Modifications and tests have been defined to establish whether Concorde can fly again safely

David Learmount/LONDON Julian Moxon/PARIS

The main reason why the Air France Aerospatiale/British Aerospace Concorde became uncontrollable before its fatal crash, according to the latest investigators' report, is that the raging fire under its left wing had set up a chain of events that robbed the left engines of power, and had begun to damage the control surfaces. Regaining the aircraft's certificates of airworthiness, therefore, depends primarily on preventing such fires in the future.

The programme basis

On this basis, the two Concorde operators, Air France and British Airways, have been cleared to go ahead with an engineering and flight trials programme to fit and test a system which will prevent a high-rate fuel escape if the wing tanks rupture again. The system adopted is to install more than 100 Kevlar/rubber panels between wing ribs in the aircraft's fuel tanks which, had they been fitted to the accident aircraft, would have reduced the 100litres/s (26 USgal) fuel escape rate to 1litre/s, says BA Engineering technical and quality director Jim O'Sullivan. BA says the safety modification programme will cost it £17 million ($25 million).

The close correlation between the fire and engine power loss was not apparent in late August last year when the first of two interim reports from the French investigators, Bureau Enquêtes-Accidents (BEA), was published. Then it was believed that the tyre explosion, caused by a strip of titanium debris on the runway, might have projected matter into the engines, causing failure. Two new points emerged when the BEA's more recent inquiry progress report was published on 5 January.

First, in the 25 July accident, the engines had suffered only "soft-body" damage which would not have caused significant power loss. The crew shut down No 2 engine when its fire warning sounded 11s after rotation, but examination showed that there had been no engine fire. Meanwhile, tests suggest that the initial power dip in both engines resulted from ingesting fuel vapour and hot gases from the fire. The BEA report says that No 1 engine recovered 90% power before it ran down following further hot gas and fuel ingestion, and engines 3 and 4 lost power seconds before impact because of "slipstream distortion".

Second, it is believed possible that the ignition source of the escaping fuel may have been the engine reheat flashing back via the wing boundary layer - not the result of damaged electrical cables as originally suspected. Nevertheless, a stainless steel flexible "armour" is to be fitted to protect the only high-power electrical cables associated with the undercarriage - the brake fans supply, which is also to be automatically isolated during the take-off run.

Hydraulics, already armoured, were not believed to have been seriously affected in the accident, O'Sullivan says, because the gear doors began to move even though the gear would not retract. Gear retraction is electrically signalled, O'Sullivan explains, and the failure to retract is still being investigated.

BA's chosen test aircraft, G-BOAF, is forecast to be ready for flight testing within "eight to 10 weeks", says O'Sullivan, asserting that since the changes are a "passive modification" - not designed to change the aircraft's operation - the tests should be brief, just quantifying effects on weight/balance, fuel system operation and fuel quantity. The Kevlar liner material is denser than fuel, and likely to add 400kg (880lb) to the aircraft weight, but it takes up little volume, O'Sullivan says, adding that, when tests are complete, a long-planned cabin refit will replace the 100 old seats with new lighter ones.

Fitting the tank liners

The flexible Kevlar fuel tank protection sections are tailored to fit specific positions between the wing's internal ribs to which their preformed upturned edges will be bolted, held firmly by fixing plates.

High-speed taxi trials of an Air France Concorde began at the Istres flight test centre in southern France on 18 January, where the aircraft - F-BVFB - was flown from Paris last week. The trials by manufacturer, European Aeronautic Defense and Space (EADS), will take around 15 days .

The tests are aimed solely at validating the extensive computational simulations so far carried out, and to "establish the efficiency of the modifications in preventing catastrophic fuel leakages", says Air France executive director general Pierre-Henri Gourgeon.

A reservoir containing coloured water will be fitted in the cabin, and liquid released through four pipes to points under the left wing. During the taxi runs, at speeds of up to 180kt (330km/h) and with an aircraft mass of 110t, the water will be released from each pipe individually for around 7s, the resulting vapour flow around the engines and nacelles being filmed by cameras positioned on the landing gear. Each test will take a minute, the aircraft braking to a halt.

Parallel tests are to be carried out by the Centre D'Essais Aéronautiques de Toulouse. EADS says that 4kg objects simulating pieces of tyre will be shot at models of a Kevlar-lined Concorde fuel tank, and fuel leakage rates will be monitored.

Source: Flight International