| TWA 800 - Accident or Error?
Gerry Evans Member, Association of Licensed Aircraft Engineers (U.K.) Some years ago I was asked, together with the electrical/instrument group of my Squadron, to give a demonstration of the volatility of Aircraft fuel. It was summer time and we were based in central Europe. The landmass there always produces high ambient temperatures, often so hot that you are unable to touch the upper surfaces of the aircraft. The day we chose to carry out the demonstration was like this, the temperature being in the high eighties and touching 90 through mid afternoon. Initially we used a 90-gallon drum containing about 6 inches of aviation fuel. I cannot remember the fuel specification, but it was similar to what is used today. The drum having been left standing in the sun was already too hot to touch. It cooled with the added fuel, but quickly returned to a high temperature, giving rise to considerable vapour. We then applied, via two probes, a continuous spark, the source being from a 250-volt megger. The probes would only work when close together, but the spark was strong. There was no immediate result and so the probes were placed gradually nearer to the fuel at the bottom of the tank Again no result, and so we progressed through a 500-volt megger and on to a 1000-volt megger. The probes were varied in their construction and the sparks generated were magnificent. But no explosion or fire resulted. A lid was applied to concentrate the fumes and eventually holes were made in the side to improve air supply. This went on for some hours and by this time the tank temperature was over 100 degrees. We got fed up and tossed in a lighted rag, it simply went out. A week later we had another go. This time with a galvanised 400 gallon tank, the ambient temperature again in the high eighties. A similar procedure was followed, but using only the 1000 volt spark source as this gave the best sparks across a larger gap Apart from becoming adept at generating sparks we achieved no fire or explosion. This was what we had expected before we started. When I first heard that an aircraft fuel tank had exploded I was immediately interested. When it was suggested that the cause was due to a stray spark from some aircraft wiring source, I was amazed and, quite simply, disbelieving. I was sceptical because a spark, per se, has no energy. To be maintained it has to be fed from a source, and it would require a large voltage spark to be effective. Given the right conditions a spark can ignite a fuel vapour, but not this fuel, in these circumstances. In the case of TWA 800 the catastrophic failure that engulfed this aircraft happened in only a fraction of time. Such a failure required the instantaneous dissipation of a large source of energy. If it is deemed that the fuel tank exploded, it could only have done so if there had been applied across the tank a discharge of considerable energy such that the entire volume of vapour in the tank ignited virtually instantaneously. In other words a detonation took place. Combustion has a slow rate reaction, and would require precise conditions and volume to achieve an explosion. I am talking about the burning reaction of the fuel vapour. The fuel residue would not have contributed to the explosion, only to any subsequent fire. Liquid fuels only burn as vapours. Surface temperature and fuel temperature would control the vapour volume and as more vapour is generated so it mixes with the unchanged volume of air within the tank. Reaction in vapours is governed by heat. This may seem obvious, but chemical explosives are different. They are metastable compounds and so behave in a way quite unlike that of a vapour. It is essentially the rate of reaction that separates combustion from detonation. That is not to say that the eventual damage occurring from the two reactions will be dissimilar, it is more a question of probability. In this particular case we have to ask, did the volume of this tank have the capacity to generate the pressure required to cause this explosion, having been initiated by a low intensity spark from aircraft wiring? The answer from me, at this time, is no. COMBUSTION In combustion the reaction rate is relatively slow and therefore requires precise conditions to result in an explosion of such a magnitude as that which occurred to the TWA 800 aircraft. Considering the fuel vapour as the only explosive medium present, and given that the present argument provides for a spark having generated a fire in that vapour then the final energy dissipated would be dependent upon the rate of advance of the reaction through the vapour. That reaction would be relatively slow to start and would accelerate as the heat increased. That slow start would of course burn any air in the tank and at the same time remove some of the fuel vapour. The reaction would be enhanced by the confinement provided within the tank and the tank temperature. The reaction would however be restricted by the divisions in the tank. This would have the effect of slowing down the chain reaction through the vapour and the subsequent build of pressure. The build time of the reaction would also rob energy from the eventual explosion. The heat source would have to be a fire, started, as suggested by a spark. The heat generated by that fire would vaporise more fuel, but there would not be any additional air. The rate of reaction would have to be immense in order to generate the energy required to disintegrate the tank. An explosion has a transitory effect. Splitting the tank would be one consideration; breaking through the fuselage would be another. It makes the argument for the rate of reaction and therefore the source of detonation very strong in favour of another option. DETONATION Detonation of the fuel vapour would be a different story. Remember that it is the reaction rates that effectively separate Combustion from Detonation. In Detonation the reaction accelerates through the material/vapour like a shock wave. The resulting explosion is instantaneous. Detonation would require the dissipation of a large energy source across the tank, such that the fuel vapour ignited instantaneously, the rate of reaction giving rise to a very large pressure within the tank and subsequent disintegration of the tank taking place. The instantaneous pressure generated would be immense, and so we have the explosion we are looking for. But where do we look for the source of detonation? From the aircraft, such sources can only be the HF radio and engine ignition, both unlikely. The idea that a spark generated from the break up of a fuel pump is untenable. Sparks from such a source are transitional and without energy. Sparks from Aircraft wiring faults would have to be external to the tank and again without the potential that I believe would be required to trigger an explosion. Static however, I have reservations about, this subject deserves more research, especially cabin generated static. We are therefore looking at an external source, external in the sense that it was not generated within the aircraft. Firstly, the most perfect catalyst would be a high heat source such as an explosive device; secondly fork lightning. From here we move into the world of phenomena, or perhaps something more sinister. |