DALLAS — 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.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:THE NUMBERSAbout 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.