More than six decades ago, when the federal standards on the strength of airplane seats and seat belts were written, government regulations specified that seats be designed for a passenger weight of 170 pounds. But now the average American man weighs nearly 194 pounds and the average woman weighs 165.

Now, some engineers and scientists have raised questions about whether airplane seats, tested with crash dummies that reflect the 170-pound rule, are strong enough to protect heavy travelers.

“If a heavier person completely fills a seat, the seat is not likely to behave as intended during a crash,” said Robert Salzar, the principal scientist at the Center for Applied Biomechanics at the University of Virginia. “The energy absorption that is built into the aircraft seat is likely to be overwhelmed and the occupants will not be protected optimally.”

Nor would the injury necessarily be confined to that passenger, Salzar said. If seats fail, he said, those seated nearby could be endangered from “the unrestrained motion of the passenger.”

Yoshihiro Ozawa, an engineer whose company, Jasti Ltd. in Japan, has been making crash dummies for 20 years, raised similar concerns. He said he worried that there was no data proving that “seats and seat belts are safe enough” for larger passengers.

“If we don’t test with heavier dummies, we won’t know if it is safe enough,” Ozawa said through an interpreter in a telephone interview. “There is no regulation that says they have to test for heavier.”

Executives with two U.S. airline-seat manufacturers declined to comment on the issue. Dede Potter, a spokeswoman for one of those manufacturers, B/E Aerospace, said only, “We comply with all industry regulations.”

In 2005, the FAA updated the average passenger weights used in calculating each flight’s total weight and balance. Men’s weight was raised by 25 pounds to 200 and women’s by 34 pounds to 179. (That is the summer calculation; it is higher in the winter when travelers are wearing heavier clothes.)

The size of the seats is not a function of passenger weight but a legacy of airplane design from a generation ago, said Vern Alg, a former airline executive who is now a private consultant. “The restriction is the dimension, the width of the aircraft,” he said. “With Boeing narrow bodies, for example, if they are going to have six seats across, they can only be 17.1 inches wide.”

Strength and size are not the only factors affecting safety when the passenger is overweight. Use of the seat belt can also be a problem for these travelers. The FAA recommends wearing seat belts throughout the flight, though use is required only for takeoffs and landings.

Dietrich Jehle, a professor of emergency medicine at the University at Buffalo who has conducted a study of more than 300,000 serious automobile accidents, said he found that very overweight drivers faced an increased risk of death in a severe crash and that they were 67 percent less likely to be wearing seat belts, which he suspected was for reasons of comfort.

Jehle said obese air travelers may also be less likely to wear seat belts than thinner passengers. Unbelted passengers are at risk of injury and can be a mechanism for injury to others, Jehle said. “Force is mass times acceleration, and when someone is heavier and unbelted, there’s that much force that is being applied.”

He said both airlines and car companies needed to address the unique challenges of protecting overweight people. “Since a third of the population is obese, we need to be doing some of our crash testing with obese dummies,” he said.

Salzar said seat belts should also be tested to insure they could restrain heavier individuals. “You’d be amazed at how the large person blasts through that restraint,” he said.

In airplane economy seats, Ozawa said the proximity to other passengers created a higher likelihood that the heavier passenger would become a hazard by colliding into those sitting nearby. The back of the seats may not be strong enough and the spaces between seats wide enough to protect passengers from the impact of heavier passengers behind or beside them, he said.

The National Transportation Safety Board did recommend last year that the FAA begin collecting information about the size and weight of people flying in private planes to determine if testing accurately predicted the effectiveness of seat restraints for a range of people from the 5-foot, 110-pound woman to the 6-foot-2, 223-pound man. The safety board’s action was prompted by an accident in which a private pilot’s large belly prevented the inflation of an air-bag-equipped seat belt.