Investigators clear crashed Robinson helicopter of 'mast bump' fault

Investigators have combed through wreckage from the helicopter which plunged into Glenbervie Forest. Photo: Northland Emergency Services Trust.

A helicopter crash which killed two Northland forestry workers doesn't appear to have been caused by an issue that's prompted red flags about Robinson choppers, crash investigators say.

Allan Jessop, 42, and Derek Hammond, 49, died when the Robinson R44 helicopter they were in crashed and caught fire in the Glenbervie Forest, north of Whangarei on October 31.

An interim report released by the Transport Accident Investigation Commission on Thursday says it's "very unlikely" the crash was caused by "mast bumping".

Mast bumping is where the inner part of the main rotor blade hits the drive shaft in low-gravity flight.

Just days before the crash, the commission, which can make safety recommendations to the transport sector, added the issue on Robinson helicopters to its watchlist of most pressing concerns.

It said since 1996, aviation officials have investigated 14 "mast bumping" crashes involving Robinson helicopters, costing the lives of 18 people.

But the investigation of October's crash has found it's unlikely the helicopter had broken up mid-air, as occurs with mast bumping, and there were no signs of the tail boom being struck by the main blade.

It appeared the chopper had struck the ground at a "high rate of descent and a low forward speed" as the men were surveying an area of the forest ahead of spraying, it said.

An intense fire followed, destroying the cabin and fuselage.

The Commission said it would now be looking into the condition of the engine before the crash, weather conditions, maintenance history and procedures around aerial spraying operations.

There are about 300 Robinson R22, R44 and R66 helicopters flying in New Zealand, about 40 per cent of the country's fleet.

Earlier this year, the commission flagged mast bumping in its report into the deaths of flight student James Patterson Gardner, 18, and instructor Steven Combe, 42, near Queenstown last year.

The victims' families blamed the helicopter design but Robinson said it was caused by pilot error.

The October crash led to the Department of Conservation suspending all use of Robinson model helicopters.

- AAP