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Aviation History
1960
1960 - 0235.PDF
19 February 1960 235 w be approached. Any system which produces informationwav which can be readily transformed into error signals of tvi>e at present accepted by autopilots will prove adaptable to £raft presently planned for automatic landing.During an automatic landing sequence certain critical switching rirations will probably have to be performed at the correct point r the approach so that position during the approach will e to be known. The basic parameters from which position8av be calculated are vertical angle relative to touchdown, azimuth 1 de relative to runway centre-line, displacement relative to glide- ,ath and centre-line, and height or range. An indication of the areas of uncertainty likely to occur when trying (o define position along the approach, using various sources of informa- tion with and without ILS angular values. Smoothness and continuity of information are also important ILS provides the two angular values and position may becalculated from these by adding either height or range measure- ment. The diagram above shows the line of uncertaintywhich may result if the height term is used because of the considerable horizontal spread allowed by glide-path tolerancesfor a given height value. The line is extended to a plane of uncertainty by the equivalent angular tolerance in the localizer;and that plane is extended to a volume in proportion to the inaccuracy of height measurement. Nevertheless, accuracy willconsiderably improve as the vertical and lateral distance repre- sented by the angular tolerances decreases near the beam origins.Vertical spread caused by the lack of a smooth reference surface for radio or sonic altimeter is likely to be excessive until smoothground occurs shortly before or actually on the runway. If position co-ordinates were derived from an area-coveragesystem as accurate as Decca in its optimum coverage—and this accuracy would be substantially constant over the whole approach—position on the glide-path might be derived by adding the height term. Here a cylindrical area of uncertainty would remainwhose radius would be determined by positional accuracy and depth by height accuracy. Radio and sonic altimeter limitationsstill apply. Positional information might be combined with glide- path angular reference, with resultant improvement at points further out along the approach, where height-sensing is poor.Finally, the two angular values could be combined with range which is after all the most desirable direct measurement.Horizontal spread caused by angular tolerances could be greatly reduced using DME-type equipment and the system would bereasonably accurate at points further away from touchdown. The criterion is that the system should provide a level of errorsignal appropriate to the aircraft characteristics, having regard to aircraft inertia and the effect of cross-winds and gusts. At inter-mediate ranges of about one mile, ILS often requires long time- constants to overcome step errors in the beam and thereforecannot provide the information necessary for tight control. It is obviously desirable that the group of measurements chosenas the parameters for denning the approach should all be derived from one system and that their accuracy should be matched andremain reasonably constant throughout the approach. One method is a single microwave system capable of providing accurateangular and range information to be sampled in the aircraft. From this, position on the approach could be derived and almost anyglide-path angle suitable for the particular aircraft computed in the airborne equipment. A conventional radio altimeter-controlledflare-out would probably be retained and the range information would provide a clear indication of the moment at which thesystem should be armed to switch from approach to flare-out phase. This is the solution offered by Gilfillan Regal, but it iselectronically very complicated. At present, the moment for switching to flare-out can only bejudged when the radio altimeter first senses a given height above ground. With sloping or broken ground in the undershoot areathis indication may be reached quite suddenly. There is a slight possibility of the flare computer being prematurely triggered if anobstacle projects close to the glide-path, but existing obstacle- clearance limits should prevent this. Smooth range informationallows for some preparation, either for the pilot or the equipment, when flare-out is approaching. Altimeter limitations over roughground might be avoided by making the equipment insensitive at heights above, say, 50ft or 150ft. A development of GCA might allow displacement and rangeinformation to be sampled in the aircraft without the present expedients of voice or data link, but clutter at short ranges willstill be a problem. A computer may be added to a lock-on radar to detect and compare the position of an aircraft with thatdemanded by an approach programme. The Bell ALS works on this principle and controls the autopilot by means of a VHFcommand link. The ground-based equipment exists at present only in single-channel form and is very expensive, but muchcheaper equipment could be used to provide an independent monitor for automatic landings. Such a back-up facility willpossibly be demanded for civil operations. The accounts of ILS development work by Pye and StandardTelephones and Cables later in this review indicate that centi- metric systems are being developed by these companies and thatthe problem of random interference with ILS is being investi- gated. Pye also mention a new flare-out system based on the ILSglide-path beam. One such method was suggested in America some time ago and consisted of a second glide-path at a shallower 1500ft RUNWAY CENTRE-LINE DEFINED BY ILS 30Wt £=110&ft The pioneering BLEU Autoland system using Smiths autopilot and speed-control, Murphy Leader Cable, S.T.C. radio altimeter and Pye ILS. Autoland has been adopted for Bomber Command and will be the first automatic landing system ever to go into regular operation. British civil systems are developed from it. A description appeared in "Flight" tor October 17,1958 ILS GLIDE PATH TRANSMITTER SELECTED BY PILOT _EADER CABLE INITIATED BY ENTRY INTO LEADER CABLE FIELD INITIATED BY AUTOMATIC HEIGHT-OPERATED SWITCH MANUALLY CONTROLLEDRUN WITH DIRECTOR INFORMATION • i-S BARO- COMPASS METRIC "*ALIZER HEIGHT lLOCALIZER I ATTITUDE ALLOWING FOR WIND. AZIMUTH AUTOPILOT AZIMUTH PITCH AUTOPILOT AZIMUTH PITCH AUTOPILOT AZIMUTH PITCH AUTOPILOT i • . I B "— ! AUTO THROTTLE CONTROLLED BY AIRSPEED ERROR AND PITCH ATTITUDE CHANGES -- -" BY AUTOPILOTDEM/ )ED :ABLE I ALT TTH AZIMUTH PITCH AUTOPILOT AZIMUTH PITCH AUTOPILOT AUTO THROTTLE REDUCES POWER TO SAFE FLIGHT IDLING INPUT PROPORTIONAL TO SIGNAL INPUT PROPORTIONAL TO SIGNAL RATE OF CHANGE
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