MAX KINGSLEY-JONES / LONDON HEATHROW
After an intensive modification programme, Concorde has returned to the skies, and BA engineers are working to ensure it remains in there for at least another decade
When news began filtering through the chalets at last year's Farnborough air show of an Aerospatiale/BAe Concorde crash near Paris, a dark mood fell across the event. People spoke of little else for the rest of the week, despite the fact that the 100-seat supersonic transport has little relevance to today's airliner manufacturing industry.
Until the crash, the aircraft's two operators had been confidently predicting another 10 or 15 years of Concorde operations. Now some were asking: "Is this the end for Concorde?" And if not, would it result in a truncated service life for the type? Air France's five remaining Concordes were grounded immediately after the 25 July crash, and three weeks later British Airways was forced to ground its seven-strong fleet when the French and UK civil aviation authorities took the almost unprecedented step of withdrawing the airliner's certificate of airworthiness (CofA). By then, a sketchy picture was forming of the circumstances which brought the aircraft down, and there were observers, including some within the two airlines, who believed that Concorde had flown its last commercial flight.
By the end of 2000, it was established that the crash had come as result of a complex chain of events that started with a main gear tyre burst during the take-off run beyond V1, after the aircraft ran over a titanium strip. The disintegrating tyre carcass struck the underside of the wing, rupturing a fuel tank and causing its contents to escape at a flow of 100litres/s (26 USgal/s). The fuel ignited, causing a huge fireball. The crew's ability to control the aircraft in this precarious state was then hindered by the engines losing power as they inducted the escaping fuel vapour. Around 60s after lifting off the runway, the Concorde crashed in the Paris suburb of Gonesse, killing all 109 passengers and crew on board, and four people on the ground.
As the investigation gathered momentum, it became clear that the two operators faced the choice of either making major modifications to their Concorde fleets to eliminate this Achilles heel, or accepting that the era of supersonic passenger travel had ended after 25 years. The former option was chosen, and less than 15 months after the crash in Paris, modified Concordes were back plying the North Atlantic for Air France and BA.
Aggressive schedule
"Probably the most impressive aspects of the programme have been the high levels of concurrent engineering and testing undertaken," says BA Engineering technical and quality director Jim O'Sullivan. "A four-stage process - investigating the accident, designing and testing the modifications, stripping the aircraft for installation and manufacturing the kits - was undertaken simultaneously. If you laid the programmes out nose-to-tail you are looking at two-and-a-half to three years - we did it in 15 months," he says.
The bulk of the programme developed for the return of the CofA by Concorde's manufacturers BAE Systems and EADS involved the installation of a "passive modification" which did not interfere with aircraft systems. This involved the fitting of over 100 Kevlar/rubber liners to each aircraft between wing ribs in the fuel tanks. These are designed to reduce the fuel escape rate after a rupture to around 0.5litre/s.
Other changes involved the installation of a stainless steel flexible "armour" in the undercarriage bay to protect the only high-power electrical cables associated with the gear - the brake fan supply - which are now automatically isolated during the take-off run. New lighter, near zero growth (NZG) radial tyres developed by Michelin have also been mandated, which are far less susceptible to foreign object damage (Flight International, 6-12 November).
BA has invested £17 million ($25 million) in the safety modification programme, while another £14 million is being spent on a new interior for the aircraft, which was already in development before the crash. Phase 1 of the new interior - seats, curtains and carpets - has been installed during the modification programme. Phase 2, which comprises new galleys and vacuum toilets, will follow in 2002.
"Inevitably [the truncated development programme meant] there were some false starts," says O'Sullivan, which was why the modification of the first aircraft took six months - far longer than originally envisaged.
BA was tasked with leading the installation programme, which it began at its London Heathrow facilities on Concorde G-BOAF in January. The first major problem to emerge was that the individual build of each aircraft had not been allowed for. "Every Concorde was effectively hand-built, so we could not install a standard liner size for the same position on each aircraft," says O'Sullivan.
This factor was further hampered by the toughness of the Kevlar liners, says O'Sullivan. "We had anticipated that we would be able to do any rework on the liners locally, but they were so strong that that had to go back to the manufacturer when alterations were needed."
The overlapping, testing, development and installation of the modifications took a further twist after a third of the liners had been installed in the first G-BOAF. "There are small holes in the liners to allow fuel to circulate between them and the wing skin to facilitate Concorde's skin cooling process, and they were too big," says O'Sullivan.
The hole size is a compromise between being large enough to allow enough fuel to circulate, and small enough to prevent excessive flow rates in the event of a leak. "The liners were removed, patched and then had smaller holes drilled in them," says O'Sullivan.
Flow rate tests were conducted on the liners, while in Shoeburyness, UK, a Rolls-Royce/Snecma Olympus engine (Concorde's powerplant) was used to establish that it could withstand ingestion of fuel at a rate of around 0.5litres/s without sustaining damage.
Impressive testing
While BA and Air France were preparing their fleets for return to service, other tests were being undertaken to understand the circumstances surrounding the accident and verify the modifications. Air France and Michelin worked together to verify the robustness of the NZG tyre, while in the windtunnel at BAE's plant in Warton, UK, a full-scale mock-up of Concorde's main landing gear bay and underwing area was used to establish what levels of fuel leak would be ignitable. This test involved an ignitor plug from an engine to provide the ignition spark.
"The level of testing was very impressive, as were the results - each one came back with grade A marks," says O'Sullivan. "Nothing like this had been done before - we had to invent the tests from scratch," he says.
Around 150 BA engineering staff have been employed on the modification programme, with engineers that were tasked with the installation of the liners in the cramped and claustrophobic fuel tanks being "hand picked from volunteers", says O'Sullivan. "Installation in some of the tanks was very, very slow, as people could only work for a limited time."
BA flew the first modified Concorde on 17 July, and tests proved that it was meeting or bettering predictions. Concerns about the liners hampering fuel cooling proved unfounded: "It was a non-event," says O'Sullivan.
On 5 September, the UK and French authorities issued the airworthiness directives for modifications to secure the return of the type's CofA.
With weight critical on Concorde - "every pound of weight equates to a pound more fuel", says O'Sullivan - BA engineers were concerned about the effects of the modifications on performance.
The Kevlar liners add around 400kg (880lb) to the aircraft's empty weight, and result in around 400kg of unusable fuel capacity, explains O'Sullivan. "Against that the NZG tyre is lighter than the old one, and we will also have a lighter interior when the second phase is installed, so we will not be that far from where we were," he adds.
Since returning to service, BA's Concordes have each shed between 100 and 200kg, as moisture that had collected in soft furnishings and the insulation during the year or so on the ground "burned off". The aircraft are consuming around 250kg more fuel per trip between London and New York than before modification, but the difference will be negligible once Phase 2 of the interior revamp is completed.
"There will be several weekends every year where we will be capacity limited on New York flights due to prevailing winds," says O'Sullivan. The London-Barbados service, which operates once a week in the winter schedule, is at the edge of Concorde's payload/range envelope and has a limit of 80 passengers as it did before the modifications. Occasionally, in adverse weather conditions, technical stops will be required in Shannon or Lisbon.
O'Sullivan says that, after studying performance from early flights, BA is happy that there are no "single sector events to worry about" due to the modifications. However, for the first 50 flights, G-BOAF is flying with two passenger seats removed to allow temperature and strain gauges to be carried, to gain a clear picture on what if any long-term effects the modifications will have on performance and airframe fatigue life.
Modifications
The second, third and fourth BA aircraft have been modified, with the process now taking about 60-70 days each. Three aircraft were available for the return of commercial flights to New York on 7 November, and the fourth aircraft will re-enter service in January. "This aircraft will provide more robustness in the scheduling, and give us the option of adding a second flight to Barbados," says O'Sullivan.
Modification of the fifth aircraft will begin next month, and should be completed in time for the start of BA's summer schedules in April. "This will allow us to begin double daily frequencies to New York," says O'Sullivan.
BA needs five Concordes available to run two daily transatlantic services, as it always has a back-up aircraft for every flight. The airline says it is experiencing "load factors in excess of 70%" on its transatlantic Concorde services so far against a break-even load factor which is believed to be 40% (in other words, 40 seats).
BA remains committed to the modification of all seven Concordes, but the timing of the last two has not yet been determined as the airline assesses the long-term impact of the 11 September attacks on its North Atlantic operations. "It is a case of when, rather than whether, we do the last two," says O'Sullivan.
The completion of the last two aircraft will give BA more flexibility with its fleet heavy maintenance programme, and help it accommodate the installation of the new interiors. Phase 2 of the interior installation is a major programme requiring around 20 days downtime per aircraft, which is why it was not undertaken prior to the return to service.
As part of the effort to extend airframe lives and limit utilisation, BA has no plans to re-introduce charter flights, which accounted for around 10% of operations prior to the grounding. This could change once the fleet is back to full strength.
Source: Flight International
