Experts: Planes Vulnerable to Bombs Built on Board

Challenge is finding the ingredients in screening, since they can be hidden in everyday objects

And the increasing probability that terrorists will try to strike with explosive components hidden in hand-luggage has been accompanied by a trend among discount airlines to encourage passengers to bring more carry-on baggage. In recent months Europe's market-leading airline, Irish budget carrier Ryanair, has imposed a mandatory charge on all check-in luggage; an Irish competitor, Aer Lingus, has announced plans to follow suit.

"I'm really surprised the Irish aviation authority hasn't stepped in to moderate this rush to hand luggage by airlines," said aviation expert Gerry Byrne. "All our airport security has been geared towards baggage going into the hold. ... It will overwhelm security if the emphasis is suddenly switched to hand baggage."

A British security expert, Steve Park, said the likely scenario would involve a two- or three-member terror team boarding the same flight, each carrying a different part of the bomb to be made. "They could combine resources on the plane. That would be perfectly possible on a busy flight," he said.

Critical to conventional bombs is a power source to trigger a detonator. Clonan said cell phones could provide an ideal power-timer unit for a bomb.

"In midflight you could go into the toilet, attach the mobile phone to the explosives and, as the plane makes a final approach over a densely populated urban area, you detonate it," he said. To puncture an aircraft's fuselage would require an explosive charge "half the size of a cigarette packet," he said.

Hatcher said "liquid bombs" were not the most likely explosive. He said it was far more likely that a terrorist cell would try to smuggle on board explosives in crystalline or powder form and to combine it with an acid-based compound.

Homeland Security Secretary Michael Chertoff warned Thursday of precisely that threat: "benign" materials smuggled on board and mixed to create bomb. He said authorities were analyzing to see how to protect against such a threat.

Hatcher said terrorists might also construct an on-board incendiary bomb based on paraffin or petrol, which if ignited in the mid-Atlantic could destroy an aircraft before it could land.

None of these items, he noted, could be detected by a typical US$5 million (euro4 million) X-ray. Hands-on inspection was the only way to tell if a dark-plastic medicine vial really contains what it says on the label.

"You'll have to carry your prescription and prove to security that the medicine really is what it is. But for 20 million people a year going through Heathrow? How do you do that?" Hatcher said, foreseeing a future airport arrivals hall with five-hour security checks.

And that scenario, he said, points to a future likely target for terrorists - detonating bombs in an airport terminal, not on a plane.

"You can carry a bag into the center of an airport with thousands of people around you before you are ever screened. That, too, must change," he said.