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Aviation History
1956
1956 - 0528.PDF
528 FLIGHT, 4 May 1956 IMPACT OF THE HEAVY JETS . . . may be gained from the fact that an airport able to supporta hypothetical single-wheel aircraft of 200,000 lb is, generally speaking, strong enough to take a four-wheel bogie aircraft ofsomething like twice the weight. A rough-and-ready calculation of the equivalent i.s.w.l. of a four-wheel bogie may be obtained bymultiplying gross weight by 0.22: this is an I.C.A.O. (Annex 14) recommendation, and assuming 290,000 lb for the gross weight ofeach jet, we arrive at approximately 64,000 lb equivalent i.s.w.l. This at once enables us to see how the two aircraft match upto die main airports which quote their bearing strengths in terms of i.s.w.l. (which is in fact the majority). And if we assumefor each type a tyre pressure of 150 lb/sq in we arrive at an L.C.N. number of the order of 80 or 90. For comparison, L.CN.'s forthe Comet 4 and the Britannia 310 lie between 50 and 60. The strength required of the alternates is not so severe, beingbased on landing weight. The landing weight of 180,000 lb assumed earlier gives an equivalent i.s.w.1. of 40,000 lb for theseairports. Some airport strengths quoted in the table simply give a gross-weight figure, and in each case for lack of further information it is assumed that this is for dual main-wheel aircraft; by applying afactor of 0.35 (I.C.A.O., Annex 14) an approximation for equiva- lent i.s.w.l. can be gained. It should be emphasized that this isonly an approximation. It is in fact a reversal of the Annex 14 DC-8 AND 707 Summary of Assumption! Take-off weights ... 290,0001b Equivalent isolated single-wheel loads 64,000 Ib Take-off field lengths, I.S.A 9,000ft Landing weights 180,000 1b Equivalent isolated single-wheel loads 40,000 Ib Landing field lengths with reverse thrust 6,000ft correction factor—which, as previously stated, is intended forapplication to an aircraft rather than to a runway. Pavement resistance—which depends on its flexibility, aircraft tyre pressure,tyre contact area, aircraft wheelbase and so forth—is an imper- fectly understood subject, and each runway has to be consideredon its merits. But the 0.35 factor has been applied because it gives the benefit of the doubt. If the airports which quote theirrunway strengths in terms of maximum acceptable gross weight were not analysed in this way, and their "gross weights" takenas the only measure of their strength, it would be found that very few indeed would be able to take the two aircraft. If, then, the suitability of each airport is assessed according tothe foregoing assumptions, we arrive at the conclusions listed earlier. The writer believes that, in the light of published know-ledge about the DC-8 and 707, the assumptions are fair and in certain cases possibly generous; throughout, the benefit of anydoubt has been granted. For example, the great majority of the i.s.w.l.s quoted in the table are for considerably lower tyrepressures—often 100 lb/sq in—than the 150 lb/sq in assumed for the DC-8 and 707. The conclusion, in a nutshell, is thatbefore the world can take full advantage of these mighty air- craft, it will need about 150,000ft more runway (of appropriatestrength), and 140,000ft of runway overlay (of anything up to a foot thick) to bring existing pavements up to strength. The totalcost, assuming round figures of £300 per foot for new runway and £100 per foot for overlay, is of the order of £60 million. Andthis excludes the imponderable cost of toughening taxi-tracks and aprons. Here it is appropriate to make a few general observations aboutairfield pavement design, a branch of engineering which, because General arrangement of the DC-8, with a diagram of the revised four- wheel bogie (changed from the co-axial quad illustrated in "Flight" of July 29th, 1955). Note steerable rear pair. of the different conditions—e.g., climate, availability of materials,soil strength and so forth—that occur in practice, is far from being an exact science. The extent to which the world's runwayscomply with any standard is difficult to assess; many were laid in the war with tarmacadam or 6in-slab concrete surfaces, whenthe requirements of the future were ignored in the urgent needs of the moment, and very few were designed to calculated stan-dards. London Airport—with its double-slab concrete surface (8in below, 12in top) and heavy base and subgrade—is typicalof postwar runway engineering at its best. Strengthening a runway may be carried out in either of twoways. If the extra strength needed is relatively small, a bitu- minous surface will suffice. For an existing strength of, say,40,000 lb to 50,000 lb i.s.w.l., about four inches of overlay would be needed to take the heavier loads. For weaker runways,resurfacing with concrete slabs, which may have to be anything up to a foot thick, provides the only satisfactory solution. Resur-facing, it should be pointed out, is always an awkward undertaking because of the serious dislocation to traffic it causes. A point that has to be seriously considered in runway designis the effect of repeated loads on strength, and here it is a sobering thought that taxi-tracks, which—because of their more confinedarea—receive more repetitions of maximum load than do main runways, must be at the very least as tough as the runways theyserve. Statistics show the actual movement-repetition ratio of taxi-track to runway to be about 10:1 Taxi-ways, and evenmore so the aprons and hardstandings, in fact have to bear the full brunt of the worst possible loading condition—that imposedby the slowly moving fully-laden aircraft. And if, as shown, the majority of runways are below strength, it is fair to assume thatan even greater majority of subsidiary airport pavement is too. It seems that there are good business prospects for the world'srunway contractors—and for the makers of medium-weight airliners. -.'•••-.•.- CONCLUSIONS Of the 58 main airports considered: Number suitable ... ... ... ... ... ... ... ... 1 Number suitable after completion of current work ... ... ... 2 Number deficient in length or strength ... ... ... ... 30 Number deficient in length and strength 25 Of the 80 diversion airports considered: Number suitable ... ... ... ... ... ... ... ... 57 Number deficient in length or strength ... ... ... ... 20 Number deficient in length and strength ... ... ... ... 3 AIRLINER IDENTIFICATION "Know Your Airliners," by John TV. R. Taylor and Roy Cross. Published for Shell-Mex and B.P., Ltd., (though not available from that company), by Perry Colour Books, Ltd., and Educa- tional Productions, Ltd., 11 Buckingham Street, London, W.C.2. Illustrated. Price 2s 6d.r " would require a very long search of the aeronautical book-shelf to find a better half-crown's worth than Know Your Airliners, a joint effort by John W. R. Taylor and Roy Cross.The purpose of the book is to describe briefly and illustrate 28 leading types of transport aircraft, and it contains three-viewsilhouettes of each. In one respect, however, it is a recognition book with a difference: the conventional photographic illustra-tions have given place to a superb set of Roy Cross paintings, which are reproduced in full colour. Also shown in colour, for thefirst time, are the badges of all the major airliners serving British airports. The book's attractive appearance and low price shouldguarantee it a wide sale among the airport-visiting public. "FLIGHT" PHOTOGRAPHS MANY of the photographs reproduced in Flight, i.e., thosetaken by our own photographic staff (as indicated on pub- lication) are subsequently available in the form of high-qualityprints. Prices are as follows (dimensions given in inches): 5i x 3i, Is 4d (glossy or semi-matt, unmounted) or 2s (tonedsepia, unmounted); 6x4, 2s 6d or 4s; 8x6 3s 6d or 5s 6d; 10 x 8, 5s or 7s lOd; 12 x 10, 6s or 9s 8d; 15 X12, 9s 6d or 13s;20 x 16 (semi-matt only), 13s or 18s. Also available are a postcard series, totalling over 200, depici-ing both historic and modern aircraft of the R.F.C., R.N.A.S., R.A.F. and Fleet Air Arm. Complete lists of the series are obtain-able on application. Prices are 8d each card, or 7Jd each for quantities of a dozen or more. One view only—in most cases agood three-quarter front or side—is available for each aircraft type- Available, too, as postcards, at the same prices, are reproductionsof general-arrangement drawings of some 20 British aircraft that have appeared in our "Aircraft Intelligence" pages.
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