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Aviation History
1960
1960 - 0234.PDF
234 FLIGHT, 19 February SYSTEM SURVEY system. Although Britain is first to apply the technique to bothmilitary and airline operation, the concerted American programme, backed by considerable financial resources and good publicity,may threaten the British lead in a relatively short space of time. Clearly ICAO will have to examine the various approachesin order to establish some form of international standard. Here there is real danger that an American system might be adoptedregardless of its merit relative to any British counterpart. History has proved this all too clearly. The only hope of achieving anequitable solution lies either in the fortuitous case of the two systems being sufficiently similar to be covered by a single standard(this is not as unlikely as it might seem) or in coordinating the two countries' efforts by means of early technical co-operationof the jt*ad..begun**last«year between the MoA and the FAA. Another headron collision- such as that at Montreal last yearcannot provide any satisfactory solution. Certain it is that ICAO will have to begin considering the matter within the next fiveyears and British companies clearly must recognize the implica- tions of this fact. A British ground installation is unlikely to be supported effec-••tively-by -either of the airline corporations because BO AC is deliberately not committing itself yet to any system requiringspecial (and therefore standardized) ground equipment and BEA has a special localized requirement which could initially be satis-fied by a single ground installation at London Airport alone. As far as a time-scale is concerned, it would be precarious tomake any firm prediction of when automatic landing will become a routine operation, even at one or two airports. The cleareststatement is that by de Havilland (Flight for January 22) to the effect that they hope that automatic landing in the D.H.121 inBEA service might become routine during 1970—ten years hence. In the meantime it should not be forgotten that Bomber Com-mand aircraft are being equipped for Autoland now—a world "first" which has so far received less recognition than is due. In parenthesis, it may be added that blind landings were suc-cessfully made many years ago and that several thousand success- ful automatic landings—many of them also blind—have beencompleted on both sides of the Atlantic using a wide variety of systems. The commercial situation is still that, although literallymillions of blind or automatic approaches are made in scheduled service each year, not a single normal blind landing has yet beenmade in scheduled service. The subsequent sections of this review give details of Britishand American systems, a summary of the general requirements, a study of the D.H.121 and VC10 projects and a study of ILS whichundoubtedly will be the first guidance element for auto-flare. THE PROBLEM INSTRUMENT approaches using the purely angular informationderived from ILS have hitherto raised no very great problems so long as site-induced ILS errors have not been excessive. Theaircraft is flown down the beams to a decision point where the pilot can either see the approach lights to take over and landvisually or fails to see them and overshoots. This point can be defined with acceptable accuracy either by the emergence of theaircraft below a cloud-base of known height or, in the overshoot case, by barometric altitude indications which can be renderedsufficiently accurate by appropriate altimeter setting. Only at the Configuration of an automatic landing, showing distances, heights and times along the glide-path down to the beginning of the flare-out. The distance represented by the glide-path angular tolerance at various ranges is also shown. It is assumed that a radio-altimeter-controlled flare-out must begin over the runway TIME (s«c) at 12O (lOO)kt 15(1B)-+— '20(24)-*+*- 50(60)—-f*~ 50(60)~ O-2 GLIDE-PATH TOLERANCE FLARE-OUT transition point is it imperative that the pilot should know bisexact position along the glide-path as opposed to his angular deviation from it. Marker beacons and the intersection of the levelapproach with the glide-path provide convenient approximate references. The drawing, below left, illustrates the configuration of anapproach in the glide-path dimension from 1,500ft downwards. This height will be some 30,000ft from glide-path origin]Distances from origin are given at 300ft, 150ft and 50ft heights because at these points the present automatic landing systemswitches respectively to Leader Cable, attitude-hold and flare-oat. Distances along the bottom of the drawing show how far theseglide-path heights are from origin and the times at the top show- how long, in seconds, an aircraft will take to fly through thesesectors at 120kt and, in brackets, at lOOkt, The first speed allows for a slight headwind or still air, according to the aircraft, and thesecond for a headwind component which is perhaps as high as will apply in really low visibility. The D.H.121, for example, is tohave a steady approach speed of 127kt. The ICAO ILS specification allows an angular tolerance of0.2° in glide-path (0.6° for the localizer) and the vertical spread in feet at various ranges is shown in the drawing. It is assumedwith present ILS that glide-path guidance will have to be released at about 150ft and replaced by open-loop autopilot guidance inpitch down to 50ft where auto-flare takes over. This phase will last for between 10 and 12sec and aircraft must not be signifi-cantly displaced from the glide-slope if an undesirable spread in touchdown points is to be avoided. The flare-out, which begins atabout 50ft (the actual height depends on the aircraft and its systems), relies on radio altimeter measurements for which smoothground is required. It is therefore presumed that the aircraft will be established over the runway by the time it reaches this height,but that it will still be some 1,000ft from glide-path origin. The exponential flare-out curve is followed to a touchdown point somedistance beyond glide-path origin, this distance being established to some extent by the touchdown rate of descent selected in theflare computer and by the programme set in the automatic throttle control. As the ICAO standard approach aid, ILS is an obvious basisfor extending automation down to touchdown. Yet it can be argued that ILS is not the right form of guidance for landing. Itsfrequencies, for one thing, are unhappily chosen: if they were lower, site errors caused by reflections from ground objects couldbe avoided; and if they were higher, the signals could effectively be beamed away from the ground. As it is, site errors and randominterference significantly degrade accuracy and reliability of information in the majority of existing installations. The systemin any case employs one VHF and one UHF frequency, each with its own problems. It has been shown that accuracy can be very considerablyimproved by new aerial systems and that monitors and correctors can be applied without departing from the ICAO specification.But the provision of this accuracy may well entail alterations which would be as expensive as new equipment. Other alterationswill be required to provide a degree of stand-by equivalent to that demanded in airborne equipment and to ensure instantaneouschangeover to stand-by channels. It is questionable whether ILS in fact provides the kind ofinformation which is needed for automatic landing. Localizer and glide-path give only angular information. Establishment ofposition along the approach is assisted in the BLEU Autoland system by the reception of the Leader Cable signals which per-form one of the major switching operations at 300ft, some 6,000ft from touchdown. Other key positions are established by radioaltimeter signals. Either a microwave ILS incorporating distance-measuring—something like an aircraft-sampled GCA—or, conversely, some medium-wave, phase-comparison positioning system like Deccaappear to be attractive alternatives. Both these lines are being followed and at least four microwave ILS systems are beingdesigned, two in Britain, one in France and one in America. They are still far from completed and there is no guarantee that majordifficulties may not arise. Years will certainly pass before any of them can be adopted for civil operations. ILS has been in servicefor many years and its characteristics, both from the performance and maintenance point of view, are well known. It has beenshown that its shortcomings for automatic landing can be over- come with additional equipment. There seems to be no reason to suppose that ILS will be super-seded as a guidance element for automatic landing before 1970. The ICAO standard will more probably be extended for someyears, perhaps with rather tighter accuracy requirements. In early post-war years there was some discussion as to whetherILS or a British Rebecca/Eureka (BABS) system should be developed as a standard civil landing aid. It is not without interestto speculate what the automatic landing situation would now be if the British and not the American system had been adopted. As a long-term prospect, an entirely new system is certain tobe developed and it is worth while to consider in which way tins 0 1.000 3000 6000 10,000 20.000 50,000 DISTANCE FROM GLIDE-PATH ORIGIN (rt)
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