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Tuesday, 5 September, 2000, 08:41 GMT 09:41 UK
Concorde: What went wrong?
Investigators have not yet been able to pinpoint the exact cause of the 25 July crash of Air France Concorde flight 4590.
But the French Air Accident Investigation Bureau (BEA) did confirm in a preliminary report issued at the end of August that they were continuing to focus on a burst tyre triggering a chain of events that brought down the plane.
"The 25 July accident shows that the destruction of a tyre, an event that we cannot say will not recur, had catastrophic consequences in a short period of time, preventing the crew from rectifying the situation," the BEA said.
The investigators also confirmed that a 40cm (16in) metal strip, probably from another plane, had been found on the runway at Roissy-Charles de Gaulle airport.
US-based Continental Airlines has said one of its DC-10 aircraft was missing a metal piece similar to the one found.
Investigators said the metal piece fitted the shape of a cut in one of the tyres.
However the report said that the sequence of damage and the links between the various events had not yet been fully established.
The report does not identify whether the fire broke out in one of the plane's engines, or started in the main wing fuel tanks.
Early theories suggested that the fire might have been caused by debris from a burst tyre puncturing the fuel tanks at high speed causing a stream of highly inflammable kerosene to pour out.
Neither does the initial investigation answer questions about the Concorde undercarriage's failure to retract and the problems in two engines.
Sequence of events
Experts began their investigation by recovering the "black box" flight data recorder and cockpit voice recorder.
Thanks to their safe recovery, the unusual amount of eyewitness testimony and even an amateur video, this is what they quickly established:
What caused the fire?
Following early speculation that there had been an "uncontained failure" of engine parts - effectively a jet compressor turbine shattering - the investigators moved on to looking at how the tyre blow-out could have led to the rupture of one of the fuel tanks in a high-velocity impact.
The US's National Transportation Safety Board (NTSB) released details of four incidents in which Air France Concorde's had blown tyres on take-off.
In each of the incidents which took place between 1979 and 1981, the NTSB said that tyre debris and wheel shrapnel damaged one of the engines, punctured wing fuel tanks and severed hydraulic control lines.
The board went on to recommend that each of Concorde's tyres and wheels should be carefully inspected before each take-off.
Once the cause of an air accident has been established, air investigators move onto examining what role the design of the aircraft had in the incident.
Before this accident, Concorde was widely regarded as having been one of the most carefully designed aircraft in the world.
Safety testing of Concorde took years to complete as designers and engineers went to great lengths to make supersonic passenger travel a reality.
For instance, the twin-mounted engines positioned under each side of the delta wing are separated by a relatively thin but strong wall.
This is designed to protect a fully operational engine from damage caused to its neighbour.
Concorde was also designed to ensure that areas most likely to be affected by external damage such as the break-up of an engine are clear of essential mechanical parts.
The aircraft's three hydraulic command systems were also designed to operate independently of each other.
In theory this means that the aircraft can still be piloted even if one of the systems had been knocked out.
So why did it take off?
Given that the disaster began on the Paris runway, one of the key questions that the relatives of the dead have asked is why did Concorde AF4590 take off at all?
Former pilots say that Concorde is "very tough" and has an extremely large capacity to generate additional power when needed.
However, the pilot was aware of problems but had given the "V1" signal - the point beyond which take-off cannot be aborted.
Once in the air, he was attempting to stabilise the aircraft by increasing power and lift (the relationship between an aircraft's speed, wing design and angle of flight).
Had he done so, Mr Marty would have attempted to pilot on the fully-operational engines to the allotted emergency landing area at nearby Le Bourget Airport.
Pilots agree that this is the best approach to this kind of emergency.
Shortly before take-off, pilot Christian Marty overruled an earlier decision not to replace part of a faulty thrust reverser on the number two motor, the engine under the port wing nearest the fuselage.
The repair is reported to have taken around half an hour. It was the same engine which Mr Marty then reported to be at fault following take off.
While this has formed part of the investigation, it is unclear whether this played any exacerbating role in the unfolding disaster.
While establishing the facts may appear an almost impossible task, some of the world's best aviation experts and aviation engineers have studied Concorde for years.
It was this approach of painstakingly piecing together almost all of the Pan Am airliner blown up over Lockerbie, Scotland, in 1989 which proved that a bomb had been planted in a suitcase on board the aircraft.
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