Tom McCready, an engineer and veteran accident investigator, explains.
More than 20 years after United States safety authorities began a special safety investigation into Robinsons, debate is raging following a spate of crashes in New Zealand. Phil Taylor reports.
Farmers in the valley reported no discernible wind. Without wind, you don't get turbulence and turbulence is often a factor in helicopter mast-bump crashes where a main rotor blade can slice through the cabin or tail boom.
Yet, there on a mountainside near Queenstown, strewn among the trees lay the wreckage. The helicopter had broken up in flight. Two lives were lost.
It was a Robinson.
The company, founded by Frank Robinson, made its first helicopter in 1979 and 12,000 more have since rolled off its production lines. Robinson is acclaimed, the recipient of the Daniel Guggenheim Medal for his "conception, design and manufacture of a family of quiet, affordable, reliable and versatile helicopters".
Yet, according to crash data, people are dying in these helicopters at an inordinate rate.
The families of those who died on the mountainside say Robinson helicopters have pushed too far towards lightness and affordability for New Zealand conditions.
It is a vexing issue. Whether the high accident rate is due to operator error, or because the unique Robinson rotor design makes them more prone to catastrophe when things go wrong, or both, is a question that carries obvious commercial implications and an emotional load for those who are grieving loved ones.
But officials expected to be dispassionate also hold concerns. In a decision heard around the world of aviation, New Zealand's Transport Accident Investigation Commission (TAIC) last October put Robinson helicopters on its watch list: the highest alert it can give.
The commission's role is to inquire into a marine, rail or air incident when it believes safety lessons can be learned. But it has no power to compel entities to apply its recommendations, and there is room for doubt that our other aviation safety agency, the rule-making Civil Aviation Authority (CAA), sees eye-to-eye with the accident investigation commission.
Citing 14 mast-bumping accidents costing 18 lives since 1991, the commission called for renewed testing of Robinson helicopters (among other recommendations aimed at promoting safe handling of the machines).
The commission's action was described as an "outburst" in an email exchange between a staff member of Robinson Helicopter Company and, surprisingly, a CAA official. After Newshub published the emails, the CAA said the comment was "banter".
The Department of Conservation (DoC) has stopped its staff flying in the helicopters (pending receipt of a report it has commissioned), so have Tourism New Zealand and TVNZ.
Mast bumping is contact between the inner part of a main rotor blade or the rotor hub and the drive shaft or "mast". Helicopters are not yet required to have recording devices and there is rarely eyewitness testimony or other direct evidence about what led to a mast bump.
Statements in investigation reports about probable cause are essentially the same: "The divergence of the main rotor from its normal plane of rotation for an undetermined reason." Many mast-bump accidents are known to have occurred in a low-G situation (a feeling of lightness or weightlessness, like when a car speeds over a hump). That can result from turbulence or large or abrupt flight-control movements.
A Herald review of a decade of helicopter accident data released by the CAA under the Official Information Act shows Robinson helicopters are involved in a disproportionate number of crashes.
Wreckage of a Robinson R44 helicopter at Lochy Creek near Queenstown. Photo/Supplied
Robinson helicopters and New Zealand crash data
Robinson helicopters make up 35 per cent of the New Zealand fleet but have been involved in 49 per cent of accidents in the 10 years to last November, including 64 per cent of fatal accidents. Notably, all seven fatal mast-bump accidents were Robinson aircraft.
35 per cent of fleet 49 per cent of accidents 64 per cent of fatal accidents 100 per cent of fatal mast-bump accidents.
California-based Robinson Helicopter Company makes lower-cost, lightweight helicopters. The smallest, the two-seater R22, makes up 14 per cent of the fleet and was involved in 28 per cent of accidents and 36 per cent of fatal accidents.
This model is the popular choice for training (with more than half of all training hours) and is flown by a higher proportion of low-hour pilots. It is, depending on the point of view, nimble or sensitive.
The four-seater R44, the most popular model in New Zealand, makes up 20 per cent of the fleet, a similar proportion of accidents, and 23 per cent of fatal accidents.
There has been one fatal accident involving the biggest of the Robinson stable, the five-seater R66. There were six of these craft in the country as at the end of last year.
All three models share the unique Robinson rotor head design and have all been involved in catastrophic mast-bump accidents in New Zealand and internationally.
The potential for mast bumping is inherent in all semi-rigid two-bladed main rotor systems that teeter around a central rotor mast, as Elan Head, special projects editor for the US magazine, Vertical, explained in an article about Robinson helicoters last year.
With 295 helicopters, Robinsons make up three-quarters of helicopters with teetering systems registered in New Zealand. The others are the Bell 206 (79), Bell 47 (seven), and UH-1 Iroquois (10). But only Robinsons have been involved in mast-bump accidents here in the past decade and CAA confirmed only Robinsons were involved in fatal mast-bump crashes in the past 25 years.
The Robinson rotor head differs from other teetering systems in that it has three pivot points rather than one. The rotor system is lighter and more responsive to control movements but also to turbulence. A mast bump is usually catastrophic.
Louisa Patterson and her son James Patterson-Gardner. James died when a Robinson helicopter broke up inflight after a mast-bumping event near Queenstown in 2015. Photo/Supplied
The Lochy River crash
Those who died that summer's day two years ago near Lochy River, a short hop from Queenstown, were Stephen Combe, 42, and student James Patterson-Gardner,18.
Combe had 4500 flight hours in helicopters and was regarded by former students interviewed by investigators as "a very thorough and professional instructor and pilot". He flew Gazelle helicopters with the British Royal Marines in the war in Iraq and was awarded a "Best Overall Pilot Award".
Patterson-Gardner was a trainee with a pedigree. His forbears include a World War II Spitfire pilot and a great uncle who was a pioneer with NAC (now Air New Zealand). His father, Murray "Mo" Gardner, was an Olympic skier and also an aviator.
His mother, Louisa "Choppy" Patterson, is renowned in the helicopter industry. She is one of five pilots in New Zealand to have gained a platinum safety award for 25 consecutive years without a serious accident. Her company, Over The Top, had a similar safety record.
Among framed certificates on the walls of its Queenstown office is a Gold Safety Award recognising 19 years of continuous service by the company without an accident. The award covered the period ending 2013. On another wall is the framed photograph reproduced above. In February 2015, 13 months after the period covered by the safety award, her senior pilot and her only child died in a Robinson R44 operated by her company.
The accident report concluded that the helicopter broke apart in mid-air when a main rotor blade struck the cabin after a mast bump. It was flying across mountainous terrain at relatively high speed, estimated at 102 knots.
Patterson and staff flew into the Lochy River area about 20 minutes after the R44 broke up. They searched without thinking about which way to hover. You hover into wind but there was no wind, she told the Herald. "Without wind, you don't get turbulence."
Patterson suspects something happened to the aircraft to cause it to instantaneously roll to the right and mast bump - a pitch link failure, a blade crack?
An early suspicion it may have been the result of main rotor-blade fatigue was not borne out by the investigation - a metallurgist concluded that blade fracturing was a result of the accident rather than a cause.
The head of Robinson Helicopter Company, Kurt Robinson, visited Patterson last year. He believes it was the result of pilot error.
"I do believe it was a very tragic training accident and that the instructor did not get back on the controls soon enough," Robinson told the Herald by phone from Torrance, California.
"They were flying through an area [where there was] turbulence, a student pilot on his second training flight, and something happened. It was tragic but there was nothing wrong with the aircraft."
The finding of the investigation, however, was that it was "as likely as not that the aircraft had hit a pocket of light to moderate turbulence". The report was equally inconclusive about who was on the controls.
An abrupt control movement (particularly in turbulence) can result in a sudden roll to the right and a low-G situation from which recovery can be tricky and can result in mast bump.
Patterson, whose company had operated a Robinson helicopter without incident for some years before the accident, says her son was "a measured boy" unlikely to make rash control movements.
In her opinion, the aircraft is not fit for purpose and should be redesigned. How come, she asks, other types are not as problematic?
Mast-bumping and Robinsons' unique rotor head
Air accident investigator and engineer Tom McCready, with his diagram of Bell and Robinson helicopter rotor hubs, in Palmerston North. 06 March 2017. New Zealand Herald photograph by Mark
"The Robinson rotor head will mast bump sooner than other types and the reaction from mast bump will be much quicker and more severe," Tom McCready, an engineer and veteran accident investigator, told the Herald.
In other helicopters, you may walk away with a fright and a dented mast, says the former CAA investigator. Mast-bump accidents in Robinsons in New Zealand have been catastrophic.
Robinson Helicopter Company does not believe there is anything wrong with the design. Its view is these accidents can be avoided by pilots following the flight safety guidelines for these aircraft, and that has also been the tenor of CAA's safety training recommendations.
The helicopter company has responded to TAIC putting Robinsons on its watchlist by posting safety alerts stressing what pilots must do to avoid low-G situations that could lead to mast-bumping, such as slowing down to 70 knots when distracted and in turbulence and to respond to buffeting with gentle control inputs.
The commission, noting that the three Robinson models share the same rotor system, recommended that United States safety authority, the FAA, "reinstate research into the dynamic behaviour of the Robinson rotor system under conditions of low-G".
McCready has attended about 30 fatal accidents, done the Robinson factory course for maintenance and made a study of the company's helicopters.
"I've worked on teetering systems since 1978. Of the helicopters with teetering systems you will be in trouble in a Robinson way before the others and you will have a nastier reaction."
He is concerned that the Robinson rotor head has been "normalised" and many pilots are unaware that it is "unusual". If it was up to him, he would redesign the head, he says.
McCready points out that he has concentrated on what happens when a mast-bump occurs rather than the conditions or type of flying that may lead to it.
A mast-bump could occur so fast, McCready says, an instructor may not be able to recover the situation when a trainee is flying.. "When it turns to shit, it turns to shit very quickly."
Robinson purposely built a helicopter for the masses and its machines have a place, says McCready. It may be that the helicopter is sometimes pushed beyond what it is designed for.
The owner of a flight instruction company that uses Robinsons, but who did not want his name published, notes they were designed as an executive commuter. Kiwis were the first to put a hook on a Robinson. They are used for mustering and deer recovery. "Kiwis being Kiwis have taken it to a whole different level. And in the right hands they are capable of doing that, but you just have to know where those limits are."
Back to McCready. "It is not a big, tough helicopter. If you relate it to motorbikes, which are also about balance and power and handling, it's a scooter. I ride a good 1200cc motorbike. They handle well, have got good brakes, the power to get you out of trouble, got all sorts of things going for them. But a scooter doesn't.
"You're on a scooter and you get between two logging trucks. Now relate the trucks to mountains where it is windy. A lot of accidents that have happened is not the fault of Robinsons but because they are Robinsons."
He says he believes there has been an over-reaction to the helicopters being put on the watchlist. "They [TAIC] are giving you a heightened awareness."
Kurt Robinson, who replaced his father, Frank, as president and chairman of the company in 2010, says placing the helicopters on the watchlist was unfair because it didn't allow time for changes by the CAA to the training required for pilots of Robinson helicopters to take effect.
Trainee pilots now have to do a Robinson safety refresher every three months, qualified pilots every two years.
"I'm hopeful now that will make a pretty dramatic difference," Robinson says.
"We have these helicopters flying all over the world and we did see a higher incidence in NZ. I think a lot of it is the particular manner in which the pilots fly." Kiwis may be more inclined, he says, to do a cyclic "pushover" that can risk low-G, a manoeuvre banned in the United States because it can induce mast bumping.
A Robinson aircraft is like a high-performance sports car, he says. "It needs to be handled properly. If done so, in the right hands, the aircraft performs beautifully. But if the proper training isn't done or if people handle the aircraft incorrectly then, yeah, you can get into trouble real fast."
Mast-bumping isn't the cause but part of the accident sequence, he says. "You've gone too far."
A Robinson helicopter flies over Queenstown. Photo/Ross Setford
Call to renew testing
Robinson says he doesn't know how McCready reached his view about the rotor head design risk. The Robinson company had looked at it "a lot and we can't find anything to it." And an FAA rotor craft panel had looked at the Robinson tri-hinge design in the 1990s and "explicitly found that it makes no difference to the susceptibility to low-G mast bumping".
However, research into the dynamic behaviour of lightweight helicopter main rotor systems was not completed because it was not thought safe to conduct flight tests of the response to abrupt control inputs.
Georgia Tech School of Aerospace Engineering subsequently worked on a computer simulation model but ran out of funds and the work was dropped in 1998 after the FAA decided it would have "limited application" and that "subsequent validation . . . would involve extensive testing with significant risk to flight safety".
In calling for unfinished testing to be reinstated, New Zealand's accident commission said it "is likely that mast-bump accidents with Robinson helicopters will continue to happen unless the dynamic behaviour of the main rotor preceding such a catastrophe is fully understood."
The aviation lawyer
It was suggested to the Herald that the litigious system in the US would have seen Robinson Helicopter Co sued out of existence had there been a proven flaw in the design of its aircraft, but also, conversely, that the fear of retrospective lawsuits might work against abandoning a major aspect of its design, such as the rotor head.
Ilyas Akbari is a partner specialising in transportaccidents for Baum, Hedlund, Aristei & Goldman, a Los Angeles law firm that has acted in numerous actions arising from helicopter accidents.
All of the firm's cases involving blade strikes were Robinson helicopters, Akbari told the Herald.
"Robinson Helicopters says it is operational error, you shouldn't be flying in high wind areas. But in my opinion, a lot of it also has to do with the design. If you can make the blades more robust, in addition to stiffening the vertical structure around the main rotor mast, I think it would go a long way to stabilising this helicopter."
The blades had less mass and were more flexible than comparable helicopters. "When you have less mass you have less ability to recover," Akbari says.
"I don't know how much consideration Robinsons has given that. Their position is they have 30-million flight hours and comparatively there are few accidents, when in reality per thousand hours of flight time they have by far the highest number of deaths or crashes compared to other manufacturers."
According to data collected by the independent Aviation Safety Network, the four-seater R44, for example, has been in 95 accidents internationally since January 2015, resulting in 58 fatalities. Twenty per cent of those accidents, (making up three-quarters of the fatalities), were recorded as arising from unknown circumstances.
Akbari: "One of the things that puzzles me is that there are very experienced pilots dying. It is one thing to blame the pilot but at the same time a lot of experienced pilots are dying in these helicopters."
The experienced Robinson pilot
One of the most experienced Robinson pilots in the world is Simon Spencer-Bower, owner of Wanaka Helicopters. He is in his 50th year flying without mishap having amassed 22,000 flying hours, 16,000 of them in Robinson helicopters. His company has five R22s and three R44s.
He says the debate about Robinsons can be emotive and the reporting sensationalised.
"They don't just break up in flight. Someone has done something silly or is operating in an environment they shouldn't be flying in." It's no different, he says, from a small boat that operates perfectly well but shouldn't be taken out in a storm.
"Once you get to the mast-bump it is like a car that has skidded. That's when you are in serious shit, so you don't let it happen in the first place.
"It's like a car going over a hump. If you go over fast you get that light tummy feel. Go over slow you don't get it. The emphasis in the past was teaching people how to recover from the low-G situation. I think there were a few problems, so now the emphasis is on recognising the conditions that are likely to cause it and recognising the symptoms and you don't let it establish.
"It's flying conservatively. It occurs under high power, that's why you are advised to reduce power."
His view is shared by others the Weekend Herald spoke to, notably veteran John Clements, who flew in the war in Vietnam and is a former owner of North Shore Helicopters. The crash rate of Robinsons dropped markedly after governors were fitted 20 odd years ago, he says, and safety-awareness training improved.
"It went quiet about Robinsons. And then there were a few crashes and all of a sudden there is something wrong with the Robinson helicopters. I don't believe there is. I'm an old-school pilot who thinks most accidents are caused by pilots."
In his lifetime of flying Clements says he's had only a few scares, "And each time it was my own bloody fault".
Spencer-Bower has experienced many highs in his career, such as being awarded a QSM in 2010 for services to aviation and being named flight instructor of the year by the Helicopter Association International in 2015. Lows include the deaths in Robinson accidents of three instructors who worked for him.
Wanaka helicopter instructors in September 2008. Photo/Supplied
Spencer-Bower (far right) and Andy Clayton (next to him) are the only two alive of the five instructors in this 2008 photograph.
Clayton currently works for a company that does not operate Robinsons but told the Herald he has no problem with them. "It's a sensitive area and that's why I'm reluctant to comment but I will say that if you had most people learning to drive in Ford Cortinas, then Ford Cortinas would have a bad name."
Graham Stott, Steve Combe and Jason Wright died in accidents while flying with students. Two involved mast bumps, the third occurred practising recovery from a loss of power.
There is a tendency when an experienced pilot is on board, says Spencer-Bower, to say it must be the machine. "I don't consider it [the helicopter] is a problem at all. I consider it is the operator and the sooner we get cameras or some recording device in the machines, you are going to stop all this speculation and innuendos. You are going to have some facts."
Robinson Helicopter Company hopes to have a recorder ready by the end of the year. It is adding an autopilot. "So in turbulence," says Kurt Robinson, "you can literally let go of the controls and you will be fine."
Louisa Patterson's company has a recorder ready for market. Called Eye in the Sky, it records video, audio, speed, altitude, position and information about pitch, roll and yaw.
Profits from sales are to go to the James PG Foundation, named for her son, who would have turned 20 last month.
The Robinson Helicopter Company requested a right of reply to this article. The Herald stands by the report but has agreed to publish the response, written by company president Kurt Robinson.
This article includes comments by Mr Tom McCready, former CAA accident investigator, regarding the Robinson main rotor design. The article also includes a short video featuring Mr McCready comparing a Bell main rotor head to a Robinson rotor head.
Mr McCready's comments and video claim the Robinson rotor head is more susceptible to mast bumping than other two-bladed designs, and that when mast bumping does occur the result is more severe.
There is no evidence or valid engineering theory to support these claims. Any helicopter with a teetering rotor system relies on positive G force for control and is susceptible to mast bumping during low-G situations.
The main rotor provides the thrust that overcomes the weight of the helicopter. The pilot controls the movement of the helicopter by tilting the rotor disk, and therefore the direction of thrust, in the direction the pilot wishes to move.
When the helicopter experiences a low-G condition the thrust is reduced and consequently the pilot's ability to control the helicopter is reduced.
If the helicopter begins to roll while in a low-G condition, the pilot's attempt to correct the roll by tilting the rotor disk will be ineffective. The rotor disk still responds to the pilot's control inputs, but there is no thrust to create a response in the helicopter fuselage.
If the roll is rapid the attempt to correct the roll can result in tilting the rotor disk to the structural limit. This structural limit is the rotor hub contacting the rotor drive shaft, described as "mast bumping", and when severe can lead to failure of the drive shaft or contact between the blades and fuselage.
In helicopters with the main rotor turning counterclockwise such as Robinsons, tail rotor thrust will likely cause a roll to the right during low-G.
Contrary to Mr McCready's assertions, the Robinson rotor system is in several ways less susceptible to catastrophic mast bumping than the Bell-style rotor. Heavier rotor blades (such as in a Bell) have a greater gyroscopic inertia that causes the rotor to lag behind the fuselage during attitude changes such as an uncontrolled roll during low-G.
This lag reduces clearance between the rotor hub and drive shaft, increasing the likelihood of a mast bump. The Bell Huey rotor system used as an example in Mr McCready's video includes a gyroscopic stabiliser bar, which further increases the lag of the rotor system.
The mast bumping phenomenon originally became well known due to a large number of Huey and Cobra accidents during nap-of-the-earth (low-G pushovers at hilltops) flying in Vietnam.
The Robinson rotor control system geometry provides a design feature known as "negative delta-3 pitch-flap coupling", which causes a blade pitch reduction as a blade flaps up. This acts to reduce overall teeter angles in flight and decreases the likelihood of a mast bump.
During a mast bump, the additional hinges on the Robinson design provide flexibility at the hub to partially relieve the forces imparted on the rotor shaft.
Contrary to the assertions of Mr McCready, there is no tendency for the blades to "pop up" out of track as a consequence of contact between the hub and shaft due to Newton's Third Law.
The motion of the blades are dominated by aerodynamic, centrifugal and Coriolis forces. The force imparted on the blade by contact with the shaft is closely aligned with the outboard hinges and the tendency for the blade to rotate about the hinge by this force is negligible.
Mr McCready states that often with a Bell-style rotor, a mast bump would only dent the rotor shaft and frighten the crew followed by a normal, safe landing. However, according to a 1983 article in US Army Aviation Digest, mast bumping was associated with 59 accidents and 213 fatalities in US Army UH-1 and AH-1 aircraft between 1967 and 1982.
Clearly, catastrophic mast bumping accidents are not unique to Robinson helicopters and the accident rate for Robinson helicopters is relatively low.
Helicopter rotor dynamics is a complex subject. Robinson Helicopter Company has thoroughly analysed the main rotor and control systems on each of our helicopters. We are confident the Robinson rotor system design has no unusual handling qualities and is safe when flown within the approved flight envelope.
While it is prudent to slow down in turbulence and to avoid extreme turbulence, mast bumping is avoidable in all conditions when the helicopter is flown properly.
