When TWA Flight 800 exploded off Long Island 12 years ago, the only available system to prevent ignition of fuel vapor in aircraft weighed more than 300 pounds.

Now research by the Federal Aviation Administration and the industry has produced equipment as light as 106 pounds.

That reduction in weight, with a corresponding decrease in complexity, size and cost, is one of two key technological breakthroughs that allowed the FA A on Wednesday to announce a rule to require airlines to install "inerting" devices for passenger jets with center fuel tanks. The equipment makes explosions in empty tanks highly unlikely by replacing some of the oxygen with nitrogen.

At the time of the Flight 800 crash, the only available technology was being used on military jets. It was large, heavy and expensive. But the government wasn't concerned about cost or size on military transport planes.

Size and cost would be important for commercial carriers, however, so the FAA convened the first of two committees in 1997 to look into a system that would work for passenger jets. The experts reported back in 1998 that there was no practical or affordable way to do it. The second committee concurred in 2001.

But the FAA continued with its research in laboratories in Seattle and near Atlantic City, and two eureka moments led to Wednesday's action.

"What made it affordable was the development of a lightweight nitrogen generating system" by FAA engineers aided by other experts in 2002, said John Goglia, a former National Transportation Safety Board member who is now a Washington aviation safety consultant. "That was the first breakthrough."

First, it eliminated all of the moving parts in the military system. And the weight of the units dropped to about 106 pounds for a Boeing 737 and 257 pounds for a Boeing 747 like Flight 800.

Second, a higher percentage of oxygen could be allowed -- 12 percent instead of 10 pe rcent -- without increasing the risk of an explosion. The insight was based on Navy research on its jets.

That allowed the inerting equipment to be even smaller, lighter and cheaper.

After those breakthroughs, the FAA was able to announce in 2004 that it planned to require a system to reduce the vapor risk and make a proposal the following year. It then took the agency three more years to adopt the rule over the objections of aviation trade groups.

An increasingly small price tag helped seal the deal. When it first raised the idea of eliminating the vapor threat, the FAA calculated that retrofitting all of the existing aircraft and installation of equipment on future jets would cost $35 billion. The cost of the final proposed change is estimated at $1 billion, FAA spokeswoman Alison Duquette said.

The rule will affect 2,730 aircraft built after 1991 -- 55 percent of the U.S. passenger airliner fleet -- and all similarly designed jets built in the future. Boeing and Airbus, the only aircraft makers who used the center-fuel-tank design after 1991, now have nine years to make the retrofits. By 2010, all new aircraft with center fuel tanks must have the new safety equipment.

To get the industry even reluctantly onboard, "the FAA also gave up a lot," Goglia said. "Airplanes built before 1991 are exempt" and that covers almost all of the McDonnell Douglas MD80 fleet and much of the Boeing 747 fleet.