First published on Transport Safety Blog NZ
The safety record of Robinson helicopters in New Zealand is again in the spotlight following the release of a coroner’s report into a fatal crash of a R-44 helicopter near Queenstown on 19 February 2015. TAIC published their investigation report AO-2015-002 in August the following year. Following the receipt of new information the investigation was reopened and a new report published one year after the original, AO-2015-002R. This resulted in some new safety recommendations, but the finding of the original report as to the probable cause of the accident did not change. The conclusion is that the most likely cause of the accident was a mast bumping scenario where parts of the mast head (the assembly consisting of the drive shaft and blade attachment fittings, where the blades are attached to the shaft) contacted the mast itself. In this type of scenario, the mast head can snap off the shaft, which results in the helicopter unable to keep flying (equivalent to the wings falling off a regular plane) and so this is followed by immediate crash, which is generally fatal.
The Robinson helicopters have a unique mast head design which is more prone to this type of problem than most other types of helicopters. The blades are hinged to the top of the mast and are free to change their angle relative to the mast at any time in flight, this is called teetering. Both blades move together in doing this, a bit like a playground seesaw. In normal flight, the blades are at 90 degrees to the mast. In any helicopter, to change attitude of the helicopter chassis, the angle of the blades is changed, then the idea is that the helicopter body moves to the new attitude and puts the blades back to the 90 degree angle again. But because the blades can move very freely out of line in a Robinson, the risk is if the blades are moved too quickly or too far at a time out of this 90 degree angle, they can go into this “mast bumping” scenario and crash the helicopter. There have been quite a number of Robinson accidents where the rotor blades have actually diverged enough to chop off the helicopter’s tail.
By contrast the majority of other helicopters have much more rigid rotorheads that do not diverge all that much from 90 degrees and therefore they are much less likely to have this mast bumping situation happen. These choppers are also therefore much more able to handle undulating flying and therefore are able to have higher performance overall. The Robinsons see mast bumping happen a lot because a strong wind gust can push the helicopter body quite rapidly out of line with the blades in a very short space of time especially at high speed, or if the operator makes large abrupt control movements. Another factor with these helicopters is that everything is engineered to be lightweight. The design of the Robinson rotorhead is quite similar to some of the early Bell 200 series helicopters (the famous Hueys of the Vietnam War). The risks of the rotorhead design surfaced in large measure during that conflict when it was found the helicopters would crash more often when they were flown close to the ground following all of its contours in hilly areas, as would be the case in combat zones where aircraft are operated low to escape detection from radar etc. This really exposes the limitation of abrupt changes in attitude (such as up and down) that is inherent in this head design.
The safety challenges of Robinson operation are well known enough worldwide to have attracted regulators’ attention going way back and resulted in various requirements for more advanced training standards for people flying them. Unfortunately in NZ the CAA has been slow to apply all of these expectations across our helicopter industry that has undoubtedly led to a higher accident and death rate than should have been the case. So whilst in the rest of the world for the most part the Robinson accident rate has dropped back to more comparable levels as other types of helicopters, they have remained high in NZ, which is a combination of inadequate safety regulation and the greater prevalence of mountainous terrain here which means much more hazardous wind conditions around hills and gullies. It remains to be seen therefore whether this latest report really changes anything in the helicopter industry in this nation. The large attraction of Robinsons is that they are relatively cheap to buy, operate and maintain. However the unavoidable conclusion from our relatively high accident rates compared to other jurisdictions is that these lower costs attract operators who are more willing to cut corners on safety. When it comes to chartering helicopters for personal flight it is very much a case of “buyer beware” in that the public have to be confident enough to insist they will not fly in a Robinson if there are any possible risk factors that might be involved. Not all Robinson crashes are down to the mast bumping issue but it is the fact that the rate of incidents caused by this particular problem remains unreasonably high in New Zealand that needs to be recognised.
At the same time, the debate continues about whether the helicopters are inherently unsafe or whether the real issue is that they are often being operated in unsafe conditions by pilots. If the industry had widespread acceptance they were unsafe they would not be as ubiquitous as they currently are. The overall safety culture of our helicopter industry therefore has to come under increased scrutiny at some level. In the particular case known the helicopter was on a training flight and it was flying in a valley at the time, so the risk of wind turbulence was higher, which could have been avoided by keeping in flatter terrain. A lot of these helicopters get used by tourism operators including for hunting which means they are also used to lift the dead animals out of a kill zone, which means the risks of overloading can be experienced as well. Of course, people want to fly in the scenic hilly areas which also creates risks.
It’s somewhat unlikely that the Robinson helicopters will be forced out of NZ by safety edict but the questions over the tension between low cost and safe operation will remain as long as this particularly unique design of helicopter continues in production.