Turbulence can strike terror into even the most seasoned flyer. But do we actually have reason to be worried? Julietta Jameson investigates the things that go bump in the flight.
While plane disaster movies have fallen out of vogue since the September 11 terrorist attacks, you only have to search “flight turbulence” on YouTube to find out just how much of a bogeyman mid-air bumpiness is.
There's good reason why Hollywood has exploited it, newscasters have reported it and amateur video makers have captured it. Whether a person is the type of flyer who's snoozing the minute they board, or one who needs Valium to even step on an aerobridge, most will contemplate their mortality the minute the plane starts to jerk enough to make that free drink choppy.
Says Captain John Holmes, who trains pilots for Virgin, Jetstar and Tiger through Ansett Aviation Training, “You're sitting back relaxing and suddenly you're jolted. It's psychological. And an plane is an unnatural environment. A lot of people are a bit nervous already.”
Turbulence damages more aircraft and injures more passengers and flight crew than any other aviation misadventure - besides crashes.
But it would take some pretty amazing turbulence to take down a commercial airliner. As a Boeing spokesman says, “Boeing planes are designed to withstand G forces that are one and a half times greater than the typical turbulence you would encounter.”
Serious injury figures are low. Between January 1998 and May 2008, there were 339 turbulence occurrences in Australia reported by airlines to the Australian Transport Safety Bureau. These resulted in over 150 minor and serious injuries to passengers and cabin crew. The most recent occurred in January this year, when seven passengers were injured on board a Qantas flight from London to Singapore.
Stats from the US Federal Aviation Authority show that there have been three fatalities from injuries sustained during turbulence between 1980 and 2008 in the US, resulting in 298 serious injuries and three fatalities. Of the 298 serious injuries in that time, 184 involved flight attendants and 114 involved passengers.
At least two of the three fatalities involved passengers who ignored the seatbelt sign and did not fasten theirs. On that note, the FAA says, “Each year, approximately 58 people in the United States are injured by turbulence while not wearing their seat belts.” (They're not joking when they say 'buckle up', people.)
So just what is turbulence? It is irregular fluctuations in air, usually caused by weather.
“There are varying degrees of turbulence and most of the time we don't notice it,” says Professor Sylvester Abantoriba of RMIT's School of Aerospace.
There are also various types of turbulence including: thermal, which is vertical currents of air rising to meet cooler descending air: mechanical, which is caused by interference with the horizontal air flow by things such as mountains or buildings; convective, which is air currents moving up and down in clouds or thunder storms; and aerodynamic, or wake turbulence, caused by another aircraft. These are all visible and/or predictable.
Then there is shear.
This occurs when the direction or speed of wind changes radically within a short distance.
Often it changes from a slower speed to a jet stream – a band of high wind caused by the heating, cooling and rotation of the earth. The aircraft is jolted as it moves between the different wind speeds.
And it's this that is the most troublesome form of turbulence because it tends to be invisible.
“On of the biggest problems for long haul aircraft is the cruise altitude, 30-33,000 feet,” says Abantoriba. “That's where you have the jet streams that are not really visible to anybody, so you don't know about the turbulence till you are caught in it. It's called clear air turbulence (CAT). Sometimes it can be very strong.”
Often pilots will go looking for jet streams: on the route east across the Nullarbor, for instance, where one is known to exist and it cuts flight time and saves fuel. (It's avoided going when heading west.)
But the unpredicted jet streams are the troublemakers.
The new Boeing 787 Dreamliner is a equipped with a 'turbulence dampening system'. It has sensors that read the air in front of the plane and attempt to predict the effect the air will have on the aircraft. Computers then send signals to the control surfaces to lessen the impact of the turbulent air.
And Abantoriba says “researchers are working on further models which they hope will be installed in aircraft and will use the environmental data to predict such occurrences.”
But a lot still depends on pilots who, in the event of turbulence, will slow the plane.
“The pilot slows down to set figures we call turbulence penetration speed, which is the optimum speed for that particular plane when it enters turbulence to smooth the ride. You can hear the engines being retarded when they slow down,” says Holmes.
Regardless of the noise, which may alarm some, “Handling it is taught, studied and practiced by pilots. The danger is minimal.”
The Pacific is prone to serious levels of turbulence as are the Equator and Central Africa, California and Alaska.
“And generally if you're flying up to Singapore or Hong Kong, you'll often find when you get to the subtropics area that the seatbelt sign will come on because they know you're getting near the jet stream area,” says Holmes.
Once an aircraft finds turbulence, the pilot will report it so others can avoid it.
But how bad is bad? The experts (and the statistics) say most flyers will never experience severe and prolonged turbulence.
“We're only talking about a very small amount of flight time,” says Holmes. “And most flights you won't encounter much at all. In actual fact the plane doesn't really move much at all. It appears to. You might be only talking centimetres of movement. The plane can feel like it's moving a lot but your drink will stay in your glass. If you were dropping a lot, that liquid would actually come out of your glass.
“Most aircraft will be travelling over the ground probably about one and a half football fields a second. If in that first length there's an updraft, it's more than possible that there is a downdraft on the next length. So it's evening itself out ever so quickly. The aircraft is not actually moving much.”