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Aviation History
1951
1951 - 0459.PDF
(Left) An example of failure which was not due to ground impact, but occurred in the air: stripping of a plywood wing skin as a result of damage from fitters' boots on a too-narrow walk-way. (Right) A second illustration of blade-failure from centrifugal loads in the region of the tip-sheathing. they could be dangerous on larger and faster aircraft: people wereapt to think of the slat as a device which operated according to flying speed, whereas actually it opened as a function of the angleof attack. The result of such a misconception could be serious damage to the mainplane, or perhaps its complete disruption,when the slat moved forward violently during some high-g manoeuvre. .: As regards skin failures, cases had occurred of a plywood main-plane top-skin coming off in the air, and in a certain type of aircraft they were traced to the use of an insufficiently wide tread-mat,which had permitted fitters' boots to damage the surface. Enlarge- ment of the mat and a strengthening of the wing in this region putan end to the trouble. During the war, a large number of structural failures in a certainfighter type were traced to aileron instability, and it was recom- mended that an up-float limit should be introduced, and that thecontrol-cable circuit should be accurately tensioned. An improve- ment in the accident rate for this type—from one in 16,000 hoursto one in 82,000 hours—followed the implementation of this requirement. In the past, locked controls had caused many accidents. In onecase, an aircraft which had been completely destroyed by fire after impact was found to have the aileron locking pin still in itslocked position in the rod between the control-wheel sprocket chain and its under-floor attachment; and this evidence remainedin spite of the fact that the control casing had been entirely burned away. Accidents of this kind were now guarded against by theintroduction of nuisance bars or other foolproof devices. Examination of wreckage had also shown up various weaknessesin wooden-airscrew construction, especially as the result of centri- fugal loads in the region of the tip-sheathing, in conjunction withtoo-thin or too-dry glueing. The majority of such accidents resulted from failure of a scarf-joint after a new tip had been fitted.Modification of assembly and repair processes reduced such failures to insignificant figures. Enumerating a surprisingly large number of ways in whichcarbon monoxide had entered cockpits and incapacitated the pilots, the lecturer said that nowadays fume-checks were carriedout on every prototype aircraft, and percentage checks made on those coming off the production line; since 1943, there had beenno record in the A.I.B. of any accident from this cause. In earlier days, also, carburettor icing had caused many accidents, whichhad included a forced landing by an Empire flying-boat in the Cause of engine stoppage and a crash: oil filters so choked with •carbon that the gauze has begun to deform under the fluid-pressure. Atlantic; here again, subsequent design requirements had virtuallyeffected a complete cure. A very important part of the A.I.B.'s duties was the investigationof aircraft fires, though when wreckage was burned-out the chances of finding the primary cause of the conflagration weresomewhat remote. In crashes, hot exhaust systems and broken electrical circuits were among the commonest means of ignitinginflammable fluids. The whole question of crash fires was receiving continuous attention, which involved a study of the useof inertia switches, skin-deflection switches, and various types of crash-resistant tanks. As regards fires in the air, the generalintroduction of supercharged engines during the early part of the war made it apparent that no matter whether the fire developedslowly (following, say, a pipe failure) or rapidly (e.g., after a major engine failure) the final result was in most cases catastrophic;collapse of a wing could follow the start of a boosted-mixrure fire in 90 seconds. Fires were usually caused by (a) major engine-failures causing leakage of oil, hydraulic fluid or fuel/air mixture; (b) leakage of fluids through faulty pipes or joints; (c) location ofpipes too near the exhaust system, or bad location of oil or fuel pressure pipes combined with over-filling of tanks; (d) exhaustpipe failure or displacement. A combined report by the A.I.B. and R.A.E. issued in 1943 was given a wide circulation; it illus-trated many causes of fires and a number of recommendations were made which were subsequently put into effect. On the workand results of investigations during the war years and afterwards, the British Military and Civil Airworthiness Requirementsregarding fire precautions were largely based. Among design modifications introduced with the object ofminimizing fire risk, the lecturer cited sheet steel (instead of light alloy) firewalls, fireproof fuel and oil pipes, introduction of high-melting-point material for extinguisher spray-piping, and the locking of pipe union nuts with stainless steel wire. (In connectionwith this last point, A. Cdre. Brown mentioned that during the war, many unions were not wire-locked—this was due to a pro-duction concession necessitated by the high breakage-rate of i/i6in drills used by female operatives; this size of drill was invery short supply.) Continuing with his review of modifications, the lecturer instanced the enclosure of generator brush-gear anduse of flame-trap vents in such components; leak-proof fuel filler- caps ; and the sealing of wing roots so that fumes from an incipientfire in the wing could not reach the pilot. The air commodore said that accidents were not very oftenfound to be due to bad maintenance or inspection. Nevertheless, examples occasionally arose, and he illustrated a petrol cockblocked by cotton waste, oil pressure filters so choked with carbon that they began to collapse (this caused the loss, with 13 lives, ofa four-engined aircraft), and blockage of tank interconnecting balance-pipe valves by the slushing compound sometimes usedto seal metal tanks (suggesting that the tank had, wrongly, been treated with the valve assembly in position). There were, said the lecturer, very good reasons for the existenceof licensed ground engineers, and he went on to quote some evidence from crashes involving private-owner aircraft. In oneinstance, a man had built a sailplane; the woodwork was excellently done, but the flying wires, instead of being spliced, were attachedby tying knots and securing them with bell-wire. The joint did not fail but, owing to the snatch which resulted from an inabilityto tighten the flying wire, it broke at the eyelet. At the other end was a turnbuckle fixed directly on the mainplane lug, where thecable was tied in a similar way. The drawing specified 20-cwt cable; on test, it broke at under 6 cwt, and was found to consistof commercial wire washing-line bought at the local ironmonger's. The lecturer, in his summing-up, said that his department'smethods did not involve crystal-gazing or black magic; they worked on principles arrived at as the result of 30 years ofexperience. There was no easy road to the cause of an air accident, and it was very dangerous to give a quick answer. Theilhtstrations are R.A.F. official photographs, Crown Copyright reserved]
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