FAA Fix For Fuel Tank Explosions
The Federal Aviation Administration said today that it hoped to propose a rule this fall that would require the airlines to cut the risk of fuel tank explosions like the one that destroyed T.W.A. Flight 800.
The rule would take effect in 2006, 10 years after the T.W.A. Boeing 747 crashed off Long Island, killing 230 people, and would have a seven-year phase-in period. It would cover about 3,800 big jets registered in the United States and built by Boeing and Airbus. The agency is in discussions with European regulators, but has reached no agreement with them
For years after they understood the broad outlines of the T.W.A. accident — ignition of the fuel tank by an electrical spark — regulators have discussed a variety of approaches. They talked about altering jet fuel so that it would be less prone to turn to vapor, the form in which it is flammable, or pumping inert nitrogen into the empty space in tanks when planes are on the ground. Both were rejected as too expensive.
But in December 2002, the F.A.A. demonstrated an onboard system that takes compressed air from the plane engines and uses it to remove some of the oxygen from ordinary air, thus raising the level of inert nitrogen, if only fractionally. That nitrogen is then pumped into the fuel tanks.
"We're taking this step because we have found a practical solution," the F.A.A. administrator, Marion C. Blakey, said. "Once planes are equipped with inerting technology, we can close the book on fuel tank explosions," she added. "It's a major moment in the safety of aviation."
The safety fix is also notable for the extent to which the agency used in-house engineering to figure out how to solve the problem.
Airlines could still oppose the F.A.A. solution, however, because of the costs. Installation would probably cost $140,000 to $220,000 per plane, officials said, plus another $14,000 annually for operating costs, which is high but not a record for the agency. And $14,000 would represent only a small fraction of the annual operating cost of an aircraft.
Depending on the size of the plane and its fuel tanks, the system will weigh 100 to 200 pounds, and will require some extra fuel use by the engines to provide compressed air, according to John Hickey, director of the F.A.A's aircraft certification service.
Boeing has said it will use inerting technology in its new 7E7, which is still being designed. The F.A.A. intends to require it on American-registered Airbus A-380's. A prototype of that plane is supposed to fly later this year.
Ms. Blakey, asked why some planes would not be equipped with a preventive system until 2013, or 17 years after the accident, said that the installation could only be done during major maintenance. But she pointed out that the F.A.A. had already issued scores of orders for inspecting or replacing wiring or other electrical components, to reduce the risk of tank explosions. The F.A.A.'s original strategy for guarding against fuel tank explosion was to exclude any possibility of spark, but Ms. Blakey said that with three fuel tank explosions in the last 14 years, the agency did not believe that that approach was reliable.
"We all know that one accident of this type, one accident of any type on board an aircraft, is simply one too many, both for the families of victims and for our airlines," she said. "Reducing commercial fatal accident rates is our No. 1 objective."
But she added that fuel-tank explosions were occurring at the rate of roughly one every five years, raising the possibility of another before a fix is installed.
Flight 800 was initially thought to have been downed by a bomb or a missile, and aviation engineers took some time to come to full recognition of the fuel tank problem. At the time of the explosion of Flight 800, when the Boeing 747 left Kennedy International Airport for Paris on a hot July evening in 1996, many experts believed that it would be difficult for the tanks to explode, because the fuel/air mixture had too much fuel or because the fuel was cold and would not readily turn to a vapor, the form that burns or explodes.
But extensive tests by the National Transportation Safety Board after the crash determined that the fuel/air mixture was often within the range that could sustain an explosion. In the case of Flight 800, the plane was delayed on the ground, with its air conditioners running, giving off their heat to the center fuel tank. As the fuel warmed, its propensity to turn to vapor increased.
Because the plane was not fully loaded and was making a relatively short flight for a 747 with the prevailing winds, it did not need to carry a full load of fuel. As a result, the ground crews had not filled the center tank; it had a few inches of fuel at the bottom, and a lot of air. As the plane climbed into thinner air, pressure in the tank dropped, allowing more fuel to vaporize.
The Safety Board said it could not identify the source of the spark, but examinations of the wreckage, and of other old 747's, found a variety of wiring problems.