Experts will focus on the 'catastrophic' failure of two engines

Two British accident investigators were sent to the crash scene outside Paris yesterday to take part in an investigation that will focus on what appears to have been the "catastrophic" failure of the two engines on Concorde's left wing, which one eyewitness said occurred soon after take-off.

Two British accident investigators were sent to the crash scene outside Paris yesterday to take part in an investigation that will focus on what appears to have been the "catastrophic" failure of the two engines on Concorde's left wing, which one eyewitness said occurred soon after take-off.

Among the possible causes could be one of the fanblades inside the engine breaking spontaneously - which would tear apart the rest of the mechanism in seconds, causing a fire; or a bird may have been sucked into one of the Rolls-Royce Olympus 593 engines as the plane climbed after leaving the ground.

"Bird strikes" are a serious, if rare, hazard: worldwide, since 1912 they have caused at least 200 deaths and the destruction of nine civil, many general aviation, and at least 65 military aircraft.

Sid Hare, an American pilot who witnessed the crash from a nearby hotel, told CNN: "I knew it was in trouble. There are four engines on the Concorde and on the left side one of those obviously had a catastrophic failure.

"It was trailing flames, 200 to 300 feet behind the airplane. It probably wiped out the other engine next to it, so the airplane was then trying to climb on only two out of four engines and it just couldn't gain altitude. He kept trying to get the nose up to gain altitude, which eventually caused a stall, the nose pitched straight up in the air, and the airplane just started rolling over and back-sliding down toward the ground."

The crash, he said, had been "a sickening sight. When it hit, there was just a huge fireball, like a mini atomic bomb went up, and it's really just a sickening sight."

Aviation expert Paul Beaver of Janes, a defence information group, said reports that the plane began to rotate on the air before hitting the ground made a bird strike sound more likely. "A bird strike, causing a fire which then burned through flight-control cables, and therefore prevented the pilots from keeping the plane under control, would have that effect," he said.

While the remaining two engines should have been enough to lift the plane, Concorde has a reputation for being difficult to handle on take-off, Mr Beaver explained: "Concorde is an aircraft which climbs with a remarkable amount of power and speed. The engines are reheat engines, which are more akin to a fighter aircraft than an airliner and it is a difficult aircraft to control."

The task of piecing together the aircraft's last minutes could take months. French air accident investigators will begin looking at once for the "black box" flight recorder - actually bright orange - which records instrument readings during the flight and would have been at the back of the plane.

They will also search for the cockpit voice recorder that records sounds from the cockpit where the pilots would have struggled to regain control of the damaged aircraft.

The wreckage will be located, documented and then pieced together again at a separate location to try to understand how the accident occurred.

French law dictates that the investigation will be led by a local magistrate. However, the British investigators from the Air Accident Investigation Branch of the Department of Environment, Transport and the Regions will have equal powers to the French investigators.

As Concorde was an Anglo-French project, the two countries signed a memorandum of understanding in 1978, which allowed their investigation authorities the right to take part in inquiries into such incidents.

There are obvious key questions that any investigation will have to address as a matter of course, such as pilot error.

As with all aircraft the take-off is the key moment in a flight when passengers and crew and the plane itself are most vulnerable. This is the time when pilots use all their experience to ensure that anything that goes wrong in the moments before lift-off can be rectified. Investigators will also be keen to find out whether one of the crew members, particularly the commander, became ill, or possibly collapsed, during the key moments of take-off.

Recent revelations that British Airways' Concordes had developed cracks in the wings will also be something that investigators will take into account.

BA has taken one of its Concordes out of service because of the problem and investigators will want to see whether the Air France Concordes had also been hit by these difficulties. However unlikely it may be, investigators will also have to consider the possibility of terrorism, if only to rule it out. In an age of international terror, Concorde would be a high-profile and symbolic target for any disaffected group.