Explainer video on the science behind flight turbulence

A gashed face while descending into Denver. A fractured spine while heading to the Cayman Islands. Second-degree burns from scalding water while heading to Barbados.

These were among the most serious injuries from turbulence aboard airliners last year, when the Federal Aviation Administration said the number of injuries doubled from a year earlier.

The 44 injuries in 2016 compared to 21 in 2015, the FAA announced Wednesday. During the last 15 years, the lowest total was 12 in 2006 and the highest was 107 in 2009, according to FAA.

Passengers tend to get injured more than crew members, and three-quarters of the injuries last year were for passengers. But crew members often suffer serious injuries because they frequently are standing or walking around the cabin when so-called “clear air” turbulence strikes unexpectedly.

The FAA urges passengers to listen to flight attendants and use an approved child-safety seat for children under 2 years old. The FAA also urges airlines to include turbulence in weather briefings, and to have pilots and dispatchers relay reports about turbulence.

Flight attendants said the incidents serve as a reminder of the risks of their profession – and the need for passengers to remain seated with their seat belts fastened during flights.

“The airplane cabin can be a dangerous work environment for flight attendants,” said Bob Ross, president of the Association of Professional Flight Attendants, the union representing crews at American Airlines. “Clear-air turbulence is particularly problematic and a leading cause of flight attendant injuries. By its nature, it is difficult or impossible to predict. That is why we stress that passengers keep their seat belts on whenever they are seated and pay particular attention to crewmember instructions during all phases of flight.”

The National Transportation Safety Board investigated airline turbulence incidents during 2016 including:

–On Dec. 13, a United Airlines flight from San Francisco to Denver hit mountain-wave turbulence and a “big jolt” at 20,000 feet. A flight attendant securing the galley in the Boeing 737-800 was thrown to the ceiling and then struck her face on a counter that gashed her cheek and fractured a facial bone.

–On Aug. 11, a JetBlue Airways flight from Boston to Sacramento encountered stormy weather above South Dakota. Three flight attendants and 24 passengers suffered minor injuries in the…