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Aviation History
1953
1953 - 1652.PDF
Ground installation of an experimental "narrow-beam' Crown Cop/right I.L.S. localizer system operated by the Blind Landing Experimental Unit. ALL-WEATHER LANDINGS... term errors which may have a destabilizing effect. A more reliable rate term can be inferred from the relationship existing between the actual aircraft heading and that required, allowing for drift, when tracking parallel to the runway centre-line. In practice, experienced pilots will fly a mixture of heading and I.L.S. information. To assist the pilot in making the necessary mental calculations and thus to achieve accurate approaches, various methods of simultaneous presentation of I.L.S. and rate information have been devised in which the desired flight path, or even the ideal aircraft attitude required to achieve this path, is automatically computed. These devices are known under the generalized title of "couplers." In its most advanced form the coupler feeds I.L.S. and attitude information, using the appropriate transfer func tions, into the autopilot, thus constituting an automatic approach system. In this case the pilot, having adopted the appropriate landing configuration, merely selects the approach facility required and maintains his air-speed at a specified value, taking over from the autopilot when he wishes to "go visual" and land or overshoot manually. The assessment of the accuracy of the different approach methods is being investigated by the Unit in full-scale tests, during which the aircraft position in the two planes, at different ranges from the threshold, is recorded photographically. Sub sequent analysis of the records enables a figure of merit for per formance to be assigned. The criterion of "approach success" adopted can conveniently be quoted in terms of the lowest height on the glide path from which the instrument approach could ideally be converted into a successful landing in every case with out exceeding specified safe manoeuvring and terminal limits. In all cases so far investigated it has been found that, provided an appropriate air-speed has been specified, behaviour in azimuth is the limiting factor. Where instrument approaches are being examined arrangements are made for the pilot's view to be suit ably obscured; on these occasions a safety pilot with dual con trol takes over at an appropriate time to overshoot or land. It will be apparent that, whichever coupler is used, there will be a manual phase of the approach during which the pilot will be adapting himself from instrument (or automatic) to visual flying. This finite period, known as the "transition," will be an important one, since during this time the aircraft is virtually without guid ance. Sufficient time must also be available, once the transition is complete, for the pilot either to flare out and land or to over shoot safely; and the minimum acceptable visual range of the lighting pattern denning the weather limits is therefore depen dent to a great extent on the ease with which the transition can be made. Although the tests are still incomplete it appears probable that the potential accuracy of the more erudite I.L.S. couplers will not vary very greatly from one to another or from that obtainable with good G.C.A. If this is so, the choice of coupler will depend mainly on the relative ease of transition, which will again be influenced by the effort called for from the pilot and his degree of confidence during the approach. The method of presentation adopted for monitoring the approach accuracy, as distinct from that required to fly the beam, may well be important. Visibility Experiments.—The complementary part of the prob lem is concerned with the efficiency of the approach and runway lighting systems in all weathers by day or night. This has involved the Unit in a unique series of flying experiments, carried out over the last three winters. A Devon aircraft fitted with auto matic approach equipment was flown as a test vehicle to assess the range and effectiveness of the approach lights as seen from the glide path during the landing manoeuvre. This had to be done at all hours and under the worst possible conditions, includ ing ice and snow, owing to the impracticability of inducing sufficiently "natural" effects on a large enough scale. Landings carried out under these conditions have yielded valu able information about the extent of approach and runway light ing and/or marking which must necessarily be visible in order to define the ground plane without ambiguity. In addition it has been proved beyond doubt that, if the maximum use is to be made of the approach aids and lights, it will be necessary to devise some means of forecasting the slant visual range on the approach as seen from various heights on the glide path, using, this criterion to specify the operating limits rather than the more usual meteorological parameters of cloud base and visibility. Fortunately, it has been found possible to correlate the range of the lights as seen from the air with measurements of the visual range of a light flown from a balloon on the glide path as seen from the ground. This balloon was capable of being raised and lowered quickly by a mobile winch; and the method, which obviates the use of aircraft, has therefore been recommended as a sub-standard for further meterological investigations. No correla tion was, however, obtained in these experiments between the horizontal visual range at ground level and the slant visual range, owing to the non-homogenous nature of fogs or clouds near the ground. A practical method of obtaining slant visual range directly may therefore have to be devised. On one occasion during these tests the pilot saw nothing during his landing until he was down to a height of only 70ft. At that time, when the meteorological visibility at ground level was of the order of only 35 yd, the nearest diversion airfield was Prestwick! Needless to say, on such occasions the generous expanse of the Woodbridge runway (250 yd wide by 3,000 yd long) is a comforting thought. Naturally, every precaution has been taken to provide the airfield with emergency landing equip ment. In addition to a complete stand-by microwave I.L.S. system (air and ground) a precision radio altimeter is fitted, with a "flare-out" instrument on which the pilot can (and does) land completely blind if necessary. It is of interest to note that, in recognition of his work in this field, the pilot of the Devon (S/L. R. C. Everson) was awarded the A.F.C. last January. Future Tasks.—A detailed study of the mechanism of transition is proposed in order to establish the second criterion of coupler performance. Although full-scale experiments will continue, use will also be made of action-speed simulator techniques. Such methods may be attacked as lacking in realism, particu larly when applied to that part of the manoeuvre where subjective effects and emotions are obviously important. On the other hand, experiments devoid of risk can be extended considerably in scope and the rate of collection of data enormously increased. The use of such a simulator in the evolution of approach lighting systems was found to be invaluable. Unless there are fundamental changes in aircraft design, the present tendency towards ever-increasing approach and landing speeds may swallow up improvements in couplers without reduc ing limits so long as landings have to be completed manually with visual guidance. Although the situation may be eased tem porarily by the introduction of dispersal systems such as Fido, it is possible that only by eliminating the direct visual part of the landing will weather be completely defeated. This will require either a reversion to the fully automatic landing techniques or the development of a system which will replace for the pilot, with sufficient detail and realism, the essential aspects of the visual world otherwise denied to him. Both these possibilities are receiving attention as research items. HELICOPTER ASSOCIATION JOURNAL "Vfl"ANY of the papers published in the quarterly Journal of the •"•* Helicopter Association have become standard reference- works. As a number of the 24 issues are now out of print, the Association are considering the possibility of reproducing the whole series in the form of three bound volumes for sale at £5 each, a price in line with those of individual back numbers. Before taking this decision, however, the Association wishes to assess the probable demand, and those interested are requested to advise the administrative editor, Mr. C. Colin Cooper (at the Helicopter Association of Great Britain, Londonderry House, London, W.l).
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