Is there a problem with Robinson helicopters?
Two people are dead after a Robinson helicopter crashed in a Northland forest.
It is too soon to say what caused the Northland crash. But the involvement of the Robinson, which is on a watch-list of serious transport concerns, raises questions.
Despite reviews, investigations, and changes to training recommendations, Robinson helicopters continue to crash at unusual rates in New Zealand. So what's going on?
* Two dead in helicopter crash south of Hikurangi, near Whangarei
* Owner's son killed in Queenstown helicopter crash
* Railway crossings and Robinson helicopters added to watchlist
* Investigation finds no clear reason for helicopter crash
* Robinson helicopters back in the air
The helicopters are crashing from "mast bumping" incidents and investigators are not entirely sure why. Mast bumping usually happens in turbulence, sudden movement, a slowing rotor, or a "low-gravity situation".
The rate of "low-G" accidents involving Robinsons is significantly higher in New Zealand than in other parts of the world - about nine times higher than the US, according to the Transport Accident Investigation Commission (TAIC).
Fourteen such incidents have occurred since 1991, causing 18 deaths. If the Civil Aviation Authority (CAA) finds that mast bumping caused today's Northland crash, the death toll would rise to 20.
The rate of accidents prompted the TAIC to place Robinson helicopters on a watch-list, but there are no industry calls to ground all Robinson helicopters.
There are around 300 Robinsons registered in New Zealand - mostly R22 and R44 models - making up 40 per cent of New Zealand's helicopter fleet. The three main Robinson helicopter models all use the same rotor system.
What is mast bumping and low-G?
Low-G is the feeling of no gravity or weightlessness you feel when you go over a bump on a country road, or when turbulence makes an airplane drop slightly and you feel that rush in your stomach.
In airplanes this is okay. In helicopters it can be fatal because it causes the rotors (blades on top of the helicopter) to bend or move beyond normal limits.
This is when mast bumping can occur. The blades separate from the attachment to the hub and hit the main body of the helicopter, usually cutting through it.
What's being done?
In February last year, Stephen Anthony Nicholson Combe, 42, and James Louis Patterson Gardner, 18, were killed when the Robinson they were flying crashed near Queenstown.
After that crash, the TAIC released a report which said the aircraft's main rotor blade system could fail in high winds or turbulence. The CAA subsequently warned pilots of Robinsons to avoid turbulence.
Last week, the TAIC released a report on mast bumping accidents from Robinson helicopters in New Zealand. It said part of the problem was there wasn't enough evidence from crashes to determine why mast bumping occurred.
But it also said many of the accidents have occurred in "low-G" flight conditions, usually caused by turbulence making the pilot and aircraft feel temporarily weightless.
The TAIC report said pilots must undergo high quality training, and have knowledge of the helicopters' limitations and risks. It also recommended cockpit video recorders - similar to a blackbox - become a requirement in order to gain better knowledge of why crashes occur.
Spokesman Peter Northcote said: "The commission will not speculate or comment on any potential relationship between this afternoon's accident and its watchlist item concerning Robinson helicopters".
But they would monitor the Civil Aviation Authority's investigation into the Northland crash, "to see whether the circumstances may warrant a separate inquiry by the commission".
Is pilot training to blame or something else?
US lawyer Ladd Sanger has represented many victims' families connected to Robinson helicopter accidents around the world. He told RNZ earlier this year there was a systemic problem and pilot training was not the answer.
"Either bad pilots are selecting Robinson helicopters or there is something broader going on to have the Robinson accident statistics be so much worse than competing helicopters."
Should they all be grounded?
After Combe and Gardner died last year, their families called for Robinson helicopters with the same rotor design to be banned.
In a joint response, their families said other Robinson accidents were blamed on turbulence or pilot inexperience.
"In our case, the instructor was experienced and from the tracking data, the aircraft was being flown in the cruise, in calm weather and at a safe altitude."
"We firmly believe that had Steve and James been in any other aircraft type the accident would not have occurred."
New Zealand Helicopter Association executive officer John Sinclair wouldn't answer questions about the accidents but said the association followed the CAA recommendations on pilot training.
Civil Aviation Authority (CAA) 2015 report
The CAA conducted a review recommended by the TAIC between March 2014 and March 2015.
It found that although the CAA had updated the training requirements for new pilots wanting to fly Robinsons, the CAA had no way to regulate this training.
There was also no way to ensure those providing safety training were suitable to conduct that training.
The CAA made a number of new conditions including:
* Increasing the number of hours in training a pilot needed to fly solo in a R22 from 10 hours to 20 hours.
* Training must be done with organisations that have an approved Robinson safety course delivered by approved and qualified instructors.
* Those instructors must be approved by a general aviation examiner with Robinson safety awareness privileges.
* Ongoing training will be required every 24 months.
Fatal mast bumping accidents since 2010
* October 2010: Robinson R22, Bluff Harbour, 2 killed
* April 2011: Robinson R22, near Mt Aspiring, 2 killed
* November 2012: Robinson R22, Cardrona Valley, 1 killed
* March 2013: Robinson R66 Kaweka Range, 1 killed
* October 2014: Robinson R44, Kahurangi National Park, 1 killed
* February 2015: Robinson R44, Lochy River, 2 killed.