MAX KINGSLEY-JONES / LONDON
Supersonic services are being re-introduced by Air France and British Airways this week following extensive modifications to their Concorde fleets
The brief hiatus in the ability to travel at twice the speed of sound while sipping champagne and eating caviar comes to an end tomorrow (7 November), when Air France and British Airways return their Aerospatiale/British Aerospace Concordes to scheduled service after a 15-month interlude.
The 12 100-seat supersonic transports (SST) were grounded in the aftermath of the crash of an Air France example shortly after take-off from Paris Charles de Gaulle Airport on 25 July last year. All 109 people on board were killed, as well as four on the ground, when the aircraft crashed in the northern Paris suburb of Gonesse.
The sequence of events that resulted in Concorde's first crash in its 30-year history were as complex as they were freak, but the catalyst to the event was a main gear tyre's failure after it struck a titanium object on the runway, and debris from the tyre then ruptured a fuel tank. Air France immediately grounded its five remaining aircraft, but BA flew its seven Concordes until 16 August when the UK Civil Aviation Authority and its French equivalent the DGAC took the almost unprecedented step of withdrawing the type's certificate of airworthiness (CofA).
Since the grounding, the investigation has established a relatively complete picture of the accident's circumstances, allowing Concorde's manufacturers EADS and BAE Systems, in conjuction with the aviation authorities and the operators, to draw up a modification programme to enable the re-instatement of the CofA. BA says the modifications to its fleet are costing £17 million ($25 million). Unlike Air France, the UK carrier is upgrading the aircraft's cabin at a further cost of £14 million.
Air France has modified three of its five SSTs for entry into a daily service five times a week between Paris Charles de Gaulle and New York Kennedy from 7 November. Simultaneously BA will reinstate six weekly flights from London Heathrow to Kennedy with three modified aircraft. While the French flag carrier says it has no plans yet to restart charter services with Concorde, BA aims to fly one scheduled flight a week from December on the "holiday" route between London and Barbados.
Michelin stars
Among the stated requirements for the renewal of Concorde's CofA are Michelin's new near zero growth (NZG) radial tyres for the main gear, which offer greatly increased resistance to foreign object damage.
The construction of Concorde's tyres is significant due to the much greater forces they experience compared with those on conventional aircraft, as Concorde behaves quite differently during its take-off run. Whereas the wings of conventional aircraft start to generate sufficient lift to relieve some of the weight from around 40kt (73km/h), Concorde's delta-wings do not provide any significant amount of lift until the point of rotation and lift-off.
Therefore Concorde's entire 185t of take-off weight is borne by its 10 tyres during the whole take-off run until lift-off speed is reached - typically a 41-sec roll of around 2.8km to an unstick velocity of 230kt (430km/h). Additionally, Concorde tyres develop some 30% more centrifugal force than those on a conventional aircraft. According to Michelin, whereas the mass acting on the tyres equipping a conventional aircraft decreases from the start of the run, it actually increases on those on Concorde between the point of rotation and unstick.
Although before the accident Michelin did not supply tyres to either Concorde operator, Pierre Desmarets, general manager of Michelin's aircraft tyres division, says the French tyre manufacturer was approached by EADS along with the existing suppliers Goodyear (Air France) and Dunlop (BA) to see if it had a tyre for Concorde that would offer a much greater resistance to damage.
"We had started working on the NZG concept in 1999 to develop a tyre with fewer plies which could take heavier loads - not for Concorde but for new, larger aircraft in development like the A340-600 and A380," says Desmarets. After the approach from EADS, Michelin started developing the NZG concept for Concorde in August 2000.
While the existing Concorde tyres were of conventional bias construction, the NZG is a development of the radial concept invented by Michelin in 1946. The combination of heating and centrifugal force, says Michelin, creates a 12% increase in a bias tyre's radius during an acceleration to 100m/sec (195kt). The resultant elongation of the tyre rubber and tread makes it less resistant to cuts and Michelin has developed its radial concept to reduce this significantly.
Compared with a bias construction, the radial design incorporates a reduction in the number of bead wires from six to two and fewer nylon cord casing plies which are postioned at 90í to the bead wires, as opposed to 45í on the bias design. The stability of the radial's tread also enables a reduction in the number of grooves, giving the advantage of a larger surface area in contact with the ground.
The crown of the radial tyre, which is made of nine plies, is able to "roll" on the ground like a bulldozer's caterpillar track, says Michelin, as it is independent of the sidewall. The radial structure produces a lower level of heat generation, while the 90í disposition of the plies restricts the growth of the tyre, says Michelin, limiting the radius increase to 8%.
To create its NZG radial, Desmarets says a new, high tensil fibre cord developed in-house by Michelin is used in the casing, enabling the tyre to have around a third as many casing and crown plies as a standard radial (see diagram). This much reduced mass combined with the use of the higher strength cord limits the radius increase to just 3% - or a fifth of a convential bias design (see table 2). This impressive reduction in the tyre's growth is what makes it so resistant to damage and offer greater residual endurance if damage is sustained. Michelin cites, as an example, the difficulty in cutting an unstretched elastic band compared with when it is stretched.
"In conjuction with EADS, we have tested the tyre hundreds of times at low and high speeds of over 210kt, with a variety of objects including steel and titanium blades, and it has never exploded," says Desmarets. He adds that the NVG achieves its greater resistance to damage by being "more supple" than a bias tyre which is "extremely solid" with thicker sidewalls. The NVG is therefore less prone to causing damage if it bursts and the tyre carcass disintigrates.
Less weight, more durable
Michelin's NVG also has the advantage of being 20% lighter than the equivalent Goodyear tyre and more durable, but Desmarets concedes that it is around "20-30%" more expensive: "We were asked to develop a Concorde tyre that could complete around 40 flight cycles - roughly what the existing tyres did - but we expect to achieve a much lower wear rate and many more cycles than that target."
So Concorde is poised to begin the next chapter of its career in a far healthier and more robust state than it ended its last. It now remains to be seen whether the aircraft has managed to overcome the vivid memories of last year's tragedy and can re-establish itself as a travel industry icon.
Source: Flight International
