Plot thickens in hunt for clues to TWA crash

Any explanation of the cause of the TWA crash a week ago which killed 230 people is going to be fairly unbelievable, filling a new chapter in the history of aviation disasters.

While the most far-fetched theories such as a missile attack or a meteorite can probably be ruled out, the accident and the ensuing explosion are unprecedented in aviation history. Any explanation, therefore, will have to show how such an unlikely event took place. Kieran Daly of Flight International magazine said: "We are now in the realms of what is plausible, rather than what is likely."

The two most plausible causes remain a bomb or an uncontained engine failure. And both are highly unlikely. The bomb theory has a number of drawbacks. First, there is the difficulty of getting a bomb with a timing device on to the plane at an airport - New York's JFK - which is one of the most security conscious in the world. The theory about a device having been put on board at Athens does not hold water since there was no reason why it should not have been set to explode on the first leg of the aircraft's journey.

Second, the devastating explosion means that either the terrorists hit very lucky or they had amazing technical knowledge and good access to the aircraft. Christopher Ronay, a former head of the FBI's bomb unit, who investigated 30 aircraft bombings until his retirement two years ago, says he cannot recall any similar incident."You could blow the hell out of a cargo compartment with a luggage bomb but you have to blow up a fuel cell or an engine to get an explosion like that."

Any mechanical failure which caused such instant devastation would also have to involve an engine and the fuel around it. The death of a woman and her child in a MD-88 when part of an engine disintegrated during take-off in Florida only a week before the TWA crash shows how destructive such accidents are.

Although the TWA aircraft was carrying some 48,000 gallons of fuel, the kerosene used by jets is not very explosive. It needs to mix with air which means that one of the tanks would have to be breached.

While there have been several instances of aircraft blowing up in flight, either through mechanical faults or because of bombs, none of the explosions has been as cataclysmic as last week's event. Even at Lockerbie, where a Pan Am Boeing 747 blew up at 31,000ft in December 1988, there was no fiery explosion until fuel-laden parts hit the ground.

In that case a small bomb, weighing just under 1lb, caused the explosion but the terrorists were very fortunate. If a bomb that size had been in the hold surrounded by luggage, the plane would have survived. Other aircraft have managed to land despite having holes punched in them by bombs, such as a TWA flight in 1986 between Rome and Athens when four people were blown out of the plane. Two similar bombings over the Pacific resulted in the deaths of single passengers, but the planes limped home.

Michael Barr, director of aviation safety at the University of Southern California, said yesterday: "These planes just don't blow up. There's too many fire walls, too many checks and balances."

Yet, explosion there was. Frank Taylor, director of Cranfield Aviation Safety Centre, says that burning fuel oil on the surface of the sea, as well as the television footage of burning debris falling from the plane shows there was an explosion.

While some reports of chemical traces suggesting a bomb were appearing in the US media, there is still no certain evidence and all the safety experts are being very cautious about coming to a firm conclusion.

The relatives of the dead on the TWA plane will probably have to wait until the discovery of the cockpit voice recorder - which will probably be more revealing than the separate flight data recorder - before the reason for the disaster is revealed.

But Mr Taylor is certain that the cause of the crash will be discovered: "It may take several weeks, but they will find out in the end," he said.