wallis also died in a chopper crash - was caused by mast bumping.
The Transport Accident Investigation Commissions (TAIC) today released its findings into the July 2018 crash.
It found mast bumping caused an in-flight breakup of the Robinson R44 helicopter as it cruised over mountainous terrain in conditions and at a speed that put the craft at risk of an "adverse outcome" from strong turbulence.
Mast bumping is contact between an inner part of a main rotor mechanism and the main rotor drive shaft.
It can happen in a split second, usually resulting in the helicopter breaking up in flight, and is fatal for those on board.
"In this case, mast bumping caused a main rotor blade to bend down so far that it struck and entered the helicopter cabin, the helicopter broke up in flight, descended, impacted the lake, and sank," said TAIC chief commissioner Jane Meares.
It was likely strong unexpected turbulence caused the main rotor disk to teeter excessively and the mast to bump, she said.
"Operators, owners and pilots of helicopters with this type of main rotor need to know that the risk of mast bumping increases with the likelihood of turbulence, mountainous operating environments, high power settings, higher speed, and light weight."
Wallis, 39, was the sole occupant of the craft when it crashed. His body was found in lake Wanaka three days after the accident.
He had gone on what was only supposed to be a short, 15-minute flight, but was reported missing by his family's company, Alpine Helicopters, when he did not return as expected.
Wallis' brother and two other people were killed in another helicopter crash near Wanaka Airport in October 2018.
The final report on that accident is still some time off being published, but an interim report published earlier on the crash highlighted a major safety issue regarding the risk posed by loose items in the cabins of helicopters.
"There is evidence that a pair of over-trousers that had been packed in the cabin came out of the helicopter and became entangled in the tail rotor," Meares said at the time the interim report was published.
TAIC has previously raised concerns about the number of accidents in New Zealand in which Robinson helicopters have experienced mast bumping.
It did not make any new recommendations in today's report, having previously made three recommendations addressing safety issues.
Robinson Helicopter Company has updated its R44 Pilot Operating Handbook to define the non-standard term "significant turbulence" after TAIC noted the potential for misinterpretation.
Investigations into loss-of-control or mast bumping accidents in Robinson helicopters continued to be hampered by a lack of data, the report said.
There was also a lack of understanding of how the main rotor performs in adverse conditions.
The key lesson from the investigation was that pilots needed to exercise caution when planning and conducting flights into areas of potential turbulence, and that they should seek to avoid these situations.
"Should turbulence of any strength be encountered, pilots need to take immediate action to minimise its effects," the report said.
They should also be familiar with Safety Notice 32 and the associated video, and avoid flying in high winds and turbulence.