Ramon Lopez/Washington DC
On 17 July, as the first anniversary of the Trans World Airlines Flight 800 crash passed, US Government and industry officials were still waging a war of words over what lessons have been learned and what, if anything, should be done as a result of the accident. Meanwhile, friends and relatives of the 230 passengers and crew who died when the Boeing 747-100 exploded and fell into the Atlantic Ocean off Long island looked on with increasing frustration.
The issue of improved fuel-tank safety, advocated by the US National Transportation Safety Board (NTSB) since it was determined that an explosion in the aircraft's centre-wing fuel-tank had caused the aircraft's structural failure, was politely discussed in a Congressional hearing on 10 July. It was also the subject of "background" press briefings by senior aviation safety officials and airline industry executives who refuse to be identified.
In the year since the accident, the NTSB, Federal Aviation Administration and Federal Bureau of Investigation (FBI) have mounted the most extensive and expensive investigation in aviation history, but still do not know why the 747's centre-wing tank exploded. Investigators have not found the ignition source, and six theories remain under investigation.
"There is no evidence of a bomb or a missile impact in the wreckage," says NTSB chairman James Hall. James Kallstrom, chief of the FBI's New York office, has testified: "All our efforts to date have failed to uncover any credible evidence that the loss of Flight 800 was the result of a criminal act."
The criminal investigation continues, but Kallstrom hopes to end the probe within two-to-three months. Meanwhile, Hall plans to hold a public hearing on the accident in December, once the criminal probe id officially closed.
Of the six scenarios, four involve mechanical or electrical problems and two, criminal acts, but those are considered unlikely. NTSB investigators are examining each, using experiments and flight tests.
The fuel-tank's scavenge pump was never recovered, and investigators are trying to determine whether this type of pump might have overheated or suffered an electrical surge. Researchers, meanwhile, are investigating whether static electricity from an ungrounded component in the tank would ignite a fuel/air vapour mix.
The third scenario involves an electrical shortcircuit in wires outside the centre-wing tank that led to a spark or overheating of components inside it. Other investigators are examining whether the fuel vapour could have been 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 wing tank.
Meanwhile, the NTSB continues to investigate the possibility that a small explosive charge went off near the centre-wing tank, igniting the fuel/air vapours. Later this month, investigators will set off small explosive charges in locations around the centre wing tank of the 747 which, in May, was used by the FAA and the UK Defence Evaluation and Research Agency for baggage container explosive hardening tests.
Meanwhile, the NTSB has begun a series of ten test flights using a 747 leased from Evergreen, and fitted with temperature and pressure sensors, vapour sampling equipment and associated instrumentation. On 14 July, it took off from New York Kennedy Airport on a mission to repeat the conditions of the tragic flight of a year before. The tests are intended to determine the temperature profile and chemical composition of the fuel/air mixture in the centre wing tank.
Recommendations from the NTSB, if fully adopted, could lead to costly changes in the way the FAA certificates commercial aircraft fuel tanks. These include the installation of nitrogen inerting systems, additional insulation between heat-generating components and tanks, and revised fuelling procedures.
Existing FAA certification standards are designed to ensure that the ignition of fuel vapour by lightning strike, hot components, or component/systems failure is precluded. "Any (additional) measures taken to address these safety concerns can produce uncertain safety benefits, which may in fact decrease safety," the FAA says, and it has called for further research before decisions are made. Ultimately, the FAA argues, the solution is to eliminate all ignition sources, since it is "not possible" to eliminate potentially explosive fuel vapour from the tanks. The NTSB, however, also wants control or elimination of flammable vapours.
Hall and the airline industry agree that the safety record of the Boeing 747 is good and that fuel-tank explosions are extremely rare. Hall believes, however, that "extraaordinary steps may need to be taken to prevent similar accidents."
An airline executive says that costs associated with the NTSB's "radical" recommendations, which would affect all fuel tanks in all transport-category aircraft, are staggering. He says that installation of nitrogen-inerting technology alone, which introduces its own risks and is "operationally impractical", would cost US airlines $19 billion.
The airline industry says it will testify to the FAA that elimination of ignition sources is the best solution and that more research is required.
A senior FAA official says: "The TWA 800 probe is proceeding at the right pace…You must be patient in this business."
Source: Flight International
