IT MUST have been a harrowing experience being on board the Air Canada plane flying between Shanghai and Toronto on New Year’s Eve. The aircraft was forced to make an emergency landing after it hit some terrifying air turbulence over Alaska. Twenty-one passengers were taken to hospital, some after being thrown around the cabin like rag dolls. One passenger spoke of a sleeping girl “flying up” into the ceiling. It would only be human if, for those few minutes, a good number aboard the flight started to make their peace with God. But should air turbulence put the wind up flyers?
According to Lauren Reid, aviation-business development manager at the Met Office in Britain, there are three main types of naturally occurring air turbulence that affect planes flying at altitude. The first, convective turbulence, is caused by localised thunderstorms. This is relatively easy for planes to avoid because the weather is visible, says Ms Reid. The most common is clear-air turbulence (CAT). This is caused by wind shear—streams of air travelling at different speeds or in different directions. This can jostle planes from side to side as well as up and down. The last, mountain-wave turbulence, is similar to wind shear, and occurs downstream from a mountain range. The terrain can sometimes cause the wind to oscillate, resulting mostly in up-down movement of the plane.
As meteorologists have become better at predicting the likely location of bad turbulence, so relatively fewer passengers now experience it. The Met Office, which is one of two global centres providing forecast information for long-haul flights, says that over the past ten years its models, backed by greater computing power, have improved markedly. But, while planes have access to better information in order to avoid potential turbulence, the phenomena themselves may be becoming more common. Although there is no consensus amongst academics—“scientific papers go either way” says Ms Reid—some believe that global warming is making matters worse. A warming planet potentially means that jet streams are becoming more intense, increasing the likelihood of CAT.
This is not a particular problem for the planes themselves. Aircraft are well-engineered and robust pieces of machinery. Even in the bumpiest conditions, wings do not shear off and fuselages do not get flipped upside down. Indeed those sudden, sickening drops, when you are sure you have plunged hundreds of feet, are mere pebbles in the road. The most severe turbulence usually buffets planes by no more than 100 feet (30 metres) or so. In fact the main reason pilots try to avoid such weather pockets is because it upsets passengers. In an informative (and hugely comforting) piece on his Ask the Pilot blog, Patrick Smith writes:
Altitude, bank, and pitch will change only slightly during turbulence—in the cockpit we see just a twitch on the altimeter. [...] I remember one night, headed to Europe, hitting some unusually rough air about halfway across the Atlantic. It was the kind of turbulence people tell their friends about. It came out of nowhere and lasted several minutes, and was bad enough to knock over carts in the galleys. During the worst of it, to the sound of crashing plates, I recalled an email. A reader had asked me about the displacement of altitude during times like this. How many feet is the plane actually moving up or down, and side to side? I kept a close watch on the altimeter. Fewer than forty feet, either way, is what I saw. Ten or twenty feet, if that, most of the time. Any change in heading—that is, the direction our nose was pointed—was all but undetectable. I imagine some passengers saw it differently, overestimating the roughness by orders of magnitude. 'We dropped like 3,000 feet in two seconds!'
If you imagine the crew at such moments, with leather gloves tightly clenched around their yokes, struggling manfully to wrest back control of the aircraft, you could not be more wrong. One of the worst things a pilot can do, says Mr Smith, is to fight turbulence. Instead they are taught to ride it out by cutting their speed and making fewer manoeuvres. In any case, some pockets of turbulence are so localised, a diversion of just a few thousand feet can often be enough to pull the aircraft into calmer skies.
Turbulence around the world is classified as light, moderate and severe. Moderate turbulence is enough to spill your drink, with the plane deviating by perhaps 40 feet. Even severe turbulence does not put the aircraft in jeopardy. According to Steve Allright, a British Airways pilot writing in the Daily Telegraph:
Severe turbulence is extremely rare. In a flying career of over 10,000 hours, I have experienced severe turbulence for about five minutes in total. It is extremely uncomfortable but not dangerous. The aircraft may be deviating in altitude by up to 100 feet (30 metres) or so, up as well as down, but nothing like the thousands of feet you hear some people talking about when it comes to turbulence.
The biggest danger of turbulence, as borne out by the Air Canada incident, is passengers not wearing their seatbelts, even when they are instructed to by the pilot. Mr Smith makes an excellent point: American carriers, in part driven by that country’s litigious culture, are quicker to illuminate the “fasten seatbelt” sign, and tend to leave it on for longer. But one unintended consequence of such over-zealousness is that flyers have become so desensitised that they end up ignoring it. Presumably you only have to experience a horror show like the Air Canada flight once before you decide to change your ways.
