A door flying open allowing a pair of over-trousers to become caught in the tail rotor is the suspected cause of the triple fatal helicopter crash near Wanaka Airport, investigators say.
The Transport Accident Investigation Commission (TAIC) has revealed more evidence into the two separate helicopter crashes that killed brothers Matt and Nick Wallis.
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Matt Wallis was killed after the Robinson R44 helicopter he was flying crashed into Lake Wanaka in July.
Almost three months later, his brother Nick died when the Hughes 500 he was piloting crashed shortly after take-off from the Wanaka Airport.
Department of Conservation senior rangers Paul Hondelink and Scott Theobald were also killed in the crash.
The helicopter was just two minutes into its flight when it crashed. Evidence suggests the left rear door opened in-flight allowing the over trousers to exit the helicopter.
"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," TAIC Chief Commissioner Jane Meares said.
"Paint marks on the over-trousers matched the colour and profile of the tail rotor blades, and marks on a tail rotor blade match a zip and dome connector on the over-trousers."
Evidence has also revealed doors on this helicopter had flown open three times the month prior to the crash. None of these incidents were reported to the Civil Aviation Authority (CAA).
Ms Meares said it is too early to state if the door opening contributed to the cause of the crash, however she said the previous incidents should have been reported.
TAIC supports the CAA's warning to pilots to stow away any loose items inside the cabin before take-off.
Interim evidence from the helicopter wreckage of the crash that claimed Matt Wallis' life shows evidence of mast bumping.
Mast bumping is contact between an inner part of a main rotor mechanism and the main rotor drive shaft. It usually results in the helicopter breaking up in flight.
"Although we have not come to a conclusion about what caused this accident, evidence of mast bumping is always a concern for the commission," Ms Meares says.
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"There is also evidence that a main rotor blade has struck and entered the cabin in flight. There are score marks on the blade that match screws on the canopy bow; there is scoured paint on the screws. The same rotor blade has marks matching damage to the flight instruments panel."
No formal findings have been conclude as TAIC continues to investigate the circumstances surrounding both crashes.
The Alpine Group and Wallis family have reviewed the reports.
In a statement, Jonathan Wallis, brother of Nick and Matt, said some of the findings into Matt's crash were 'disturbing' and the findings into the crash that killed Nick, Mr Hondelink and Mr Theobald were of equal concern.
He said the family respect the release of the reports so early into TAIC's investigation.
The family also support the safety message about stowing away loose items.
They are aware of the incidents where cabin doors have opened in-flight and are fully co-operating with TAIC and CAA in their investigations.
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