But beginning Oct. 27, new airplanes of models certified before 1988 must also comply. In practice, aircraft manufacturers have been delivering many planes that meet the new rule for some time. So the deadline is more of a milestone than an actual change-over date.

But while Boeing has been delivering 737s for years with the 16g seats, the 747-400 — the current model of the 747 — still had the old 9g seats. But Boeing produced its last 747-400 a few weeks ago. A new model, the 747-8, will be introduced soon with the sturdier seats. Sandra Angers, a Boeing spokeswoman, said that few planes had been delivered lately with the old-style seats because few designs from before 1988 were still being manufactured.

The new rules do not affect older planes with the old seats. But those planes are disappearing from airline service because they are less fuel-efficient than newer models and because the total number of planes is down as airlines cut service in the recession.

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Safety regulators decided to impose the new rules after they found that passengers might survive a crash were they not crushed to death when the seats tore loose from the floor. The 16g standard was chosen because a deceleration more severe than that is not survivable. A stress of 16 times the force of gravity is equivalent to a velocity change of 30 miles an hour almost instantaneously.

In four recent accidents involving big jets, everyone on board walked away. One of those happened in August 2005 when an Air France Airbus A340 slid off the end of a runway on landing in Toronto. All 309 on board got out before the plane was consumed by fire. In other cases, the seats make no difference, as in the Air France crash in the Atlantic on June 1.

In contrast, an investigation by British authorities into the crash of a Boeing 737 in the Midlands in January 1989 showed that some passengers died of crush injuries when the floor broke up and the seats came loose. Stronger seats might have increased the number of survivors and reduced the severity of injuries, the report suggested. Forty-seven passengers died and 74 of the other 79 on board were seriously injured.

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Airplane seats are tested using some of the same methods highway safety authorities use on car seats. The plane seats are tested on sleds, using crash-test dummies borrowed from automobile testing. There are a variety of test criteria, including one for head trauma that is the same as that used by the National Highway Traffic Safety Administration.

The changes to the seats themselves, and the floor structure that supports them, will not be obvious to passengers. But the air bags are visible. They are built into one side of the lap belt, which appears a little thicker.

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The air bags also borrow technology from automobiles. They are set off by a shock meter that comes directly from cars. And like the systems used in cars and trucks, the seat belt air bags in planes are designed not to deploy inappropriately — in cases of air turbulence, for example.

In fact, said Mr. Hagan, this is simple, because the air bag sensor system watches for shocks on the axis on which the plane is traveling; it does not monitor up-and-down or side-to-side movements of the kind produced by turbulence, he said.

The air bags are widely used in first- or business-class cabins, where the seat in front is too far away or angled in such a way that it cannot function as a cushion. In coach class, the air bag has started out for use in front rows, exit rows and bulkhead seats, near galleys or toilets. In other seats, the passenger gets some protection from the seat back directly ahead, which is designed to break in a controlled fashion, providing a cushion.

Singapore Airlines flies 777s with wide intervals between seats and uses the air bag, Mr. Hagan said. JAL, Cathay Pacific and Virgin also use them, he said.