Report by Andrew Doyle, Max Kingsley-Jones, David Learmount, and Guy Norris/LONDON

As Flight International went to press French investigators were considering the possibility that a burst tyre started a sequence of events which led to the catastrophic fire which brought down the Air France Concorde soon after take-off in Paris on 25 July.

French investigators say a tyre burst late in the take-off run and rubber debris was found on the runway. According to the flight data recorder (FDR), investigators say, the No2 engine ran down shortly after rotation and subsequently stopped. The No1 engine ran down temporarily but then recovered full power.

The FDR also shows that the undercarriage failed to retract, suggesting the subsequent damage affected the aircraft's hydraulic systems. Sources say tyre bursts on the Concorde have previously punctured fuel tanks, resulting in British Airways fitting tyre deflator sensors. It is not known whether Air France followed suit.

By 27 July no evidence had emerged to support initial theories that a failure of the No2 engine triggered the fuel leak, although this has not been ruled out.

All 109 passengers and crew on board the aircraft and four people on the ground were killed in the first fatal accident involving a Concorde in its 30-year history. Air France's Concorde fleet was immediately grounded by the French Government, but British Airways (BA) resumed services the following day after cancelling two services in the immediate aftermath of the accident.

Fire took hold of the Concorde's port wing during the take-off roll, which began at 15.44 at Paris Charles de Gaulle (CDG) airport. The sources say the pilots were unaware of the problem until after rotation, when they received a fire warning in the cockpit at about the same time as a verbal warning was issued by air traffic controllers.

The pilot reported his speed was too fast to abort the take-off.

Several pilots awaiting take-off apparently saw fire erupt from beneath the aircraft but were unable to warn the Concorde crew because they were on different radio frequencies.

Investigators have confirmed that the pilot radioed he was attempting an emergency landing at nearby Le Bourget, but lost control of the aircraft and crashed into a small hotel at Gonesse. The aircraft was airborne for about 2min and travelled 7km (4nm).

The aircraft departed CDG at its 185t maximum take-off weight carrying more than 90t of fuel. The pilot reported a problem with the No2 engine shortly after take-off but investigators are examining the possibility that this may have been a consequence, rather than the cause, of the fire. Air France has confirmed that an inoperable thrust reverser on the No2 engine was repaired before departure.

Witness reports filed by pilots of a taxiing aircraft adjacent to the runway suggest that "fluid" was leaking from the aircraft on the runway during the early part of the take-off roll but had not ignited, according to the sources.

One possibility being considered by investigators is that debris thrown up by the port main landing gear punctured the wing and possibly a fuel line. At take-off Concorde's four engines consume around 1.5t of fuel per minute.

The investigators are trying to determine whether the fire severed hydraulic lines in the wing, causing the pilot to lose control of the aircraft. The ignition source of the fire has not been determined but the engines create a large amount of heat energy at take-off power.

Amateur photographs and video footage of the incident just after take-off show the fire extending about 50m aft from the trailing edge of the port wing. Pilots that witnessed the crash say the nose pitched up almost vertically and the aircraft rolled left, before falling to impact, tail low with very little forward speed.

The Concorde was chartered for a flight to New York by a German tour operator.

The oldest operational Concorde, the accident aircraft was built in 1975 and entered service with Air France on 23 October, 1980. It had accumulated 11,989 flight hours and 3,978 cycles. Its last "C" check was performed on 28 April this year, while a more extensive "D" check was completed in September last year. The aircraft spent six years mothballed during the 1970s and 1980s.

BA has achieved a much higher utilisation of its seven Concordes - 23,000 hours and 8,000 cycles.

The accident leaves Air France with five operational Concordes, plus a sixth airframe which has been cannibalised for spares.

Though micro-cracks reported two hours earlier were found in wing spar No 72 throughout BA's fleet of seven Concordes, and in some of the French fleet, Air France has already discounted this as a contributory factor. It says no cracks were found in the aircraft involved in the accident.

Manufacturers European Aeroautic Defence and Space and BAE Systems say the cracks pose no threat to the aircraft's safety, but BA has grounded one of its fleet because a crack continued to propagate. It is expected to be returned to service in September.

Source: Flight International