The Concorde accident occurred outside Paris soon after takeoff on 25 July 2000. The sequence of events appeared to be the disintegration of a tyre, followed by the impact of the tyre fragment on the number 5 fuel tank. Fuel from the tank, together with air, was then drawn through the engine creating a fire that brought down the aircraft.
The key factors for investigators in this accident were:
- What caused the tyre to burst and had there been previous incidents of this type?
- When the tyre hit the aircraft structure, how did this impact affect the fuel tank?
- How did the fuel ejected from the tank get burnt in and around the aircraft engines?
- How did investigators reach their conclusions about the bursting tyre?
Impact dynamics contributed to explaining why the tyre burst. The initial thoughts were that the tyre fragment had penetrated the fuel tank. This was shown not to be a complete answer, as the hole in the fuel tank was at the other end of the tank from the potential impact site.
Previous incidents with Concorde had revealed potential problems with the tyres. The burst tyre had a cross-ply construction, but a radial ply design has now been designed to increase the strength, and hopefully prevent any future problems.
What conclusions were made about the tyres impact on the fuel tank?
Computational fluid dynamics were needed to resolve this problem. A suggestion was made that a shock wave had been set up in both the tank material and the kerosene. This shock wave was of such a magnitude that the thin wall of the fuel tank was punctured with a hole that indicated that the fracture went from the inside of the tank to the outside, the reverse to a hole created by penetration. This hypothesis was confirmed by impact dynamics simulations, which also indicated that a hole, so created, would produce a large fuel flow.
A note of reservation to the report made by other investigators suggests that a small penetration could have occurred and contributed to the fuel flow from the tank to the outside. An aramid fibre-reinforced rubber fabric has now been developed to improve the strength of the fuel tanks, and has been inserted into all the fuel tanks of Concorde.
How was the burning of the fuel investigated?
The problem of the burning fuel was solved largely by identification of the "foreign" material, which was suggested had come from an aircraft that had previously used the same runway at Paris Charles de Gaulle airport. The most reasonable hypothesis (others were considered) was that the fuel from the tank mixed with the hot gases around the rear of the engines and created a fuel fire external to the aircraft. This would prevent the landing gear from being retracted and would also in the 75 seconds that the fire was visible for (eye witness reports), soften the aluminium alloy structure of the aircraft. To test the hypotheses, work was commissioned about whether self-ignition of the gases could occur, as well as a numerical simulation (using computational fluid dynamics) of the temperatures reached during a 75 second fire with a flame temperature of 1100_C. The latter gave good predictions of temperature, correlated to pieces of wreckage found. The hypothesis of self-ignition appeared feasible, but the researchers carrying out the study on behalf of the BEA (Bureau d'Enquêtes et d'Analyses - the French accident investigation agency) indicated that their results would be too dependent on the model of turbulent combustion selected and that further research was needed to be more certain. This research would have used computational fluid dynamics models as part of their approach. Another possible explanation for the fire was the ignition of the escaping kerosene by an electrical discharge, a hypothesis backed up by simulation and trials. The final report mentions both as possible explanations for the fire. It should be noted that no aircraft could be expected to survive if large quantities of fuel are channelled towards the hot areas at the rear of a turbojet with reheat.
One of the difficulties that the investigators faced was that not all of the key part of the aircraft structure survived in a recognisable form for analysis. With a more complete reconstruction, competing theories can more easily be evaluated.
Reports (preliminary, first and second interim and final) are to be found on the French BEA (Bureau d'Enquêtes et d'Analyses) website. Except for the final report, which is still only in French, all other reports are in both French and English. The definitive text is, however, in French.
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