A small "piece of metal," lying unnoticed on the runway used by the Air France Concorde that crashed last month, appears to have set in motion a chain of events that quickly brought the plane down in a blaze of burning fuel, French investigators said yesterday.

The metal piece--described no further by the investigators--apparently slashed into a tire as the Concorde neared takeoff speed at Paris's Charles de Gaulle Airport on July 25. The tire burst, sending fragments ramming through one or more of the seven fuel tanks in the left wing of the supersonic aircraft, investigators said.

That resulted in "very considerable leakage of fuel" and fire, investigators said in a brief statement yesterday. A spectacular 100-foot-long sheet of flame trailed the crippled jet as it crashed into a small hotel just beyond the airport, killing all 109 aboard the plane and four in the hotel.

The French Transport Ministry's Accident and Inquiry Office said it had not yet determined where the piece of metal came from but said it matched the profile of a cut in the ruined tire. Much work needs to be done before the investigation is complete, it said, noting that the exact sequence in which flying tire fragments damaged parts of the plane remains unclear.

The statement did not address what exactly brought the plane down--the fire or some other damage. Flight data recorders show that one of the plane's four engines failed and that another lost power, indicating the plane suffered damage other than punctured fuel tanks, possibly ingestion of debris by the engines.

Problems with blown Concorde tires have been reported since the first of the supersonic passenger jets went into service in 1969. The Concorde lands and takes off at higher speeds than subsonic planes, placing the 47-inch-diameter tires under such extreme stress that a blowout can have enough explosive force to damage engines and other nearby structures, such as fuel tanks. Blown tires on subsonic planes can cause damage, but generally do not have enough energy to pose serious danger.

The French accident office said the Concorde was rolling down the runway at 170 knots--about 195 mph--when the tire burst. Concorde takeoff speed is about 220 mph.

Peter Duffey, one of the eight original British Airways Concorde pilots, noted in his 1999 book "Comets and Concordes" that takeoff speed at maximum weight "is near to the maximum which the tread carcass can stand. Should the aircraft be allowed to overspeed on the runway, one or more of the tires may be 'cooked,' weakening them for subsequent takeoffs."

Duffey said a Concorde tire once burst as he was landing the aircraft, disabling the plane's hydraulic systems. He said he did not know that the leakage of hydraulic fluid had left him with no brakes until, while the plane was taxiing, an alert maintenance man saw a trail of fluid, ran out onto the taxiway to wave him down and signal he should coast to a stop.

"We were within a few moments of making a very sharp-nosed entrance past the gate right into the main terminal building," he wrote.

The French findings also raise an issue that requires constant vigilance by airports and airlines everywhere--"foreign object debris," usually called FOD. Runways are regularly inspected for foreign objects, and waste cans marked "FOD" are often placed near airport gates and maintenance shops so that employees can toss in anything that could damage an airplane tire, be picked up by an engine or even cause people to trip. At some airports, airlines have promoted anti-FOD campaigns with posters and bulletins.

The French statement did not describe the object found on the Paris runway, calling it only a "piece of metal." In other communiques, however, investigators noted having found a 16-inch-long strip of metal that did not come from the Concorde.

"After comparison with the profile of the cut in a tire that was destroyed at the start of the accident, it is probable that the piece is at the origin of the cut in the tire," the statement said. "Expert analysis will attempt to confirm this point definitively. Determination of the origin of the [metal] piece is still underway.

"The bursting of the tire caused the projection, at a moment the aircraft was moving at high speed (around 170 knots), of large fragments (more than four kilograms for the heaviest) and with high energy.

"In a process that remains to be determined, damage was then caused very rapidly, to one or several left-side fuel tanks causing a very considerable leakage of fuel and fire. The piece of fuel tank that was found on the runway has been identified as coming from tank No. 5."

The statement noted that each of the large delta-shaped wings of the Concorde has seven tanks. Tank No. 5 is located almost directly over the left landing gear, stretching slightly forward and to the left of the gear.

Although the statement did not say so, it is significant that the tank is in front of of the left wing's two engines. Any spray of fuel would tend to be be ignited as it flowed back through the flame produced by afterburners that give the engines extra thrust on takeoff.