A worldwide inspection of electrical wiring inside the wing fuel tanks of early Boeing 747s, such as the one involved in the explosion and crash of Trans World Airlines Flight 800, found numerous hidden wiring problems that might have caused future explosions if left undetected, according to Federal Aviation Administration officials.
FAA officials said they believe the problem could have played a role in the Flight 800 crash off Long Island, one of the country's greatest aviation disasters, which killed 230 people last year on July 17. But an official with the National Transportation Safety Board (NTSB), in charge of the investigation, said it is only one of many possibilities being explored.
Despite a year-long investigation, investigators still do not know why the center fuel tank of TWA 800 exploded violently. Theories of a possible bomb or missile are fading, however, in favor of a mechanical explanation.
Nonetheless, the many precautionary 747 inspections prompted by the TWA crash did find some chafed wires and numerous damaged protective conduits in early versions of one of the world's most popular long-distance airliners.
None of the wiring had burned through its protective aluminum cover to expose the wires to explosive fuel vapors, and officials stressed that all damaged wiring has now been replaced. The wires lead to fuel pumps inside the wing tanks -- although not to ones inside the center fuel tank that exploded on Flight 800 -- and the highly shielded wires are the only 110-volt electrical source allowed inside a tank.
However, the extent of the problem appeared to surprise officials.
In December, as a precaution, the FAA reviewed all previous 747 problems and decided to order another inspection of the wires. It was this inspection that found current problems.
FAA officials said the wiring problems also offer support for a theory developed by the FAA as to what might have caused theInvestigators have been mystified as to what caused vapors in the nearly empty center tank to explode. Unlike the wing tanks involved in the FAA inspection, the center tank has no 110-volt interior electrical lines, and tests so far have failed to find an ignition source.A theory, developed by the FAA's Northwest Region in Seattle, involves an unlikely chain of events in which an electrical problem causes a fire to burn outward from the wing tank to the wing tip through a vent tube that is designed to allow vapors to escape from the tank. At the wing tip, the flame front then reverses direction and travels back down another vent tube into the center tank."We are and have been aware of the potential scenario," said Bernard Loeb, chief of the board's aviation safety division. "It is one of the potential ignition FAA officials did not quantify how many airplanes were affected, only that "numerous" instances of chafing were found. Gardner noted that Japan Air Lines reported problems with 14 of their 29 older 747s.
Under the FAA's calculations, a hole in the wiring protection could have been hidden under fuel -- and thus unable to cause an explosion -- as the plane climbed. After leveling out at 13,000
An exchange of letters between the FAA and the NTSB yesterday illustrate their disagreement on fuel tank safety. The FAA has maintained that the only way to prevent explosions is to prevent sparks in fuel tanks, while the board says that more must be done to prevent the buildup of explosive fumes. A June 27 FAA letter, released yesterday, promised cooperation with the board in seeking solutions, but an NTSB reply expressed disappointment that the FAA is not taking immediate operational steps to hold down explosive vapors such as filling tanks with cooler fuel before takeoff. CAPTION: ONE POSSIBLE THEORY The Federal Aviation Administration's discovery of faulty wires in some Boeing 747 fuel tanks lends support to one possible explanation the agency developed for the crash of TWA Flight 800: An electrical problem in a wing fuel tank could have sparked a "flame front" that burned through the tanks' vent system and eventually detonated the center fuel tank. Here is how such a chain of events might have occurred: 1 As plane climbs, air and fumes expand and travel down air vent to vent surge tank. 2 Tank #3 feeds fuel to three engines while the plane accelerates and gains altitude. As the fuel level drops, air is sucked into the tank. 3 As the fume-laden air venting from the center tank reaches the vent surge tank, it gets pulled into the air vent toward fuel tank #3. 4 A spark from an exposed fuel pump wire in tank #3 ignites fumes. A flame front forms and follows the fumes through the air vent in the opposite direction as the oncoming air flow. 5 The flame, traveling against and faster than the air flow, makes its way back to the vent surge tank and back to the center fuel tank. Tank explodes.
