An investigation into a helicopter crash that killed two men near Queenstown last year says it was one of many incidents involving the type of Robinson helicopter they were flying.
The Transport Accident Investigation Commission has identified "mast bumping'', which caused the rotor blade to hit the cabin, as the cause of a crash that killed Stephen Anthony Nicholson Combe (42), of Wanaka, and James Louis Patterson Gardner (18), of Queenstown.
The men were killed when the helicopter in which they were travelling crashed in the Lochy River basin in the Eyre Mountains, southwest of Queenstown.
The Robinson R44, operated by Queenstown company Over the Top, crashed on February 19 last year.
The commission said there have been "many other fatal mast bump accidents involving Robinson helicopters in New Zealand and around the world that have gone largely unexplained''.
"It is difficult to identify the lessons from an accident and make meaningful recommendations to prevent similar accidents if the underlying causes cannot be determined.
"This is a serious safety issue that the industry, including pilots, operators, the manufacturer and the regulator, will need to address.''
Both Over the Top and the families of the men who died in the crash have laid the blame squarely on the helicopter.
A joint statement from the families of the men who died said they "firmly believe that had Steve and James been in any other aircraft type the accident would not have occurred''.
"Our families are traumatised and have paid the ultimate price. We do not want other families to suffer this too.''
Over the Top said the aircraft in the accident was a properly maintained R44 which had been used as part of its commercial fleet.
However, it said: "Over The Top has no intention to use Robinson aircraft again for any future operation.''
A report released today from the Transport Accident Investigation Commission said the helicopter broke up in mid-air when one of the main rotor blades struck the cabin.
That was caused by a phenomenon known as mast bumping, when the inner part of a main rotor blade or the rotor hub contacted the main rotor drive shaft.
The report said mast bumping was typically caused by one or a combination of factors. Those were: low main rotor revolutions per minute; the helicopter entering a low-gravity condition; turbulence, or the pilot making large and abrupt movements with the helicopter controls.
The commission could not conclusively determine what caused the mast bumping event.
"We found that it was unlikely to have been a low main rotor RPM event and could find no mechanical defect or failure that could have contributed to the accident.
"The student was about as likely as not to have been flying the helicopter when the accident occurred and the speed of the helicopter was about as likely as not to have been 102 knots or greater as it flew down the valley, returning to Queenstown.''
There could have been light to moderate turbulence in the area, but that should not on its own have caused mast bumping.
But when combined with a relatively high speed and a pilot's control response to any turbulence, there was a high risk of the helicopter entering a low-gravity condition, rolling rapidly to the right and suffering a mast bump event before the pilot could react.
The commission had made a recommendation to the secretary for transport and given notice to the director of the New Zealand Civil Aviation Authority to promote the need for cockpit video recorders and/or other forms of data capture in the cockpits of certain classes of helicopter to address the safety issue.
"The key lesson arising from this inquiry is that helicopter pilots must be fully aware that a condition of low-gravity can result in a rapid right roll, mast bumping and in-flight break-up before even the most experienced pilot can react and recover the situation.
"Pilots need to fly in a manner that avoids low-gravity conditions rather than allow them to develop and then expect that they can recover from them.''