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Aviation History
1952
1952 - 0354.PDF
i6o FLIGHT RADIO and TRAFFIC CONTROL Abstracts from a Notable Lecture by an M.C.A. Specialist DURING the fourth session of the 1951 Radio Con vention, held at Southampton under the auspices of the British Institution of Radio Engineers, one of the most informative papers was that delivered by Mr. G. W. Stallibrass of the M.C.A.'s Directorate of Navigational Services (Control and Navigation). Abstracts from the first part of the Lecture are given here, and others will be included in a further instalment. The author discusses the application of radio to certain naviga tional and air-traffic control problems which are now being encountered or are likely to arise in the foreseeable future. He introduces his subject by referring to the economic difficulties and international tension which have been responsible for creating the present melancholy situation in which most of the en route navigational and traffic control procedures throughout the world are still based on radio facilities that were recognized as being ready for replacement as long ago as 1947. The failure of certain new equipment to reach the standard of development originally envisaged and the realization of the high costs of providing numerous new installations have combined to introduce such an element of uncertainty that the absolute necessity of world-wide standardization on aids—at one time considered axiomatic—is now being re-examined. The airlines themselves now have doubts of the economical justification of installing certain airborne equipment, the develop ment of which has previously been strongly urged. In this paper the author reviews the present situation under the general headings Of high and low density areas. In high density areas [he said] a high proportion of services will be flown on short stages, there will be a density of traffic along a network of routes, and congestion may occur at the main airports. Navigation is no longer simply a problem of flying direct from one airport to another, but has become a matter of timing and routeing the flight, in accordance with a previously notified flight plan or the in-flight requirements of air traffic control. In short, the problems of navigation along the main routes have become inseparable from the requirements of an effective air traffic control system. Limitations of Present Equipment "En Route" Navigation.—On several European routes there are now corridors of controlled air-space known as airways, and aircraft desiring to fly within them under instrument flight rules must do so in accordance with an air traffic clearance specifying route, timing and altitudes. The radio ranges to mark them are medium-frequency radio ranges and M.F. non-directional beacons used in conjunction with automatic direction finders on the aircraft, although some aircraft still use Gee where cover is available. There are still some parts of Europe without airways, and others where their demarcation is by non-directional beacons only. The airways scheme which now serves major routes in the United Kingdom is marked by M.F. ranges. The fact that a shortage of frequencies on the M.F. band seriously restricts the number and power of M.F. installations is probably well known; but the feature which is of particular note is that, even when skilfully used, ranges and beacons by themselves can only give an indication of track and not of position. If an indication of position is required along a leg of a radio range, it requires the installation of a non-directional beacon or a V.H.F. fan marker. There is obviously a limit to the number of marker beacons that it is practicable to install and the use of non-directional beacons for this purpose is limited by the frequency difficulty just mentioned. This severely limits the position information obtainable from aircraft as they pass along the airways. So far as lateral tracking separation is concerned, beyond a distance of about 15 miles from the range station, it is practicable to establish two tracks in opposing directions by means of what is known as "right side separation"; but on the whole it is fair to say that, with airways marked by radio ranges, air traffic control cannot count on being able to give aircraft lateral separation wherever it is needed. Within these limits, radio ranges can and do give very useful assistance. It is apparent, however, that their services cannot meet all requirements when two busy airways join and it is impracticable to assign vertical separation to a succession of aircraft passing through the junction point: when fast aircraft are overtaking slower aircraft and, at some stage, descending through their altitude to reach a holding pattern ahead of them : and when aircraft with long ascent and descent paths, such as the later models of certain piston-engined types and probably all turbine-engined aircraft, require to go through the successive altitudes of an airway steadily during a period of anything from 15 to 40 minutes. Initial Approach.—For the initial approach from the airway to the holding pattern, the objective is to get the incoming aircraft into the lowest vacant altitude in the stack. Since the flight from the last reporting point should be short, the accuracy of the E.T.A. in the stack is generally fairly good; but there is need for an effective and foolproof means of checking that the assigned altitude in the stack will be vacant and of notifying this information to the air traffic control officer responsible for giving the aircraft clearance to the stack. Another problem is that, with existing navigational aids, aircraft departing from the airport have to be routed over the same facilities as those used for marking the holding pattern. This means that the lowest altitudes over the radio facility concerned will be reserved for departing aircraft, but, once past that facility, they will require to climb as rapidly as possible to their efficient operating altitude. This often requires passing through the altitudes of incoming aircraft and, even with a more flexible navigational system than radio ranges, radar surveillance of the air-space concerned is essential. Holding Pattern.—Typical patterns are of a racecourse shape based either on two non-directional radio beacons or on a radio range with a non-directional or fan marker beacon situated a few miles along one leg from the range station. Present navigational aids do not enable aircraft to maintain their assigned stack-leaving times with sufficient accuracy, mainly because the pilots can only know exactly where they are twice during each pattern, i.e. when they are passing over one or other of the beacons concerned. The performance noted from radar observations taken so far may improve as aircrews obtain further experience on the system; but, at present, allowance has to be made for leaving errors to vary between one minute early and nearly one and a half minutes late. Many errors will be less than this, but, on occasions, they will be more. Intermediate Flight from Holding Pattern to "Gate."—Various methods have been pursued to take out the effect of these variations and to achieve a smooth flow. The G.R.S. Timer, an American system which required all aircraft to be fitted with twin A.D.F., was withdrawn from use at La Guardia Airport _a few months ago, reputedly because it contributed delay to the 'operation of control rather than reducing it. Presumably this means that, when traffic was dense and the controller was most busy, he had no time to feed in the wind speeds and air speeds and operate the equipment. The Americans have recently started to use surveillance radar at New York, which is of interest since we ourselves began using surveillance radar for controlling aircraft from the stack some two years ago. The landing systems in use at London and Northolt are designed to take advantage of the fact that, with radar, control can see when an aircraft leaves a stack and can assign it one of a number of flight paths from stack to "gate" whose variations in length will reduce the effect on runway arrival of errors in stack-leaving times. These tracks are precomputed to give the desired variations and the aircraft is assigned the one appropriate to the particular variation required. They can also be used to reduce the effects of wide variations in approach speeds, but cannot eliminate them entirely since the approach speeds of different aircraft types vary by as much as 60 knots in extreme instances. A remaining difficulty is that when an aircraft has been given a selected track to fly from stack to "gate" there are still appreciable variations between the actual and estimated times taken by aircraft to cover even this short distance. Over a period of about 6£ minutes' flying the variations may be as much as a minute. At the moment there is no available computer that will enable the controller to give changes of heading during the intermediate stage in order to meet these final variations. This problem is one of a number now receiving attention. Final Approach.—On the final approach phase the traffic control problem arises from the fact that operators of large aircraft prefer to have a straight-in final approach of not less than six nautical miles in order to give the pilots time to settle down on their various instruments, including I.L.S. localizer and glide path receivers, and to complete their preliminary landing drill. The navigational problem, as distinct from the control one, is to provide an aid on which it is practicable for the pilot to approach the runway confidently and accurately down to a low altitude. Present practice has shown that, used in conjunction with good approach lighting systems, either well-operated G.C.A. or I.L.S. equipment is reasonably adequate for this, and the
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