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Aviation History
1974
1974 - 0095.PDF
FLIGHT International, 17 January 1974 87 getting sparks from a build-up of static electricity due to fuel movement may also be considerably reduced by care ful design and eliminated by the use of an anti-static additive. Furthermore, ignition due to lightning strike may be rendered less likely by correct positioning of fuel tanks and vents and by the bonding to main structure of all removable items such as access covers and filler caps. I shall return to the crash case later. Although all fuels do (and must) burn certain con ditions are needed before combustion will occur. Princi pally fuel and oxygen must be present within a certain well defined range of proportions. The fuel must usually be in the form of a vapour although very small droplets of liquid fuel are often more or less equivalent to vapour. The amount of fuel vapour depends on the vapour pressure which in turn depends on the fuel type and the fuel temperature. The amount of oxygen present, approximately 21 per cent of the air, depends on the atmospheric pressure which falls as altitude increases. The amount of oxygen may also be reduced by replacing, as far as possible, air by an inert gas such as nitrogen. In a conventional subsonic jet or turboprop aircraft, inflammable mixtures can be eliminated by ensuring that there is either insufficient fuel vapour (and/or small drop lets) or insufficient oxygen to support combustion. The former is achieved for all practical purposes by using a low-volatility fuel such as kerosene. The latter can be achieved by nitrogen inerting. If there were no difference in price between kerosene and JP4 there would be no argument, one would retain a simple system and use kerosene. However there always has been a difference in some parts of the world, with JP4, a "wide cut" fuel, being slightly but significantly cheaper than kerosene. The wider the "cut" the more fuel is directly available by distillation of a given quantity of crude oil. If the price differential increases, as it may due to the oil shortage, more airlines may be tempted to change to the less-safe JP4 since the difference may already amount to over £1 million a year for a large airline. It is possibly with this in mind that the American Federal Aviation Administration has, over the past few years, been pushing the airlines towards adopting nitrogen inerting systems. Not unnaturally these moves have been resisted strongly by both manufacturers and airlines as being heavy, expensive and, with kerosene, unnecessary. Since a comparable measure of safety may be achieved between JP4 with a nitrogen inerting system and kerosene with no inerting it might appear that the FAA has been both wise and far sighted. However, such a comparison ignores the crash case where fuel tanks are ruptured and many of the precautions taken within the tanks are no longer of any avail. Of course the FAA itself has not ignored this and much of the research into rendering JP4 less hazardous under crash conditions has been FAA-spon- sored. Nevertheless I believe that any acceptance of JP4, even with a reliable inerting system, seriously underesti mates the crash fire hazard and is therefore totally unacceptable. So let us assume that the price differential still favours JP4 even when the inerting system and all that goes with it (reduced payload and increased spares holding) has been paid for. Why should we still fight and, in Lhese assumed circumstances, pay for another approach? The answer is that accidents during the take-off and landing phases of flight are the most numerous and yet potentially the ones that a passenger stands the best chance of surviving. It is precisely in these potentially survivable accidents that fuel properties play such an important part. Here again volatility is the key factor. If tanks are ruptured and fuel spilled there is a good chance that it will come into contact with an ignition source. If the fuel does not ignite or if the rate of flame spread is slow then passengers still have a good chance of escaping and the fire services a good chance of arriving in time to help. This has generally been the situation when using kerosene fuel and the entire concept of escape time as affected by aisle widths, the number of emergency exits and other relevant factors, has been based on the time taken for a kerosene fire to develop. If, on the other hand, JP4 is used then almost invariably the spread of flame across the spilled fuel will be extremely rapid—to the extent that if a fire does occur then the chances of escape are very small indeed. The reason for this distinct difference is that if a fuel is above its flash point then the vapour/air mixture above the fuel surface is inflammable and flame spread is almost explosively rapid. If it is below this temperature the fire will only spread as fast as, by radiation and convection, it can sufficiently heat up the fuel ahead of the flame front. This is a very much slower process. Under virtually all survivable crash con ditions kerosene will be well below this critical temperature and JP4 well above. The difference in rate of flame spread is between centimetres per second and metres per second. So nitrogen inerting is not the complete answer even though it could still help under some crash conditions by reducing the likelihood of a JP4 explosion in an unruptured tank. What then are the other approaches that have been tried and is there now a viable alternative? Initially the principal effort was to thicken JP4 by emul sifying or gelling it by the addition of a small percentage of some other substance. This was to reduce the amount of spillage and with it the likelihood of contacting an ignition source. However, the difficulties of handling these thickened fuels have proved to be considerable, particularly those of emptying the tank without increasing unusable fuel weight. Moreover, although any thickened JP4 showed up as safer than JP4 itself, kerosene still remained the safer fuel. The next question is "can we improve on kerosene?" Thickened kerosene does show some improvements but not T mXm w • LL', m^m M 'T Bftftgf mm ftuL* fc#Mto*Iii *..WTOHB isw^wwiiaii
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