WASHINGTON - Nearly nine years after a fuel tank explosion caused the fatal crash of TWA Flight 800, safety officials say little has been done to reduce the flammability of vapors in aircraft fuel tanks.

National Transportation Safety Board executive director Dan Campbell said Wednesday that while much more is known about how to prevent fuel vapors from exploding, little has been done.

"We're not significantly different than we were in '96," Campbell said during a briefing with reporters.

Boeing and the FAA disagree. Both point to progress in designing safer systems, which are based on an FAA prototype, as well as plans to begin producing them next year.

FAA spokesman Greg Martin said the agency has moved aggressively to eliminate the factors that can cause jet fuel vapors to explode: sparks (or flames) and a deadly combination of concentrated oxygen and fuel.

"Although a rigid formal rulemaking process takes time, we've moved aggressively to remove both ignition sources and flammability levels," Martin said.

Boeing spokeswoman Liz Verdier said aircraft manufacturers are working to reduce fuel tank explosions even though they're extremely unusual.

"They're getting more and more rare because the industry constantly works on safety," Verdier said.

Campbell acknowledged that the FAA has reduced sources of ignition that can cause fuel vapor explosions. The FAA has ordered airlines to make more than 60 changes to eliminate potential ignition sources, such as faulty wiring.

In the past 15 years, there have been three fuel tank explosions: the TWA accident, resulting in 346 deaths, and two in Asia while the aircraft were on the ground.

All 230 people aboard TWA Flight 800 perished when the Boeing 747 crashed off the coast of Long Island, N.Y., on July 17, 1996, en route to Paris.

The NTSB said a spark in the wiring ignited vapors in the Boeing 747's partly empty fuel tank. Air conditioning units underneath the fuel tanks are believed to have heated the vapors inside the tank — making them more vulnerable to explosion — during the plane's two-hour delay at New York's John F. Kennedy International Airport.

After the accident, FAA researchers developed a system called "fuel tank inerting" that reduces the oxygen in the fuel tanks, making an explosion much less likely.

"We did not take 'no' for an answer when initial technical reviews said that fuel inerting systems would be too costly and too heavy," said the FAA's Martin.

The system pumps air flowing from the plane's engine into yard-long canisters. A ropelike substance in the canisters filters oxygen and water from the air. The result, a nitrogen-rich mixture, is pumped into the fuel tanks. The filtered-out water and oxygen is dumped off the aircraft.

Campbell acknowledged that Boeing is designing new planes with fuel inerting systems, but said the safety board is frustrated with the lack of progress on requirements for the existing fleet to be retrofitted. The FAA estimated that about 3,800 planes in the United States will need to have the new systems installed at a cost of between $140,000 and $220,000 per aircraft.

But the Air Transport Association, the lobbying group that represents major airlines, said it's not possible to assess the effectiveness of the fuel inerting systems because the FAA hasn't yet approved any for commercial use.

According to Boeing's schedule, 737s and 747s with the systems will go into service in the middle of next year, followed by 777s and 767s in early 2007, Verdier said.

Shortly after the new jetliners go into production, Boeing will be offering to retrofit existing aircraft with fuel inerting systems, Verdier said. Retrofits for the 757, which is no longer in production, will be available sometime in early 2007, she said.

The 787, a new long-range plane dubbed the Dreamliner, has already been designed with a fuel inerting system and will start flying in 2008, Verdier said.

Airbus, on the other hand, says its fuel tanks are safer because they aren't near heat sources and have no high-voltage wires running through them.

"Should the rules change, we will continue to work with the authorities, and evolve as necessary pending FAA rulemaking," Airbus spokeswoman Mary Anne Greczyn said in an e-mailed statement.

The NTSB's Campbell also criticized lack of progress on moving air conditioning units away from fuel tanks, which also reduces the likelihood of an explosion.

In a small percentage of cases, airlines now use ground units to cool planes before they take off, NTSB investigators. link