Turbulence is the most common cause of injury to air passengers, and unsettles even the most frequent fliers - as passengers on an Air Canada flight this week found out to their detriment when the aircraft dropped violently, injuring 37 - but is it really something to be afraid of?
British Airways captain Steve Allright, and Patrick Smith, a US pilot and author of Cockpit Confidential, a book on air travel, explain.
What causes turbulence?
Steve Allright: Many different things may cause turbulence, but each and every one of them is known and understood by pilots. Every day I fly, I expect a small amount of turbulence, just as I'd expect the odd bump in the road on the drive to work. Turbulence is uncomfortable but not dangerous. It is part of flying, and is not to be feared.
Different aspects of the weather cause different types of turbulence. CAT is an abbreviation for Clear Air Turbulence – the most common form of turbulence you are likely to experience.
Air tends to flow as a horizontal snaking river called a jet stream. A jet stream can sometimes be thousands of miles long but is usually only a few miles wide and deep. Depending on the direction of travel, our flight planners either avoid (into a headwind) or use (into a tailwind) these jet streams to cut fuel costs, as they can flow up to 250 mph. Just like a fast-flowing river swirling against the riverbank, where the edge of the jet stream interacts with slower moving air, there may be some mixing of the air which causes turbulence.
Patrick Smith: Everything about turbulence seems dangerous. However, in all but the rarest circumstances, it's not. For all intents and purposes, a plane cannot be flipped upside-down, thrown into a tailspin, or otherwise flung from the sky by even the mightiest gust or air pocket. Planes are engineered to take a remarkable amount of punishment, and they have to meet stress limits for both positive and negative G-loads. The level of turbulence required to dislodge an engine or bend a wing spar is something even the most frequent flier won't experience in a lifetime of travelling.
So I'm not accused of sugarcoating, I concede that powerful turbulence has, on occasion, resulted in damage to aircraft and injury to their occupants. With respect to the latter, these are typically people who fell or were thrown about because they weren't belted in. About 60 people, two-thirds of them flight attendants, are injured by turbulence annually in the US. That works out to about 20 passengers. Twenty out of the 800 million or so who fly each year.
Can it be avoided?
Steve Allright: You cannot see CAT, you cannot detect it on radar, and you cannot accurately forecast it, but there are other ways of avoiding it. In the main we rely on reports from other aircraft, which we hear either directly or which are passed on by air traffic control. We then consider the options available to us. Our endeavours to fly at an altitude that has been reported as smooth may be prevented by several constraints such another aircraft occupying that level, or the weight of the aircraft at that time.
Whatever the circumstances, your pilot will find the most comfortable path to your destination without compromising your safety. Just like you, we experience the movement and would prefer a smoother ride.
Patrick Smith: Predicting the where, when, and how much of turbulence is more of an art than a science. We take our cues from weather charts, radar returns, and, most useful of all, real-time reports from other aircraft.
Is turbulence more likely on certain routes?
Steve Allright: Any airport is at the mercy of strong winds on any given day. The same applies to jet streams on any given route, although there is generally more chance of turbulence crossing the ITCZ (Intertropical Convergence Zone) when flying south across Africa, for example.
Patrick Smith: Those burbling, cotton-ball cumulus clouds are always a lumpy encounter. Flights over mountain ranges and through certain frontal boundaries will also get the cabin bells dinging, as will transiting a jet stream boundary. But now and then it's totally unforeseen. You just don't know. Because turbulence is so unpredictable, I am known to provide annoying noncommittal answers when asked about how best to avoid it.
"Is it better to fly at night than during the day?" Sometimes.
"Should I avoid routes that traverse the Rockies or the Alps?" Hard to say.
"Are small planes more susceptible than larger ones?" It depends.
Which is the smoothest part of a plane to sit?
Patrick Smith: Ah, now that one I can work with. While it doesn't make a whole lot of difference, the smoothest place to sit is over the wings, nearest to the plane's centres of lift and gravity. The roughest spot is usually the far aft - the rearmost rows closest to the tail.
How bad can turbulence get?
Patrick Smith: I remember one night, headed to Europe, hitting some unusually rough air over 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 40 feet, either way, is what I saw. Ten or 20 feet, if that, most of the time. Any change in heading - the direction our nose was pointing - was all but undetectable. I imagine some passengers saw it differently: "We dropped like 3,000 feet in two seconds!"
You're liable to imagine pilots in a sweaty lather: the captain barking orders, hands tight on the wheel as the ship lists from one side to another. Nothing could be further from the truth. Indeed, one of the worst things a pilot could do during strong turbulence is try to fight it.
Steve Allright: Flight crews around the world share a common classification of turbulence: light, moderate and severe. The definitions are laid down in our manuals and help us to make an assessment as to what our course of action should be.
For the fearful flyer, even light turbulence can be upsetting. For pilots, light turbulence is no different to a bumpy road for a taxi driver or a slightly uneven section of track for a train driver – a small, but totally safe, inconvenience and very much part of our daily lives. In light turbulence, the aircraft may be deviating by just a few feet in altitude.
Moderate turbulence strikes no fear into pilots, as they will experience this level of turbulence for a few hours in every thousand hours they fly. It usually lasts for no more than 10 or 15 minutes, but occasionally may last for several hours, on and off.
This sort of turbulence will unsettle even some regular travellers and will cause drinks to spill. The aircraft may be deviating in altitude by 10 or 20 feet. No action is required by the pilot to control the aircraft, but the flight crew may decide to try a different altitude if the turbulence persists.
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.
I should stress that this level of turbulence is so rare that leisure travellers will almost certainly never experience it and nor will most business people.
Steve Allright is a British Airways training captain and co-author of the book Flying with Confidence. British Airways regularly runs "Flying with Confidence" courses at airport around the UK. See flyingwithconfidence.com
The Telegraph, London