Andrew Doyle/LONDON
THE BRITISH AEROSPACE/Aerospatiale Concorde is a unique airliner in many ways, not least because it has been in revenue service for two decades and there is still no new aircraft available to replace it. For its operators, British Airways and Air France, the choice is simple: either keep the Concorde flying or abandon supersonic services.
In terms of flight cycles and hours, the aircraft are remarkably young, despite the physical age of the fleet. BA's lead aircraft, which entered service in January 1976, has accumulated just 6,800 cycles and around 18,000h. The youngest Concorde has completed fewer than 4,000 cycles.
BA operates each aircraft for between 300-350 cycles and 900-1,100h a year. For comparison, a Boeing 747 is likely to be in the air for 4,000-5,000h a year.
According to Dennis Morris, BA's fleet/project manager for the Concorde, the aircraft has also benefited from the fact that it is flown in the dry stratosphere, with its hull heated by the friction of supersonic flight. "The structure is very good because we see virtually no corrosion compared to that of a subsonic aircraft," he says.
APPROACHING LIMITS
The current airframe life is defined as just 6,700 "reference" flights, a limit, which the UK airline believes is conservative. A transatlantic supersonic flight with a take-off weight of more than 170t would count as one reference flight, whereas a shorter flight with a take-off weight of less than 120t would score 0.5. BA's lead aircraft has accumulated 5,900 reference flights, and so is approaching the current limit.
Air France, meanwhile, is not expecting its lead aircraft to reach 6,700 reference flights until 2005 because of its much lower utilisation rates, although the French flag carrier has indicated that it also plans to keep its fleet of six aircraft airborne for another 20 years. The programme to lift the current limit is therefore being driven primarily by BA.
The problem facing BA has been that, until recently, no one knew what the structural life of the aircraft was, because Government-funded testing of the major fatigue-test specimen airframe at Farnborough in the UK was discontinued in 1983.
This rig had proved costly to operate as the whole aircraft structure was encased in a series of ducts, to simulate the heating and cooling encountered on every supersonic flight. The stress-fatigue specimen at Toulouse, France, which did not have to be heated and cooled, had been purposely tested to destruction much earlier in the programme.
"The rig was turned off, with around 21,000 cycles achieved, without major failure", says Morris. "They [the regulatory authorities] did the calculations assuming that major failure would happen tomorrow. In BA's estimation, that limit was ultra-conservative."
Morris points out that a safety factor of three was used to reach the figure of 6,700 reference flights, whereas a factor of around 2.5 is usually assumed for subsonic aircraft. The original design life was 24,000 flights.
Consequently, the manufacturers, in conjunction with BA, launched a programme to "...look at the scope for extending the life of the aircraft", says Morris. The Concorde Re-Life Group was set up to tackle the problem, based at BAe's Bristol, UK, plant.
It was decided that a limit of 8,500 reference flights would probably be adequate until the end of the fleet's useful life, as far as the customer, BA, was concerned. "The programme is funded by BA, and so the output was tailored to our requirements," says Morris.
Although the fatigue rig had clocked up just over 20,000 cycles, some major components, such as the engine intakes, had undergone a much higher number of cycles. It was apparent, however, that further testing would have to be conducted, to allow an extension of the airframe life.
John Britton, BAe's chief engineer Concorde and BAC One Eleven, explains: "We divided the structure up into major components in line with the original work share [between BAe and Aerospatiale], and carried out a provisional analysis. After an initial review, we decided it would be desirable to get additional information from an in-service aircraft to see if the normal operating regime was as we expected."
As a result, one of BA's aircraft (G-BOAF) was equipped with nearly 40 strain gauges for a period of three months to provide in-flight measurements. Test flights were also carried out to ensure that the instruments were giving correct readings, and to make sure the gauges had been properly calibrated.
The resulting data was "very close to the original certification data", says Morris, allowing Aerospatiale and BAe to set up sophisticated computer models of the aircraft's structure. These models were validated using data from the strain gauges.
COMPUTING THE FUTURE
BAe and Aerospatiale already had computer models from the original development programme, although these were only of individual substructures, as the processing power necessary to represent the whole aircraft was not available at that time.
"We covered 356 structurally significant items," says Britton. "Each item was reviewed in light of test data, plus the original stress data and, where appropriate, additional information from the strain gauges [on G-BOAF]."
In addition, a large number of "fail-safe" tests, were performed on major subassemblies adds, Alan Jefferson, engineering manager for fatigue and damage tolerance at BAe Airbus. "Significant amounts of damage were introduced, and the assemblies were fatigue-loaded to see how the damage would grow."
All of the initial 28 potential problem areas, which the team thought, might require modification were eventually eliminated. "We haven't identified any areas that need structural modification," says Britton.
The team determined that only changes to the aircraft's inspection programme would be required, even while maintaining the original safety factor of three.
"In many areas, we demonstrated we've got the capability to go well past 8,500 reference flights," BAe's Britton adds.
The UK Civil Aviation Authority and its French counterpart, the DGAC, have now approved, in principle, the increase to 8,500 reference flights, and BAe and Aerospatiale are expected to issue the necessary service bulletin by the end of 1996. This will then have to be approved formally by the authorities, which should allow BA's youngest Concorde to remain in service up to around 2014.
BA, however, is assessing how it will accommodate the new inspection requirements, to identify those areas where developing permanent "fixes" might be more cost-effective. These would be particularly useful where inspections cause the duration of a major check to be increased.
"We must look at the economic impact on ourselves, and assess our response to the recommendations," says Morris. "If we have any concerns about our ability to do inspections, we may require terminating modifications."
BA is now negotiating with the manufacturers over these modifications, so that they can be incorporated in the pending service bulletin. "We haven't got a conclusion yet," Morris adds.
If BA does decide that it requires some parts of the airframe to be modified, these are likely to be designed to last beyond 8,500 reference flights, to perhaps 9,600.
This is because the airline does not want to introduce modifications which are only good for 8,500 reference flights, and then find itself having to re-modify to further extend the service life when the limit is reached.
It is also considered more cost-effective to investigate the feasibility of modifications for going beyond 8,500 reference flights while the Re-Life Group is still together, as these can be included in the same service bulletin.
Meanwhile, the Concorde's Rolls-Royce/ Snecma Olympus 593 turbojet engines are "an easier proposition" than is the airframe, according to Morris.
The design life of the engines was set at 10,000 cycles, although the vast majority of parts are inevitably replaced during the course of routine maintenance. Consequently, the engines were not a major issue for the Re-Life Group.
"We do not have any major problems in terms of the engines keeping pace with the life of the airframe," says Peter Kingston, R-R product executive Olympus 593. "Most of the major gas-path components would be replaced as the norm," he adds.
At the start of the Concorde programme, large numbers of spare engine-parts were manufactured by R-R and Snecma to support the engines over what was then considered to be the likely service life of the aircraft.
The extended operation of the fleet, however, has meant that many components have had to be re-manufactured at significant cost to the operators.
"We do suffer from considerations such as small business," says Kingston, highlighting the problems that the airlines face in trying to balance the cost of re-manufacturing small batches of parts against holding large-batch inventories for several years.
SIGNIFICANT IMPROVEMENTS
The engine manufacturers have introduced modifications to the engines over the years, such as new seals, which Kingston claims have resulted in "very significant reliability improvements". In addition, the service life of some parts has been increased.
One particular incident in 1995 highlighted the potential problems associated with operating such an exclusive aircraft. During a ground manoeuvre, one of BA's aircraft (G-BOAF) suffered a damaged nose droop, which needed to be replaced.
The airline avoided the estimated £2 million ($3 million) cost of having to have a replacement manufactured by taking the nose droop of the second production-standard Concorde built (designated 202) instead. This aircraft is actually owned by BA and has been kept in storage at Bristol, precisely to meet such eventualities.
Source: Flight International
