Interim reports into separate helicopter crashes, which killed two brothers within months of each other, shows evidence of mast bumping and highlight the major risk posed by loose items in cabins.

Nick Wallis was one of three people killed in a helicopter crash near Wanaka Airport on October 18 this year.

The incident came less than three months after his brother and fellow pilot Matthew Timothy Wallis was killed in a helicopter crash on July 21 this year.

The Transport Accident Investigation Commission has released interim reports on separate ongoing inquiries into the crashes.

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The Wallis family and its helicopter company The Alpine Group said while the reports weren't final, the characteristics identified of an inflight breakup in the crash that killed Matthew Wallis was "disturbing".

The statement said the unexplained door separation identified in the crash that killed Nick Wallis was "equally of concern."

Since that accident in October we have been made aware of numerous incidents of doors opening in-flight on the same aircraft make and model," the statement said.

The TAIC report into the July crash showed the teeter stops for both main rotor blades were crushed, indicating the Robinson R44 helicopter experienced mast bumping at some point during the accident.

The Commission has previously raised concerns about the number of accidents in New Zealand in which Robinson helicopters have experienced 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, which is fatal for those on board.
The evidence of mast bumping and the role it may have played in the accident is subject to further investigation, the report said.

"Although we have not come to a conclusion about what caused this accident, evidence of mast bumping is always a concern for the Commission.

"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", Chief Commissioner Jane Meares said.

A report into the October 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.

The Civil Aviation Authority issued a safety message in November reminding aviation participants of the risk of loose items in the cabins of helicopter.

Evidence has also revealed there were three incidents in the month prior to the accident in which doors on the same Hughes 500 helicopter opened in flight.

The Commission has recommended urgent action by the Director of the CAA to remind aviation participants of the importance of reporting incidents in accordance with Civil Aviation Rules.

The Commission is yet to complete its inquiry into the accidents, and the information contained in its final reports may differ from the information in the interim reports.