A helicopter broke apart in mid-air near Wanaka after the rotors struck the tail during turbulence, killing the trainee pilot and instructor, a crash investigation has found.
The Wanaka Helicopters' Robinson R22 crashed on the return leg of a flight to Mt Aspiring National Park on April 27, 2011.
Queenstown trainee pilot Marcus Hoogvliet, 21, and instructor Graham Stott, 32, were killed in the crash.
A Transport Accident Investigation Commission (TAIC) report, released today, found the helicopter had been operating in a high-risk situation due to a combination of factors, including moderate to extreme turbulence.
It was also flying at an altitude of 5500 feet and was close to its maximum permissible weight.
Wind on the day was 40km/h, with gusts of up to 60 or 70km/h over the mountain passes, causing turbulence around mountain passes.
The helicopter had intended to return to Wanaka along the Matukituki Saddle, near Mount Aspiring. But an internal tracking device showed that instead of passing over the saddle it turned right over the nearby Waipara Saddle, into the Arawhata River valley.
The helicopter was reported overdue later that afternoon.
The wreckage of the helicopter and the pilots' bodies were found the next day in the Arawhata River valley. The wreckage revealed the helicopter had broken up in flight.
The TAIC found the break-up happened after the main rotor blades struck the tail boom.
Causes included severe or extreme turbulence buffeting the helicopter, the pilots making large and abrupt movements of the controls, and the main rotor speed being allowed to drop below its lower limit.
The TAIC identified a number of safety issues, including a lack of knowledge within the industry, which meant Mr Stott may not have been fully aware of the risks of flying an R22 near maximum weight, at high altitude and in moderate to severe turbulence.
The report found the format of the Robinson R22 flight manual did not draw enough attention to safety instructions and conditions that could result in serious injury or death.
It also noted the rate of R22 break-up accidents in New Zealand had not been significantly reduced by a local version of US Federal Aviation Administration measures intended to prevent such accidents.
The TAIC said aviation regulations must ensure operating parameters for aircraft were clearly and consistently articulated to pilots, regardless of the country in which an aircraft was operated.
It said pilots must be aware of, and stay within, the limitations of the aircraft they fly.
The TAIC recommended the Director of Civil Aviation address the pilot and instructor ratings on Robinson helicopters.
Mr Hoogvliet's father, Henk Hoogvliet, said the report confirmed the family's view that the helicopter had been overwhelmed by extreme wind and was unable to recover.
"This is a sad day for us all, almost three years after the accident. However, we are pleased the report is silent on any negligence on the boys' behalf. They acted professionally, all things considered.''
The family accepted accidents did occur, he said.
"Those who fly in and around the mountains appreciate how unpredictable conditions can sometimes be. It appears they just got caught out.''
Mr Hoogvliet said the family was deeply saddened but did not have regrets.
"We don't live in a world of `what ifs'. Marcus lived his life to the full. He spent the happiest last four months of his life learning to fly. He was a wonderful son to Anneka and I, and a loving brother to his three sisters. We could not have asked for more.''
The family's thoughts were with Wanaka Helicopters and Mr Stott's family.
They would never forget the support of the Queenstown community, their local church and those involved in the search and rescue (SAR) operation, including police.
"In recognition of the SAR team involved our family will make an undisclosed donation to the new SAR building, under construction in Wanaka, for capital costs and associated running costs.''