THE FIRST anniversary of the crash of Trans World Airlines (TWA) Flight 800 on 17 July, 1996, has been marked by sparring between the two main US aviation-safety agencies over appropriate action on fuel-tank-safety.

US Federal Aviation Administration officials and US National Transportation Safety Board (NTSB) investigators had hoped to present a unified position on a plan for preventing fuel-tank explosions such as the one suspected of causing the inßight break-up of the TWA Boeing 747-100 before crashing off the coast of Long Island, New York, taking 230 lives. The NTSB still does not know why the centre fuel tank exploded.

The FAA has so far failed to adopt fuel-tank-safety recommendations issued by the NTSB late in 1996. They include installation of nitrogen-inerting systems, additional insulation between heat-generating air-conditioning units and the centre-wing tanks, revised centre-wing-fuel procedures, centre-wing fuel-tank temperature limitations and modifying fuel tanks located near heat sources.

The FAA says that control of fuel temperatures would be difficult to implement and would not eliminate flammable fuel vapours. "There is significant doubt that any amount of fuel added to the centre-wing fuel tank, as recommended by the NTSB, will lower the temperature to the point that no explosion could occur," says the FAA.

The NTSB still hopes to solve the mystery, but says that the ignition source for the blast may never be established. Investigators, meanwhile, are examining whether the fuel vapour was ignited by a fire started by sparks from chafed wires in the right-wing tank, which then travelled through the tank-venting system to the centre tank.

Inspection of wiring inside the wing fuel tanks of 433 747s manufactured before 1980 found chafed wires which might have caused explosions. The FAA believed that the problem, first recorded in 1979, had been fixed.

In April, the FAA asked industry to comment by 1 August on the NTSB's recommendations. Boeing officials met 747 operators on 8 July to discuss fuel-tank issues. Flight-testing of a 747-100 is to follow and is expected to generate new data on fuel-tank conditions.

"Our certification requirement is based on the fact that you can't eliminate volatile or flammable vapours in fuel tanks. Therefore, you must eliminate all ignition sources. Obviously, in the case of TWA 800, we were not successful," the FAA says.


Source: Flight International