TWA 800 inquiry lost in a labyrinth

The waters off Long Island promise to be smooth this evening, pressed into silken submission by a heatwave typical for mid-July. They were that way on this night one year ago; then, however, they were also on fire.

It is the first anniversary of the crash of TWA Flight 800 and still we are asking the same question we asked that night as, glued to our television sets, we watched the flames dance on the ocean surface. What happened aboard that Boeing 747 - an aircraft with an unparalleled safety record - to have brought it down? A mammoth investigation still goes on, which, when it is done, will probably cost $50m (pounds 31m). The FBI at one point had 700 agents assigned to it. Some 95 per cent of the aircraft, its white-and-red-liveried body twisted and torn, has been recovered and a 90-foot section has been reconstructed.

And yet, as those most closely touched by the catastrophe - relatives and friends of the 230 who perished, Navy divers and rescue workers - gather over the next few days for memorial and remembrance services here, the balm of what grief counsellors might call "closure" is still missing.

It is a continuing mystery that also offers cause for unease for all of us. Until we understand what befell the plane, how can we know what we should be doing to stop it happening again?

Some progress has been made but none of it is especially reassuring. The earliest assumptions pointed to sabotage. In a country just recovering from the Oklahoma bombing and the attack three years earlier on the World Trade Center, to incline that way was not surprising. There was the simple bomb theory, spurred by memories of Lockerbie. More exotic was the notion that the climbing aircraft had been ripped open by a surface-to-air missile. Now, however, those scenarios are fading, principally because of the absence of any evidence of an an incendiary explosion among all the parts of the plane recovered.

The FBI only recently admitted that it has almost exhausted its leads and will probably conclude its part of the inquiry in two to three months.

That points to the other, perhaps more worrying, conclusion: that the aircraft, which was 25 years old and a veritable pensioner among commercial jets still flying, suffered some mechanical mishap.

For months the focus of the mechanical investigation has been the central fuel tank. It is now thought certain the plane broke into pieces when the tank, which was nearly empty at the plane's take-off from John F Kennedy airport, exploded. What, though, sparked that explosion? Experts this week began test flights out of JFK using a 747 of similar vintage to the one that crashed and which has been laced with 150 sensors to measure conditions on board.

Most importantly, they will monitor variations of temperature and movement in the fuel tank to understand how much energy would have been needed to cause an ignition.

A variety of possible causes are under scrutiny, ranging from sparks perhaps created by chafed wires connected to a fuel pump to, more improbably, the creation of an electro-magnetic field by passengers turning on devices like lap-top computers.Investigators also want to study the role played by air-conditioning units adjacent to the tank in heating the fuel vapours inside it.

If a culprit is identified, the consequences for the industry may be far-reaching, especially if age is determined to have been a factor. Of all the jet aircraft built, 80 per cent are still in service, many flying beyond their original life expectancies.

"Boeing and other manufacturers are claiming that as long as you properly maintain them you can run these planes for ever," said Vernon Gross, a former official of the National Transportation Safety Board. "That's a joke."

Even now, a legacy of the crash is forming. Spurred by the early sabotage theories, the government has begun putting in place new security arrangements at airports, including a requirement that bags always travel on the same aircraft as their owners. New procedures have also been designed to provide better support for friends and relatives bereaved when accidents occur.

As frustration with the length of the investigation grows, it is worth noting that solving such crashes is rarely easy. It is nearly three years, for instance, since a USAir 737 ploughed into woods close to Pittsburgh, killing all on board. The investigation is still going on.