Ageing luxury jet

One of the Air France Concorde fleet

For almost 25 years, Concorde has been the last word in airborne luxury.

Only 20 of the supersonic passenger jets were ever built, of which 13 are still in service, operated by British Airways and Air France.

The standard fare for a return trip from London to New York is £6,636 ($10,070).

Concorde facts

Development cost: £1.5bn

Total built: 20

Cruising speed: 1,336mph

Cruising altitude: 60,000ft (11 miles)

London - New York: 3hrs 30mins

In general the supersonic aircraft has had an impressive safety record, but some recent mishaps have raised questions over the plane's safety.

In January, one of the BA fleet made two emergency landings in 24 hours, while in October a piece of tail fell off in mid-flight.

Between August 1998 and July 1999 130 Concorde-related incidents were reported, although the Civil Aviation Authority later defended this as being no different to other commercial aircraft.

Most recently it was revealed that hairline cracks have been found in the wings of all seven models operated by BA.

On Monday, the airline withdrew one after it found the cracks had lengthened. But the other six remained operational.

Neither BA nor Air France have shown any inclination to pull their Concordes out of service, all of which were built between 1975 and 1980.

15-year flight extension

On the contrary, both hope to keep the planes flying for another 14 or 15 years. This is much longer than originally anticipated.

Although it is more than 30 years since the first Concorde flight, political wrangling over the plane's development meant it did not come into commercial service until January 1976.

Concorde carries about 100 passengers

At the time it was calculated the aircraft had a design life of 6,700 cycles - each cycle is roughly equivalent to a transatlantic flight.

As the average fleet age nears 24 years, that has recently been extended to 8,500 cycles, which, at the current rate of usage, means the planes have a life until 2006.

Speaking shortly before Tuesday's crash, Malcolm English, editor of Air International, said: "Extending the life of a plane is a common thing these days because of advances in inspection techniques and also the better use of inspection data."

The data in question deals with the stresses suffered by an aircraft due to the sheer strain of take-off, landing and cruising.

"By examining past maintenance records, you can predict how the aircraft will fare in the future and extend its lifespan accordingly," said Mr English.

David Learmount, of Flight International magazine, pointed out Concordes do not fly as often as other standard passenger planes.

"The thing that causes metal fatigue is not age but usage. Concorde is used only about a quarter or a fifth the amount of other planes. It's a bit like saying this aircraft is not 25 years old, but five years old," he said.

1,000-plus mph

Its phenomenal cruising speed - 1,336mph - puts additional strains on the frame. But this also works to Concorde's advantage.

The historic maiden flight

The skin of the plane gets so hot in flight that outside moisture instantly evaporates against it, so corrosion is not a problem. On sub-sonic aircraft, moisture freezes to the skin before melting into the structure.

Also speaking before the Paris crash, Mr Learmount said it made commercial sense for sense the airlines to keep Concorde going.

Although Concorde makes up just a fraction of BA's overall fleet, it does wonders for the company's prestige, he said.

"They get a huge number of prosperous American travellers who will not travel by any other means."