David Learmount/LONDON

The metal strip found on the Paris Charles de Gaulle airport runway which caused a tyre explosion leading to the crash of the Air France Concorde on 25 July came from a poorly executed repair to a Continental Airlines McDonnell Douglas DC-10 thrust reverser, Flight International can reveal.

The metal was titanium, not a soft alloy as originally announced by French investigators, and the thrust reverser makers have confirmed to Flight International that it was not an original equipment manufacturer (OEM) part.

After a main gear tyre ran over the titanium strip, the tyre exploded and ignited a wing fire, causing both port engines to lose power. Less than a minute later the aircraft crashed, killing 113 people and leading to the suspension of the aircraft's certificate of airworthiness.

Continental has acknowledged that the metal strip - a wear-strip or an accompanying backplate from the fan thrust-reverser mechanism on one of the DC-10's engines - came from an aircraft which departed Paris two take-offs before Concorde.

Middle River - the Maryland, USA-based, General Electric-owned manufacturer of the thrust reverser assembly - says the unit is a "...pretty old piece of hardware, and we have no way of knowing what happens to it after it leaves the shop. We are lucky if [the operators] follow our recommendations a year after it leaves here." The wear-strips, have to be replaced more often than other reverser parts, it is believed.

The company confirms that the original 1979 design contains a titanium wear strip, but the supplier manual says operators have a choice of replacing it with an OEM part, or a locally manufactured substitute. The strip on the Paris runway, says Middle River, was not an OEM part.

A preliminary report of the French Bureau Enquêtes-Accidents (BEA) gave the first clues to the unusual nature of the strip. It describes its engineering as: "Its width varies from 29 to 34mm and it has drilled holes...the holes are not at regular intervals". The report also notes that the coating on one side was appropriate "for hot sections". A fan thrust reverser is not a hot section system.

The strip, which the original BEA report says "appeared to be made of light alloy", is in fact made of titanium, UK and French parties to the investigation were told at a meeting in London on 12 October. Investigators had been puzzled how a soft alloy strip could have sliced into a Concorde tyre as it did. Efforts to identify the coated metal or its origin had been blocked because it had been commandeered by the magistrate in charge of the investigation for dispatch to an independent laboratory.

At least one source believes the strip is a backplate used on the reverser to secure a loose wear strip.

Who fitted the metal strip remains a mystery. Tel Aviv, Israel-based company Bedek says that it carried out C-check heavy airframe maintenance on the DC-10 in April, but that it is not certificated for work on the aircraft's General Electric CF6 series engines. Since that time, says Bedek, Continental has carried out A-check and B-check work itself. The last check is believed to have been undertaken on 9 July by Continental.

The US airline declined to comment on the story.

Lawyers representing relatives of the German passengers killed in the accident say that they are pursuing a "discovery action" against Continental to find out about the DC-10's repair history. John Howie, a Texas-based specialist aviation lawyer assisting in the case says he does not believe the metal part is a genuine wear strip.

"The part appears to have a non-standard rivet pattern. This is not a machine-drilled rivet pattern, the irregular pattern suggests that whoever traditionally does this didn't do this part."

Meanwhile airline sources say that a fix under consideration for Concorde to ready the aircraft for certification is a floating, flexible Kevlar sheet in the tanks which, if fuel started to escape rapidly from the wing underside, would block the hole to limit the flow to a rate which would not cause danger.

British Airways says that it believes Concorde may fly again by February 2002, but that any recertification delay beyond mid-2001 risks permanent grounding.

Additional reporting by Guy Norris in Los Angeles and Andrew Doyle in Munich

Source: Flight International