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Aviation History
1952
1952 - 0418.PDF
190 FLIGHT, 15 February 1952 RADIO and TRAFFIC CONTROL Further Abstracts from the Lecture by Mr. G. W. Stallibrass LAST week we published abstracts from a British Institute of Radio Engineers' paper in which Mr. G. W. Stallibrass (of the M.C.A.'s Directorate of Navigational Services) discussed the problems confronting those concerned with the development of air radio and traffic-control aids. Among aspects of the subject dealt with in these further and concluding abstracts are methods of increasing landing rates; further development in instrument-approach systems; and the provision of aids over sparsely populated territories. Mr. Stallibrass also reviews the requirements for turbine-engined aircraft and has a word to say on the differences between the American and the British attitudes to current "aid" problems. AS REGARDS methods of increasing the landing rate /% it is generally accepted that the minimum safe separa tion for aircraft in a vertical holding pattern is 1,000ft. This [said the lecturer] was confirmed by radar checks carried out last year, in which errors of some hundreds of feet were observed, often caused by pilots descending below their new altitude before levelling out. The risk of overshooting the next lower altitude is generally greater at high rates of descent, and for this and other reasons, including avoiding rapid changes of pressure in unpressurized aircraft or an uncomfortable floor angle, the chosen rate of descent is about 50oft/min. This means that each change of 1,000ft altitude level will take two minutes, and this irreducible element in the stack-leaving interval may ultimately prove the limiting factor in speeding up movement rates from a single stack. Investigations are therefore proceeding into the practicability of feeding aircraft to a single runway from two stacks. If successful, the same principle could, of course, be used for feeding aircraft concurrently to parallel runways. However, when aircraft have to be phased from two stacks through a common gate, preliminary theoretical studies show that, if rapid landing rates are to be attained, present standards of time-keeping must be greatly improved. If the standard deviation of flight time is as much as a minute, it is difficult to achieve a properly separated frequency through the gate, even from a single traffic stream, if the landing intervals are to be as low as, say, three minutes. It becomes very much more difficult when the timing of an aircraft has to be adjusted not only to preceding aircraft in the same stream, bu also to other aircraft that are themselves under adjustment in the neighbouring stream from the other stack. Although a brilliant radar controller could achieve the desired result by his own personal knowledge and skill, no public service could be organized on the assumption that all its officers were artists of the highest order and were at the height of their powers for every minute of every watch; other means must necessarily be sought. In the meantime, therefore, two things are clear. First, that accurate radar surveillance will be essential. Second, that some form of computor will be required to calculate the separation which will occur at the gate between aircraft on two different and curved flight paths, and if possible to display to the controller the heading instructions that should be passed to the aircraft concerned if adjusting action is required. Given a flexible navigational system providing continuous indication of position, outgoing aircraft can join the airways at any predetermined point and need not suffer the present difficulties. The problem will still remain -of feeding them into the airways at their required airways altitudes, but it can be greatly reduced by radar surveillance. Instrument Approaches.—The step from a combination of aids that enables an aircraft to approach safely down to, say, 200ft, after which landings are completed visually, to an aid which enables the pilot to land his aircraft and finish the landing run without taking his eyes from the instruments, is a very large one indeed. The justification for it requires careful scrutiny, from the aspects of the inconvenience and dislocation caused to passengers by being diverted, the cost and trouble to operators in having to organize handling and diverted passengers and aircraft, and the expense to administrations in having to provide diversion airfields and facilities. One method of bridging the gap is in the provision of thermal fog dispersal (Fido). Assuming this is not always an economic proposition, it is worth considering what an enormous responsibility will be placed on the designers, manufacturers and operators of radio or radar facilities designed to provide completely blind landings. The present concept of a minimum break-off height during an instrument approach, after which the aircraft should overshoot and climb away if the visibility from that height is insufficient for a visual landing, is based partly on the aerodrome terrain concerned, partly on the navigational aids, and partly on the performance of the aircraft when overshooting from such an altitude. It follows that the risks involved in overshooting on instruments from altitudes lower than this optimum break-off height naturally increase the lower the aircraft comes, since the margin for some failure on the part of the aircraft or error on the part of the pilot is progressively reduced. A radio aid designed to bring a pilot right down on the runway and to keep him on it after touch-down, still on instruments, must therefore be able to guarantee that the aircraft with its 50 or 60 passengers will be able to complete its landings every time, and that the risk of a baulked landing, either from pilot error or from malfunctioning of the radio aid, will be negligible. The development of such aids to the point where they could be used regularly by civil passenger-carrying aircraft, and when the airborne equipment will be sufficiently easily maintained and reliable, is clearly a big and long task. For what it is worth (continued the lecturer), my personal opinion is that, however desirable blind landings may be, at the moment the air-transport industry would benefit more if priority were given to developing facilities and procedures for reducing I.F.R. landing intervals down to the order of IJ or 2 min. Two-minute landing rates have already been achieved from time to time both in the United States and the United Kingdom, but their achievement over a limited period is not quite the same thing as maintaining them regularly over a long period. Both I.L.S. and G.C.A. are very expensive and there are instances where the density of traffic at a particular aerodrome does not really justify use of this equipment, but where the air services concerned—-which may only amount to about six or eight movements a day—are of particular importance to the local communities. Depending on local conditions, one measure has been to install non-directional M.F. beacons on which the aircraft can make instrument let-downs with the use of A.D/F. Unfor tunately, the congestion on the M.F. band is too great to permit the number of installations desirable, and other means must be sought. A promising means of meeting the requirements for an inexpensive cloud-breaking and instrument approach aid is a ground installation combining a simple search radar on 3 cm, using an "A" scope, with V.H.F. direction finding. The addition of distance measuring information to the ordinary D/F. let-down or QGH procedure should enable aircraft to position themselves sufficiently accurately to complete their approaches in visibility conditions considerably lower than those that will be suitable for a D/F. let-down and approach only, though naturally not so low as those achievable by the use of I.L.S. or full G.C.A. A model of such an aid has already been produced as a private venture and, with certain modifications, it seems promising for the purpose outlined previously. [See page 182—Ed.] Special Aids to Control The continuing development of air-traffic control requires a number of new aids for specialized functions. Three or four examples will be given as an instance of the field to be covered. It is well known that there is an inverted analogy between the problems of air-traffic control and those of intercepting a hostile aircraft, the inversion arising from the fact that air-traffic control clearances are based on calculations leading to the assump tion that the particular clearance concerned will lead to an avoidance of other aircraft by a required margin. A computor is needed to work out the take-off clearance for an aircraft in order to avoid conflict with aircraft inbound or taking off from other airfields at about the same time. This means that information must be fed in, derived from previous flight plans and in-flight information, and the computor must either provide the details of the safe clearance for the next flight or must indicate whether a proposed clearance will be dangerous or safe when details of this clearance are in turn fed into it. The requirement quickly becomes more complicated. It will be apparent that the information fed into such a computor does not remain valid for long but must be kept up to date. Aircraft do not keep to their flight plans; sometimes they change them involuntarily and sometimes by request. This means that the flow of information to the computor must be continuous. It is unlikely that it will be practicable to feed in this information manually, from information received by R/T. during flight, and the alternative is to provide it continuously, probably by radar
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