Twin doesn’t equal safer
Like most readers of Pilot I’m an eclectic consumer of aviation products. Or to put it another way, I’m a big tart when it comes to flying stuff. I’ll get in anything if it goes up in the air: Tomahawk to Citation via P-51, and Tornado to glider and hot air balloon−been there, done that. But of all the flying things, my whole heart belongs to the helicopter, because it goes straight up, and sideways, and backwards, and because there’s nothing else in aviation quite like flying down to the pub for lunch. So this is about helicopters−but even if they’re not your bag, keep reading, because it’s also about regulation, common sense, and the fact that the price of flying is eternal vigilance.
We were all heartened when EASA embarked on Rule Making Task 0318, which would revisit the rules on single-engined helicopters flying over ‘hostile’ areas. It accepted that engines are an order of magnitude more reliable than they used to be and there was no safety basis for rules that allowed only twin-engined helicopters to fly over urban areas, over water, in the mountains or anywhere that might result in a difficult forced landing. Progress at last. But we find that RMT 0318 has been ‘postponed’ and, instead, ICAO has quietly embarked on a rewrite of its performance code for helicopters that could result in all the world’s commercial operations having to be conducted in twins.
The impact on the helicopter industry would be immense. Twins are fantastically expensive to buy, operate and maintain− they are the province of the military, the government, and industries like offshore oil, where pockets are apparently bottomless. And the rules on twins are based on an entirely false ‘safety’ premise.
To the layman, it stands to reason that they are safer than singles. But in the last ten years there have been nineteen fatal helicopter accidents in the UK, which killed 62 people, and only one of those accidents
was caused by engine failure. The eight accidents that involved twins killed 49 of those people. The only fatalities on the ground were caused by two of the twins. The main cause of accidents is pilot error, even among the most skilled and experienced pilots; those who fly twins. The Super Puma accident at Sumburgh in 2013, which killed four, was due to vortex ring, exacerbated by pilot confusion over the autopilot. The EC135 that hit the Clutha pub in Glasgow in the same year was starved of fuel, killing ten. There’s a case to be made that had the helicopter been a single, the seven people who died in the pub (eleven more were seriously injured) would have been saved because it would have autorotated, or at worst bounced off the roof−twins are high-energy crashers.
The Super Puma that crashed into the North Sea in 2009, killing sixteen, suffered catastrophic gearbox failure, which brings into focus the major problem for twinengined helicopters. Remember Bill Lear’s Lear Fan, which had two PT6 turbines feeding a combining gearbox and a pusher propeller? The claim was that it had twin-engined ‘reliability’ but none of the asymmetric thrust problems of conventional twins. The FAA flatly refused to certify it because the unduplicated drive train was deemed inadequate. But that configuration, unacceptable to the FAA in a fixed-wing, is an inescapable fact of life in twin-engined helicopters.
The regulatory obsession with engine failure causes accidents. Under Group A Engine Failure Accountability Procedures, a commercial twin climbs backwards to a given height so that in case of engine failure it can land back where it took off. (Twins cannot remain airborne on one engine at all regimes of flight.) But quite apart from the risks of flying upwards and backwards into an area the pilot cannot see, the pilot finds himself at the top of the initial climb with little or no airspeed, and pulling enormous torque. If tail rotor thrust gives out at that point (statistically more likely than engine failure, at a conservative one per 50,000 hours) the aircraft will begin to spin and the pilot will have insufficient height to recover, whatever his skill level. On the other hand, if a single-engined helicopter takes off on a normal profile, seeking to gain airspeed as quickly as possible, and suffers a tail rotor failure at the point at which the twin would be at the top of its backwards climb, the single can fly on to a safe landing.
If you study the nineteen fatal accidents in the last ten years, you’ll find all sorts of causes not related to engines: tail rotor failure, mid-air collision, loss of control in IMC or at night, training accidents, collision with a crane, CFIT, carb icing, low rotor RPM. The only instance of engine failure was a 36-year-old Jetranger that crashed at Flamborough Head in 2014, killing two. The establishment of RMT 0318 shows us that EASA understands the issue, but ICAO’S intervention proves that the quasi-religious attachment to engine failure is too deeply ingrained in those who matter.
There are dozens of reasons why singles are better. A single autorotates at about 20mph vertically, so even if the pilot utterly neglects to flare off descent rate, it will arrive like a small family car running into a lamp post at a brisk cycling speed. The very complexity of twins doubles the chance of component failure−the Glasgow EC135 had six fuel pumps, enough fuel on board to continue flight and a highly experienced pilot, but the complexity did them in: reducing complexity increases safety. Singles have a greater power margin, as twins operate two engines at low power on the basis that they’ll need to scream one engine up to max torque if the other fails; singles will also have greater tail rotor authority. Payloads on twins are often lower because of the need to carry the dead weight of a redundant engine and all its systems, so they may have to make two trips where a single would fly one, and that utterly destroys any safety argument. Cost, noise, pollution, downwash, all count against the twin. Duplication has its limits: the EC225 that got into vortex ring at Sumburgh had duplicate pilots, both highly experienced. Reliability is more important than redundancy, and what nurtures reliability is simplicity.
The ‘stands to reason’ brigade and their religious fixation should not be allowed to trump facts, statistics and science to the detriment of the helicopter world. It is whispered, however, that the loudest voice at ICAO for mandating two engines for commercial work emanates from the UK. Please god, make it not so.
The rules on twins are based on an entirely false 'safety' premise