At some point during many flights, the captain will calmly announce that there could be some bumps ahead and so passengers must be seated with their seat belts on.
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The plane might seem to bobble or bounce a bit, but rarely does it turn into a serious threat to safety. That, however, is just what happened to an American Airlines flight last weekend, when 10 people were injured as the plane plowed through turbulence on its way to landing in Philadelphia.
A rundown of statistics, recent incidents, and what pilots and airlines do to avoid hitting potholes in the sky:
About 40 people a year are seriously injured by turbulence in the U.S., according to Federal Aviation Administration figures from the last 10 years. The FAA counted 44 injuries last year, the most since more than 100 were hurt in 2009.
But the official count is almost certainly too low.
The National Transportation Safety Board requires airlines to report incidents that result in serious injury or death, and FAA uses those reports to tally the number of people hurt by turbulence. But airlines are not required to report injuries unless they require a 48-hour hospital stay or involve certain specific injuries such as major broken bones, burns or organ damage.
Saturday's American Airlines flight to Philadelphia likely won't meet those standards — the injured people were released from the hospital within a few hours and didn't suffer the types of injuries that trigger a report to the federal safety board.
The American Airlines flight from Athens hit severe turbulence over the New York coastline. Seven crew members and three passengers among the 299 people on board were taken to a hospital for treatment. American Airlines spokesman Ross Feinstein said the seat-belt sign was on but none of the injured people were buckled in.
Feinstein said the plane was inspected and suffered no damage. After the interior was cleaned up — coffee and other drinks went flying, even splashing the ceiling — the plane was put back in service Monday, he said.
It was the latest in a string of scary turbulence incidents.
— In June, 10 people on a United Airlines flight from Panama City to Houston were injured when their plane was shaken over the Gulf of Mexico.
— In May, more than two dozen passengers suffered injuries including broken bones when they were tossed around the cabin of an Aeroflot plane headed from Moscow to Bangkok.
— More than 20 people were injured in August 2016 when a JetBlue plane ran into turbulence in a line of thunderstorms over South Dakota and had to make an emergency landing.
Turbulence is classified as light, moderate, severe or extreme. The first two might be frightening to some passengers, but it is only the latter two that are dangerous, especially for passengers and crew who aren't buckled in.
Most people associate turbulence with heavy storms. The most dangerous type, however, is so-called clear-air turbulence — a wind-shear phenomenon that can occur in wispy cirrus clouds or even clear air near thunderstorms, as differences in temperature and pressure create powerful currents of fast-moving air.
Planes can sail into clear-air turbulence without warning, as appeared to happen to the American Airlines flight.
TECHNOLOGY, TERRAIN AND THE EYE TEST
Airlines rely on meteorologists to predict the location and intensity of bad weather, and dispatchers on the ground give updates to pilots during a flight.
Pilots are on notice anytime a flight goes over a mountain range or through certain kinds of weather fronts.
Modern airliners are equipped with sophisticated weather-radar technology, yet often "it's as simple as looking out the window," says Patrick Smith, an airline pilot and author of "Ask the Pilot." Indicators such as thunder clouds with an anvil-shaped top usually mean the ride is about to get bumpy. But the most helpful tool, he says, is reports from other pilots in the area.
Pilots have a few techniques for getting through turbulence safely. They can slow down to what is called turbulence-penetration speed — fast enough to avoid a stall, but not so fast that they risk damaging the plane. They can ask air traffic controllers to let them move to a lower or higher altitude or go around troublesome clouds, but those requests can't always be honored.
"If you feel the plane climbing or descending midflight, (there is a) good chance it's because of a report from fellow pilots up ahead," Smith says.
Associated Press writer Anthony Izaguirre in Philadelphia contributed to this report.
David Koenig can be reached at http://twitter.com/airlinewriter