PHILADELPHIA — A baby propelled from its mother’s arms when a United Airlines flight hit severe turbulence over Montana this week has reignited debate over the safety of allowing young children to be held on adults’ laps when traveling at 500 mph, 30,000 feet aloft.

The Federal Aviation Administration (FAA) allows children younger than 2 to fly for free as “lap children,” although the FAA website “strongly urges” parents to use approved child-safety seats.

“Your arms aren’t capable of holding your children securely, especially during unexpected turbulence,” the agency says.

Nevertheless, the FAA does not require a separate seat for young children, saying the mandate would “force some families who can’t afford the extra ticket to drive, a statistically more dangerous way to travel.”

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Airlines permit children younger than 2 to fly for free in the United States, but for trans-Atlantic and international travel, they must have “a ticket in their name for 10 percent of the applicable adult fare,” according to airline policies at American and Delta.

A 10 percent ticket does not entitle infants to their own seats, the policies state.

U.S. airlines used to offer discount fares for very young children to have their own seats, but most no longer do. Southwest has a “fully refundable infant fare”; the discount is $10 to $15 off its “anytime” or midrange adult fare, said airline spokesman Dan Landson.

Will the turbulence that occurred as United Flight 1676, en route from Denver to Billings, Mont., started to descend and sent three crew members and two passengers to hospitals — the infant was not hurt — prompt the FAA to rethink the policy?

No.

“The FAA continues to encourage the use of child-restraint systems,” the agency said Thursday. Requiring all families traveling with children younger than 2 to purchase tickets “would significantly raise the net price of travel for those families.”

Charles Leocha, director of the Consumer Travel Alliance, said his Washington, D.C.-based group has not taken a position.

“Personally, I think the FAA is right,” Leocha said. “Given that this has been the law and regulation for so long, and there are so few cases where anybody gets injured, I wouldn’t suggest that we change the rule.”