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Aviation History
1938
1938 - 0148.PDF
54 FLIGHT. JANUARY 20, 1938. ator—or vice versa in the case of requests from the control to the pilot of the machine concerned. Another small lag- reducing innovation consists of the placing, at a convenient place on the control-tower balcony, oi a series of marked switches which are connected >to a similar series of lights in the radio operators' department. With its help the look- out man can tell the operator at once whether the machine's motors can be heard and from which direction, the switches and lamps being marked in the appropriate manner. This information can thus be handed on to the pilot, via the operators, with the minimum delay for the adjustment of his final approach. In addition to the medium-wave equipment, Croydon now has an ultra-short-wave approach beacon, which is, of course, controlled from the tower, where its control panel acts also as a repeater, the correct functioning of the main beacon and of the two marker beacons being shown by means of signal lamps. Two other approach transmitters are also installed near the aerodrome boundary, but these are used only for experimental purposes by the radio people concerned. More recently—in fact, during the past few weeks—an omni-directional beacon has been put into action near Mitcham, where the transmitting station for Croydon is situated. The problem of traffic control during busy periods is only equalled in its seriousness by that of radio con- gestion on the available wavebands, and the beacon at Mitcham provides an attempt to reduce the number of messages which are being received and sent. Needless to say, this beacon can only be useful to the crews of machines fitted with some form of D/F or homing equipment, and the congestion problem can, in fact, only finally be settled by the standardisation of the radio and other equipment actually carried by the machines. For the moment only the most modern transports carry the necessary equipment to make use of a transmitter of this kind or of a short- wave approach system, by such means reducing the neces- sary communication to a mere statement of position from time to time and to the reception of permission to enter the controlled zone and to land. Inside the Mitcham omni-directional beaconwhich, used in conjunction with a similar trans- mitter at Schiphol (Amsterdam), should help torelieve radio congestion in the London-Continent airway area. The photograph shows theautomatic control. Croydon's direction-finding system is actually arranged in the control tower itself, but many other airports have this equipment placed well outside the aerodrome boundary. In fact, Croydon's equipment is of the loop type, which has the advantage of compactness, while the aerial system necessary to reduce night error effects to an absolute minimum covers quite a considerable area and may form a not unim- portant obstruction. Such is the price of accuracy. With the D/F equipment placed as it is at Croydon, however, it is not practicable to make approaches by what is known as the " ZZ" system, which involves very accurate flying methods and the placing of a D/F hut on or near the aerodrome boundary. It seems possible that equipment will be so placed at Croydon in due course, and possibly within the next few*months. So much for the radio side of air traffic con- trol. The more technical features and the likely future developments are adequately dealt with in an article by Mr. Samuelson which appears elsewhere in this issue. In the control office itself there is a large map of the Heston, Portsmouth, and London- Continent airways areas, with a bearing " rose " having its centre at the airport's position. The officer in charge keeps track of all the machines within the London-Continent area by means of identification flags, which may be pinned into the board at their approximate positions— the direction in which the flag is pointed re- minding him whether any particular machine is inward or outward bound. All messages are passed to him by the operators, the flags moved accordingly, and these messages carefully filed beside him in a series of " open " pigeon-holes. If, in the case of a machine which has its own very full navigation equipment, no message is received for a long period, the identification flag is moved now and again according to the control officer's own esti- mation of its probable progress. Modern speeds tend to make this estimation rather a difficult business, and it seems likely that there will need to be developed some kind of automatic indicating machine on which, once a machine's ground speed has been correctly estimated or reported, its progress will continue to be shown without attention. Unfortunately, it would be necessary to invent an almost impossibly complicated three-dimensional model if all the facts concerning each machine were to be indicated. Necessary Experience It would be impossible to describe in detail all the duties of a control officer of a busy airport, but it is at least obvious that he must have a very full knowledge and experience up to Second-class Navigator's standard. To mention one detail which is not often considered by the ordinary person, he must visit the meteorologist's office before coming on duty and so obtain a working knowledge of the conditions likely to be experienced within his area during his period of duty. At Croydon all the inter-office messages and general information are transmitted through the building by means of the familiar pneumatic tube. Although there is, on the control officers' desk, a minia- ture D/F plotting board, the main board on which all bearings and positions are obtained is necessarily located in the radio-operators' department. In the case of Croy- don the points of reference are Croydon itself, Lympne;| and Pulham, and strings to which are attached little lead«| '' mice'' may be moved from these three points on the | operators' chart in order to obtain a machine's position.jj Nowadays such positions are very rarely required by
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