ASHINGTON, July 29 — Seven years after Trans World Airlines Flight 800 was destroyed by an explosion in its center fuel tank, Boeing is testing a 747 in which such explosions are nearly impossible, with a protective system that it is hoping to install on planes around the world.

But in an era of airline bankruptcies, it is not clear who would pay, or how. Even so, Boeing thinks the Federal Aviation Administration could certify the system's safety in the next few months.

In a one-hour test flight today, a mostly empty Boeing 747-400 took off from Dulles International Airport and made a 300-mile loop around Virginia, as technicians in the main cabin read computer displays that measured the air in the tank.

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Normally, the air-fuel mixture in the empty space in the fuel tank changes with the altitude. Sometimes the mixture is in the range that will burn if a spark is present. On this test airplane, the air was continuously mixed with a flow of nearly pure nitrogen, which diluted the oxygen content to a level that would not support a fire or explosion, rendering it inert.

"Fuel-tank inerting promises to be a genuine leap forward in safety," Marion C. Blakey, the administrator of the F.A.A., said before the flight. The plane carried an apparatus that is common in several industries, which sorts ordinary air into oxygen and nitrogen. It was adapted to aircraft use by the F.A.A. about a year ago. If Boeing receives the F.A.A. certification, it can offer the apparatus worldwide.

The problem, on jets made by Boeing and by others, is the operation of the air-conditioners on hot days. They cool the planes' interiors while dumping their heat into the fuel in the center tank, warming it enough to turn the fuel into vapor, which can explode if a spark is present. Aircraft designers recognized the problem long before July 17, 1996, when Flight 800, after experiencing an extensive delay on the ground with the air-conditioners running, climbed out of Kennedy International Airport on its way to Paris. It exploded off the Long Island coast about 11 minutes later, killing everyone aboard.

Fuel tank explosions are unusual, but 737's exploded on the ground in Manila in 1991 and Bangkok in 2001.

The National Transportation Safety Board was never able to pinpoint the source of the spark in T.W.A. Flight 800, but it found that explosive conditions existed in center fuel tanks much more frequently than had been commonly thought. Boeing says it thinks that the atmosphere in the tanks will support combustion about 20 percent to 30 percent of the time. The board also found a variety of wiring problems on older planes that could cause a spark.

In the last seven years, the F.A.A. has issued more than 50 directives, ordering airlines to inspect wiring and replace certain parts. But the safety board declared that that approach was "fundamentally flawed" because it could not be complete.

Michael P. Delaney, a program manager at Boeing for the problem of explosive fuel tanks, described the classic "fire triangle" of a combustible material, air and an ignition source.

Inerting, he said, means "attacking two corners on the fire triangle, as opposed to historically going after one, which is the ignition."

Boeing's solution diverts some of the air generated by the engines, which is produced for use inside the engine and for pressurizing the cabin, into a filter. The filter strains the oxygen out of the side and sends the nitrogen through. Normal air is about 21 percent oxygen, but the gas coming out of the end of the filter has as little as 3.5 percent oxygen. The goal is to limit the oxygen level in the tank to 12 percent or less.

The problem is complicated because the 747's tank is the size of a two-car garage, and it has baffles inside to prevent sloshing. Boeing is experimenting with where the nitrogen is inserted, and also the flow rates. And the membrane that sorts the oxygen from the nitrogen must be protected from stray particles in the air, and from ozone.

But Boeing engineers say that the system cuts the amount of time that the tank is combustible to less than 3 percent, compared with 20 percent or 30 percent today. That includes time on the ground. The tank atmosphere would remain combustible if the system failed. It could also become combustible if a plane was forced to make an emergency landing and descended faster than normal, with the dense air flowing into the tank faster than it could be diluted with nitrogen.

Boeing is not yet estimating what the system would cost, although the price is sure to be high, because it would take hundreds of hours of labor to install each one, experts said. And the F.A.A. is not saying whether it would require such a system.

Ms. Blakey said that question was "complicated."

"We are very glad that Boeing has stepped up aggressively on this, and asked for certification of the design, without requiring rule by the federal government," she said. The agency might require the step, she said, but "we have not done the cost-benefit analysis."