Robinson Helicopter Company's warning about the risks of carrying a passenger is "bizarre", an aviation safety expert told the Government.

The warning made by the helicopter company in a safety notice was possibly unique, John Fogden, director of the company Total Aviation Quality, told the Department of Conservation (DoC) in documents released to the Herald under the Official Information Act.

"I certainly haven't seen anything similar issued by any other helicopter (or fixed wing) manufacturer. To me it seems quite bizarre."

The safety notice warns that "carrying a passenger in and of itself increases risk because passengers add workload and distractions".

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Fogden's take on it was that if the pilot was distracted by having a passenger, he may inadvertently fly the aircraft outside of its design limits, thereby inducing mast bump.

"Robinson seem to assert this 'risk' is because a passenger is on board and not in any way associated with a design fault of the helicopter."

Robinson helicopters make up 35 per cent of the New Zealand fleet but 49 per cent of accidents, 64 per cent of fatal crashes and all seven fatal mast-bump accidents.

The company has blamed many accidents on poor pilot training, while critics have contended the helicopter has a design flaw.

DoC suspended use of Robinson helicopters in November 2016 because of safety concerns and has now made the move permanent following external and internal reports.

The estimated additional annual cost of using helicopters other than Robinsons is $350,000.

DoC is the biggest user of helicopters in New Zealand, spending $15 million a year hiring them. It primarily used Robinson machines for animal and weed control.

The suspension followed several fatal "mast bump" crashes involving Robinson helicopters and the decision by the Transport Accident Investigation Commission (TAIC) to put them on its "watchlist".

Mast-bumping occurs when the helicopter's main rotor blade "bumps" the drive shaft or mast and can result in catastrophic accidents.

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DoC produced an internal report and hired Fogden's company to provide an external review.

Fogden, whose 40 years in the helicopter industry includes stints as a Civil Aviation Authority safety adviser and head of the authority's rotary wing unit, said the critical difference between Robinson helicopters and others was the rotor-head design.

The Bell series made up the bulk of other two-bladed rotor system helicopters in use in New Zealand but was not identical. "The Bell design is also susceptible to mast bump, but it seems to a much lesser degree."

Fogden also noted that Robinsons appeared to have "a different philosophy for the issue of Safety Notices".

While safety notices issued by other manufacturers were mainly about what had been found wrong with their product or what was being done to improve their product, Robinson's were "almost exclusively about what pilots do wrong with their product, another example of a different philosophy ..."

Robinsons' director of flight safety, Bob Muse, told the Herald that all manufacturers spend a lot of time educating and providing tips on operational flight safety.

Muse said Robinson is unique in that it puts its Safety Notices in the operating handbook which is required to be kept in each helicopter.

"It may seem redundant or obvious to tell a pilot to fly within the design limits of the aircraft, but when you learn that some pilots routinely exceed the limits you are compelled to remind them why this is extremely dangerous in any aircraft."

DoC's decision to permanently ban the use of Robinson helicopters went further than the recommendation of its internal review which was to restrict their use to specific circumstances.

The department's director of safety, Harry Maher, said the Government department had decided to err on the side of caution.

Maher noted that TAIC remained concerned about the risk of mast-bump accidents involving Robinson helicopters. The risk seemed highest in mountainous terrain and weather. "These are exactly the conditions that our staff frequently face on the job."

Maher said that a common theme in advice received was that Robinson helicopters were heavily reliant on pilots flying them within strict operating limits.

"We aren't confident we can rely on this consistently over time across many varied situations in a large organisation like DoC.

A spokeswoman reportedly said that Robinson Helicopter Company was "surprised and disappointed" by DoC's decision.

The Californian company believes the accident rate had improved in New Zealand in recent years due to changes to pilot training.

During a visit in March, Muse told Radio New Zealand he was hoping TAIC would conclude the watchlist had served its purpose.

Five other Government agencies have suspended the use of Robinsons while it is on the watchlist.

Robinson is the biggest manufacturer of civilian helicopters in the world. Its three basic types, the two-seat R22, four-seat R44 and the bigger R66, share a rotor design unique to Robinson helicopters.