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Aviation History
1960
1960 - 0780.PDF
788 FLIGHT, 10 June I960 JET FUEL AND SAFETY . . . explosive mixture towards the end of a long flight, gasoline/airmixtures are too rich for explosion during a large proportion of piston-engined flying. There is no particular harm in carrying an explosive mixture inaircraft tanks unless there is also a source of ignition; but, as sug- gested above, lightning may provide such a source.* It has oftenbeen stated that there is no known case of an aircraft having been destroyed by lightning strike, but it looks as though such confi-dence may no longer be justified, for it is now believed that the loss of a TWA Constellation near Milan in the summed of 1959 wascaused by a fuel-tank explosion following a lightning strike. In addition to this, tip tanks carrying JP.4 on some military aircraftare known to have been exploded by lightning. If the Constella- tion report is ever confirmed, the whole question of tank explosionwill have to be given further careful consideration. While the primary protection against this hazard must lie in careful design offuel systems, and granted that all fuels can form explodable mix- tures at one time or another, it remains evident that JP.4 presentsthe worst risk in this particular regard. Spontaneous Ignition Temperature. This is the temperaturerequired at which a hot surface, without external spark or flame, will ignite an inflammable liquid. It is a difficult quantity tomeasure, being affected by many variables, the most important of which are the local ventilation velocity and the period for whichthe fuel is in contact with the surface. The higher the ventilation velocity, the higher the s.i.t., and the longer the contact time thelower the s.i.t. Let us first consider in-flight fires. Tests carried out by a well-known aircraft manufacturer showed that with a small ventilating airflow, kerosine would ignite at a surface temperature of 400 °Cand JP.4 at 500°C, suggesting that JP.4 was slightly safer. In practice, however, engine fuel systems are confined to compressorzones, where surface temperatures are well below the above figures (although as a precautionary measure in engines of high compres-sion-ratio, the "hot" end of the compressor may be double skinned or otherwise shrouded). Fuel pipes are not generally allowed topass through combustion zones other than in fireproof, drained conduits, so the possibility of leakage onto the very hot parts of anengine is ruled out except in the case of a major catastrophe such as turbine failure. In addition, engine compartments are usuallywell ventilated; and this ventilation will in itself tend to raise s.i.t.s higher than the values quoted above. There is no evidence of which I am aware to suggest that in-flight fires are any more common with one fuel than with the other. Turning now to effects of spontaneous ignition temperature incrashes, crash fires are often started by fuel coming into contact with very hot engine components, both internal and, to a lesserextent, external. The turbojet engine also presents a particular hazard, as it is liable to continue rotating after a crash and to inhaleany fuel in the vicinity, this fuel catching fire and resulting in a torch of flame out of both ends. Kerosine and JP.4 and gasoline areall readily ignited by spontaneous ignition under crash conditions as N.A.C.A. have shown. What is so vitally important, if one mayreiterate this point, is the development of the fire after it has once started, and here the principal advantage of kerosine lies. Fuel Freezing Point This is the temperature at which certainconstituents of a fuel start to solidify; fortunately fuels do not solidify completely at their specification freezing points, but remainfluid and pumpable for at least 5°C below. The freezing point of the kerosine most commonly used in this country is —50 °C andof JP.4 — 60 °C, the latter fuel having a theoretical advantage; but it is an advantage which is most unlikely to be of use in practice.In one of the slower jet transports now flying, the very lowest fuel temperature which could be experienced on a long flight would beat least 25 °C above ambient, in which case an ambient cooler than — 75 °C would be required to affect kerosine. This temperaturecorresponds to arctic minimum at 37,000ft and is extremely rare. The faster the aircraft the greater will be the protection conferredby kinetic heating. The slower flying turboprop aircraft, though not enjoying the same kinetic protection, will be operating at loweraltitudes and therefore at higher ambient temperatures. A fuel temperature of — 47 °C is the lowest of which I haveheard, and this was on a Comet aircraft. If the old — 40 °C freezing point kerosine had been in use on this occasion, an alteration inflight plan would have been necessary to meet this rare case. So far as ground operation is concerned, the current — 50 °C kero-sine should be satisfactory all over the world. Even at the Cana- dian winter establishment at Fort Churchill, sustained tempera-tures lower than about — 42 °C are rarely experienced. Water in Fuel. All fuels contain water in solution, which isprecipitated as temperature is reduced. Normally this water (and free water, if present in small quantities) is given up to the air abovethe fuel. Should the tank contain humid air, however, the preci- pitated water will remain in the fuel and form ice crystals when the *See "Uninvited Electricity," "Flight," April 10, 1959.—Ed. temperature falls below zero. This ice will pass into the fuel pipe-, and settle on micronic filters. This problem has become much more prominent since tht advent of the turbine engine, partly because of the increasing use oi micronic filters, but owing also to higher operating altitudes and to the use of integral tanks, these last two factors resulting in lower fuel temperatures than were common in an earlier era. There is also the theory that the problem has been aggravated by die use of kerosine which, because of its higher viscosity, may tend to hola its precipitated water in suspension, whereas the less-viscous fuels will allow their water to settle out by gravity. I am not convinced that the difference in viscosity materially affects the icing risk as between the different fuels (JP.4 can hold slightly more water in solution than can kerosine) although some airworthiness authorities think that it does. One such authority actually insisted on the removal of a 20-mesh booster pump screen from a certain British powerplant; this despite the fact that such a screen has not been known to ice-up in British turbine experience. In practice, filter icing occurs fairly frequently with both kero- sine and JP.4, and the solution has been to fit fuel-heaters, which use either hot engine oil or compressor bleed air as a heat source. We are all filter-icing-conscious these days and now realize that this phenomenon was probably responsible for many unexplained power-losses in piston engines. Effect of Volatility on Flight Relighting. It is sometimes sug- gested that relighting an engine in flight is easier with JP.4 on account of the greater readiness of that fuel to vaporize. Relighting at altitude may have been a problem on some early turbine engines; but, with the advent of high-energy ignition and more highly developed fuel systems, this vital engine manoeuvre can be easily performed with either type of fuel. To summarize: from the standpoints of flight relighting, water in fuel, fuel freezing point, and spontaneous ignition temperature, there is nothing to choose in practice between the two fuels. From the standpoint of flight explosion, JP.4 presents the greater hazard when one considers world-wide operation as a whole. But from the point cf view of flash point and its effect on the crash fire risk, kerosine has a clear-cut safety advantage over JP.4. Some Crash Evidence. It is relevant to consider some crashes which have occurred to kerosine-fuelled aircraft in an effort to see how the theoretical superiority has been manifested in prac- tice. Official reports are not available for all the accidents in the following list, and, where they are, precise details of fuel spillage and fire development are usually lacking; but as crashes do not occur under laboratory conditions this is not surprising. The list is not a complete one, and accidents in which all occupants are believed to have been killed by impact have not been included. It is worthy of note that in only one of the following cases is a technical failure known to have been the cause. Comet 1 at Rome, October 1952. Aircraft failed to become airborneand came to rest extensively damaged, having spilled some 2,000gal of kerosine. No fire or casualties. Viscount at Mangalore, Australia, October 1954. Training accidentduring engine-out take-off. Aircraft collided with trees, crashed and overturned, killing the pilots. Kerosine from ruptured tanks caught fire,but the slow development of the fire allowed the five occupants of the inverted cabin to extricate themselves and escape. Viscount at London Airport, January 1955. Collided at high speedwith runway obstructions. All propellers and reduction gears and two complete engines torn off, aircraft coming to rest in a pool of overl,000gal of kerosine. Two small engine fires extinguished and all occupants (about 40) safely evacuated. Viscount at Blackbushe, January 1956. Training accident duringengine-out take-off. Aircraft struck ground, cartwheeled, came to rest within 200yd and caught fire. Five occupants escaped with slightinjuries, after which fire almost destroyed the aircraft. Viscount near Benghazi, August 1958. Flew into high ground andcaught fire. Thirty-six killed, but fire development was sufficiently slow to allow 18 occupants (mainly from rear of cabin) to escape. Handley Page Dart-Herald near Famborough, August 1958. Enginefailure resulting in uncontrollable fire; burning powerplant dropped off, leaving kerosine pouring from nacelle, fire still burning. Aircraft crash-landed and was slowly consumed by fire after the five occupants had escaped unhurt. Britannia near Hum, December 1958. Aircraft flew into ground in fogduring test flight and caught fire; nine occupants killed, three escaped. Viscount near Gatwick, February 1959. Aircraft hit trees onapproach, crashed and caught fire; 15 occupants killed, ten escaped. Comet at Asuncion, Paraguay, August 1959. Aircraft flew into treeson night approach in bad weather. Pilot and one passenger died, 63 other occupants escaped from severely damaged aircraft. No fire,although massive fuel spillage would be expected. Comet at Rome, December 1959. Wheels-up landing: no casualties.No fire, although fuel spillage and sparking from abrasion on runway would be expected. Comet at Buenos Aires, February 1960. Very heavy landing duringcrew training: pod tanks broke off and started a fuel fire which severely damaged the aircraft: all 11 occupants escaped unhurt. Comet at Madrid, March 1960. Aircraft struck escarpment on nigmapproach, losing main wheels: one wing torn off on touchdown. No fire and no casualties among crew or 24 passengers. Heavy fuel spillageand sparking through abrasion on runway would be expected in such circumstances. [Concluded at foot of page «<< ?
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