Concorde's safety modifications

Since an Air France Concorde crashed in July 2000, killing 113 people, thousands of hours and £17m have been spent preparing the remaining supersonic airliners for a return to service.

The sleek white aircraft were once a common sight around Heathrow, but when Captain Mike Bannister takes Concorde Alpha Foxtrot climbing over west London on Tuesday afternoon, the supersonic jet's progress will provoke the interest of even blasé locals.

Only six fuel tanks have been deemed at risk

The three-hour "verification" flight will be the first time a British Concorde, modified to prevent a repeat of the fatal accident which befell flight AF4590 in July 2000, has taken to the skies, and the first supersonic flight of any of the remaining airplanes since the crash.

Along with British Airways (BA) and Civil Aviation Authority technical staff, Alpha Foxtrot will be carrying the hopes of Concorde's British and French operators that the veteran aircraft will soon return to full service.

BA, owner of seven of the 30-year-old airliners, has had 270 engineers working round-the-clock shifts to strengthen vulnerable points on Alpha Foxtrot's underside.

As it sped along the runway at Charles De Gaulle airport in Paris, one of flight AF4590's wheels struck a 16-inch piece of metal, shredding the tyre.

Debris from this blow-out pounded the nearby left wing, sending reverberations through the fuel tanks powerful enough to punch out an A4-sized panel. Through this rupture escaped a deadly stream of ignited aviation kerosene.

Each Concorde requires 150 tank liners

To lessen the effect of a similar impact, six of Alpha Foxtrot's 12 fuel tanks have been fitted with flexible Kevlar liners (tests suggested the other tanks were not at risk of debris strikes). The synthetic, nylon-like material - not available when Concorde was designed - is so strong it is used in bullet-proof vests.

BA hunted out its smallest technicians for the operation, since fitting the new 150 liners per aircraft involved entering compartments as narrow as 12 inches across.

Despite its incredible properties, ground tests have shown Kevlar will not prevent the fuel tanks being breached but will restrict the amount of fuel able to escape, says BA technical spokesman Pete Middleton.

Testing times

An estimated 100 litres of fuel per second gushed from AF4590. Tests in which rubber projectiles were fired at a Kevlar-strengthened tank created holes only big enough to release a mere one litre per second - the flow also stemmed by the liners' tendency to "sag" into any rupture.

"The physics of aviation kerosene mean it will only ignite if a certain quantity is present," says Mr Middleton. "With only a one litre per second leak there is no fire risk."

To make sure, engineers squirted a considerably larger stream of fuel through one of Concorde's mighty Rolls Royce/Snecma Olympus engines. There was no fire, says Mr Middleton.

New tyres can survive a high-speed puncture

It is not known what caused the leaking fuel from AF4590 to ignite, but as a precaution electrical cabling in Alpha Foxtrot's wheel recesses has been armour-plated so that flying debris cannot sever the wires, causing a spark.

The modifications to the Concorde fleet have cost as much as £17m, but have also added weight to the already fuel-hungry airliner.

A long-planned refit of the Concorde cabin has been altered to meet these new weight considerations. Lighter seats, interior walls and even toilets now see the aircraft at a lower weight then even prior to the safety modifications.

Alpha Foxtrot is also fitted with new Near Zero Growth (NZG) tyres, so-called because they stretch less than pervious Concorde tyres.

In development before the Paris crash, BA says these reinforced tyres - which have survived repeated high-speed puncture tests - are a "bonus" feature and not key in preventing another disaster or satisfying the safety authorities.

While modifications to Alpha Foxtrot are complete, work continues on the rest of BA's and Air France's fleet.

Captain Bannister's supersonic flight is intended to gauge whether these vital alterations have upset Concorde's delicately balanced flying characteristics.