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Aviation History
1955
1955 - 0336.PDF
336 FLIGHT, 11 March 1955 AIRWORTHINESS . .Continued from page 320) margin following engine failure. A typical example of the drift-down technique is the Convair CV-340 on routes over the Alps. This aircraft cannot cross the Alps with one engine out, but, withboth engines operating, it can fly high enough to permit a safe drift-down in the event of failure. The curves on page 318 of a hypothetical airliner show thetype of flight path which would be followed in the event of engine failure in the critical sector of a particular route. Curves of thisnature are required to establish the maximum weight at the entry to the critical sector and the minimum planned altitude at whichthe aircraft must fly in order to complete a safe dr,ift-down to an appropriate airfield. Calculations of this type are quite standardizedfor modern transports where the route requires. A margin for error is necessary, for a pilot may be flying above cloud at themoment of failure and he may not know his precise position, which could lead to a wrong decision—such as turning back whenhe should have gone on. Climb gradient, which was ignored in the original Americancode, is now accepted as one of the principal criteria of margin of performance. If a machine like a DC-7 were being designedtoday, the designer would be presented with different choices at the same weight and temperature according to whether the air-craft was to comply with the C.A.A. code or B.C.A.Rs., and the resulting aircraft would have diiferent degrees of safety. As anindication of the difference between the two codes the diagram of rate of climb plotted against airspeed (foot of p. 319) shows thedivergence of the lines of best rate and best gradient. All other things being equal, the speed optimum for the best gradient ispreferable to the brochure speed for maximum rate of climb; but other factors such as cooling may demand a higher airspeed. Forexample, the Constellation normally has a mandatory fast climb on account of cooling troubles. Although this diagram emphasizesthe greater safety inherent in choosing gradient as the criteria, the U.S.-designed aircraft is not inherently "unsafe", and could bemade to meet a rational code by slight modification—such as an inter alia alteration in the flap settings. Mean Performance Another concept which has only recently become appreciatedis that of fleet mean performance—an essential in the calculation of the achieved performance of fleets of airliners actually in service.The flight manual was previously based on figures measured on a new and polished aircraft submitted on a certain date. As the air-craft develops, performance tests may be repeated at higher weights and these latter tests may be more carefully done andmay give uncorrelated results. It is now usual to undertake a periodic check on the performance of aircraft actually in service;an uneconomic alternative is to insert a "cushion" to allow for unassessed deterioration. Extra aerials, for example, may eventu-ally spoil the stall or aerodynamic behaviour of a previously clean aircraft. The power actually developed by the engines may also vary, anda fleet mean power is therefore taken as half the sum of the powers of typical new and old engines. At the same time, account hasto be taken of the accessory power being extracted, for this is naturally deducted from that applied to the airscrew, and can varyconsiderably over the life of an aircraft and between different points of a given route. Incidentally, many British prototypes arefull of experimental equipment and other abnormalities, and so quite by accident the original performance testing may be done onan aircraft representative of its successors in service. Another point wordi noting is that renewal testing teaches airline pilotswhat their aircraft should do and precisely how it should be flown. The modern code of British performance requirements is basedupon fair argument. This is considered a distinct advantage over empirical codes which virtually cannot be discussed at all. Closeco-ordination is necessary between all the parties concerned and considerable pains have to be taken on what might be termedthe ironmongery of systems and safety devices. For example, any system of auto-feathering may have inherentdangers, and, in fact, simple defects in such equipment have led to a number of serious accidents with American transports. Britishdesigners have mainly preferred a system of automatic pitch coarsening, from which full power can be regained at once in theevent of error. An alternative arrangement is the use of an auto- matic warning light which comes on to the pilot's panel if failureis signalled from one engine; the circuit also selects this engine for feathering by a single button on the pilot's control wheel. Thissystem allows die pilot to decide whether or not to feather and permits him to do so without removing his hands from the controls.The Ambassador and Bristol Freighters 31 and 32 all have auto- coarsening and selective auto-feathering of the type described.The Viscount has an additional fine-pitch stop which prevents the airscrew from assuming a pitch so fine as to invalidate theestablished minimum control speed; upon deflection of the under- carriage legs, however, this stop is removed for braking purposes and also for starting the turboprops with the minimum airscrewdrag. The altitude at which maximum power is available from theengine has a considerable bearing on take-off and engine-out per- formance. DC-3 power units have had increasing full-throttleheights from 3,000 to 5,000ft and today there is scarcely a single American transport with a full-throttle height less than the latterfigure, in order to meet the Vs2 requirement. After World War II, some British transports had much lower full-throttle heights andwould have had to reduce weight unnecessarily if operated from low-altitude airfields under C.A.A. rules. (This emphasizes thegreat advantage of an engine like the Bristol BE.25.) Vertical tail-area is another parameter with a great effect onairworthiness. Even with its triple fins, the York has a high minimum-control speed, whereas the Tudor has a better engine-out performance. The Britannia also has to have a very large fin, whereas the Comet, with its close-in thrust lines, needs arelatively small surface. The Hermes was designed with the idea of meeting the Vs2law and, although at the C. of A. the weight had grown consider- ably, the Wat curve (mx. wt., alt. and temp.) was adequate andwith the take-off flap-setting reduced from 40 to 20 deg in the Hermes 4A, the engine-out climb is good. (Use of 40 deg-flap,however, gave better field-length.) Such aircraft as the Apollo and Viscount were designed to meet the original I.C.A.O. Vs2rule, and this and/or commonsense dictated the power-loading, flaps and control system. In the event, the Viscount 700 designwas settled before B.C.A.R.s were formulated but the Vickers designers knew what was coming and this aircraft is now fullycertificated in both Britain and America. One of the main shortcomings of printed airworthiness codesat the moment is in the take-off and landing of modern high- speed transports. These aircraft have low drag and great weight,and consequently are "slippery" in the air. Such aircraft may be heavily dependent on wheel brakes and sensitive to runway con-dition. As a result, the extreme landing distances may vary more widely than before. The proposed new British rational landingrequirements consider all types of landings with one or two engines out, in a cross-wind or on a wet surface or with any other typeof undesirable combination. One invariable result of performance measurement is that landing distance is extremely sensitive, i.e.,to reduce the distance by, say, 10 per cent, one has to reduce weight by a very considerable amount. Under the old code, which assumed maximum braking and adry runway, field length stayed constant and safety decreased, as braking power and test-pilot technique improved to counter theprogressive increase in approach speed. With the rational re- quirements, anti-skid devices, and other means for achievingshort landings, can be fully assessed against safety; this was not possible under C.A.A. rules. The new code also bringsout the desirability of having high air-drag and also controllable drag or thrust on the approach to minimize landing with excessspeed. A sketch on page 319 shows the tremendous variation in landingdistance produced by varying pilot-technique. Particularly on a dry runway, it is seen that by far the shortest landings can bemade by dropping die nose wheels on to die ground immediately, retracting the flaps to put all die weight on to the wheels, andthen applying full braking. Normally, however, it is possible to keep the nose fairly high, with full flap, and conserve the wheelbrakes as much as possible. It is certain diat future turbojet transports will embody some form of reverse dirust and, whensuch devices are fitted, the rational code will enable their effect to be assessed. Today, this equipment is one of manythings which are in a state of flux and, under B.C.A.R.s, it is possible to discuss the matter dispassionately and arrive at whatis probably a good solution bearing in mind the state of the art. The Human Aspect * Elementary statistics show that an aircraft has to be exception-ally dangerous before it suffers a noticeably high accident rate. A high proportion of present accidents (as distinct from reportableincidents) are ascribed to pilot error, and rational performance requirements are coming in hand-in-hand with a re-appraisal ofthis factor. This is not to suggest that captains will be able to fly less strictly, but a fairer assessment should be possible of whata normal pilot can be expected to do allowing for die usual random drop in pilot performance from die mean value. Thistends to lead one into dangerous waters, and to point out diat it is better for an airline's prestige to say that "Captain X madean error" radier tiian that die procedure under which he was flying was at fault. Accident investigation requires real co-opera-tion from all aircrew trade unions, conducted on an informed but not partisan basis. Pilots' unions are also keenly interested insuch duties as ferrying aircraft home on three engines after a failure overseas. In such flying the likelihood of an incidentis normally much greater than the standard 10 *s ; on the other hand, if crews were given a bonus for such work it might be lessunpopular. W. T. G.
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