Sit, back, relax and enjoy the flight.

After 17 years, the Federal Aviation Administration has completed work on requiring passenger aircraft to have stronger seats, designed to increase the survivability of passengers and flight attendants in accidents.

The new rule, which affects aircraft built after October 2009, says the seats must be able to withstand 16 times the force of gravity, compared with the 9g standard in effect since 1952. Floors and the tracks the seats ride on also must be able to withstand those forces.

The new seats must undergo a battery of tests to determine their strength, similar to the crash tests that automakers must comply with to meet federal safety standards.

When the FAA proposed the stronger-seat rule in 2002, it envisioned requiring airlines to retrofit existing fleets with the stronger seats. The final rule does not include planes now in service, many of which already have upgraded seats that equal or approximate features of the new, 16g seats.

That change pleased the aviation industry, which argued that retrofitting seats on a regulatory timetable would be too costly. The FAA agreed and concluded that many of the planes with older, weaker 9g seats were retired after the business downturn the followed the Sept. 11, 2001, terrorist attacks. And new aircraft introduced in the 1990s had versions of the updated seats anyway -- so-called 16g compatible seats or neo-16g seats.

"It was not worth the minute safety benefit for retrofits," said John Hickey, director of FAA's Aircraft Certification Service. "It's the right, sweet-spot solution -- it gives passengers the safety benefit at a very reasonable cost."

Not requiring faster retirement of older seats substantially reduced the cost of the rule. The retrofit would have cost $519 million. The final version of the rule will cost $34.7 million from 2009 through 2034. The FAA estimates that the airlines will take delivery of 1,752 new planes with a total of 225,274 seats during the period -- all of which would have to comply with the rule.

The benefits of installing fully compliant seats, calculated on lives saved and injuries averted, total $78.9 million over the same 25-year period. Research done for the FAA showed that 45 fatalities and 40 injuries might have been averted in accidents between 1984 and 1998 if the seats were those called for in the rule.

Aircraft seats are an important part of the equation in making accidents as survivable as possible. Generally, over the past decade, the emphasis has been on better protecting the people in an aircraft and getting them out faster. There is no way for a passenger to know what vintage the seat is beneath him.

"From a carrier perspective, this rule makes sense," said Basil J. Barimo, vice president of operations and safety for the Air Transport Association, the trade group that represents the airlines' interests. "The FAA recognized they had a difficult case to make with retrofit design because a significant number of seats had already been replaced with 16g-compatible seats."

A 2003 report by the General Accounting Office (now the Government Accountability Office) estimated that at the time, 44 percent of commercial aircraft had full 16g seats, 55 percent were 16g compatible, and 1 percent had 9g seats. Some of the "compatible" seats may offer only minor improvements over the lowest-rated seat, the report said.

The Association of Flight Attendants, a labor union, said the agency's decision to not order upgrades of all seats is a disappointment and a safety problem. It said that there are aircraft in use with 9g seats and that many of the 16g seats in service have not been subjected to the rigorous testing the FAA will require.

"The FAA trumps Congress again," said Christopher Witkowski, director of air safety, health and security for the flight attendants union, referring to a 1987 legislative directive that told the FAA to consider improving the crashworthiness of all seats. "We will have seats for decades that don't meet the occupant-protection standard FAA could have required. It could be the difference between life and death."

The National Transportation Safety Board, which has been nagging the agency since the 1970s to strengthen seats and recommended dynamic testing in 1981, supported retrofitting the entire fleet.

"Seat strength is an essential part of crashworthiness," said Nora Marshall, chief of the NTSB's Survival Factors Division in the Office of Aviation Safety. "When a crash begins, there is a lot of energy, and it goes to the ground, fuselage, floor and seats. If the seat breaks loose, you are adding energy."

Marshall said the NTSB has documented crashes in which the seats broke loose and piled up at the front of the plane -- even though the type of accident that occurred should have made it a survivable crash. "We still see 9g seats," she said.

The August crash of an Air France plane in Toronto, which destroyed an Airbus A-340-300 when it ran off the runway, seems to be an example of the survivability of an airline accident: 12 crewmembers and 297 passengers successfully walked away from the wreck.

"The difference between 9g and 16g seats in those types of accidents can be the difference between 20 people living or 100," said the FAA's Hickey. "Or, everyone."

Out for comment: John D. Graham, who has been at the helm of the Bush administration's top regulatory post within the Office of Management and Budget since 2001, will resign Feb. 1 to become dean of the Frederick S. Pardee RAND Graduate School in Santa Monica, Calif. The school, which is run by RAND, a nonprofit think tank started by the government, awards doctorates in policy analysis.