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Aviation History
1939
1939 - 0297.PDF
FEBRUARY 2, 1939 FLIGHT. 109 JOU-AS the system opinion has changed somewhat as to the risk ol interference between adjacent systems, the practicability of the glide path principle, and the width of the beam. So far as Great Britain is concerned, it is probable that three or four different frequencies will be used, Croydon staying on the present '' international'' frequency of 33 megacycles. The primary value of the system now appears to be recognised as giving the clearest line of ap proach to the airport, together with two " spot " positions, both the actual approach from the first marker beacon and the landing being accomplished by instrumental observa tion and not radio means. The beam being a diverging one, the width of the equi-signal zone is proportional to the distance from the origin, and if an approach is started too far away, exactly in the centre of the beam, there is a risk of zig-zagging between the dot and dash zones. Hence the fact that the majority of pilots prefer to fly on the threshold of the beam—i.e., where the dots or dashes just begin to merge into the continuous note, and, consequently, where any deviation from the track is immediately noted. A recent development of the Lorenz system has been tried by Lufthansa, and consists of the use of a third marker beacon, about 25 km. from the main beacon. In some cases this is to be supplemented with a miniature medium-wave omni-directional beacon the transmission of which can be used for "homing" by means of the D/F receiver on the machine. The idea is to provide, during QBI con ditions, a rallying point outside the controlled zone where aircraft can be told to "stand off" whilst awaiting their landing turn. The third beacon also gives the pilot more The disposition of this country's radio control areas, each of which is in charge of a main station. Until comparatively recently only the Croydon, Heston and Portsmouth areas were specially defined, but during 1937 the Manchester (West) area was introduced in order to cope with the amount of traffic over the Irish Sea. time to adjust his height correctly over the intermediate beacon, which is usually a signal for the final approach. It now seems generally accepted that the ideal to aim at for real landing aid is an indication of a virtually straight- line glide path, approximating to the natural gliding characteristics of an aircraft. A beam of such a nature is much easier for a pilot to follow, and simplifies the problem of a fully automatic blind landing, especially in combination with a tricycle undercarriage. Intensive research in Germany, America, France, England, and elsewhere is now going ahead in order to achieve the straight-line approach. Several systems utilise horizontal radiators for the vertical guidance (glide path) beam, and it is claimed that the radiations from this type, as opposed to these from the vertical type, are immune from variations caused by the condition of the ground, and that they propagate a straighter field-strength pattern. A new blind landing system made by Le Materiel Tele- fonique in Paris is shortly to be installed at Le Bourget after lengthy trials at Troyes, where pilots of Air France are now under training. The usual horizontal guidance is given by means of a transmitter on 33 megacycles (the '"international" frequency) feeding a vertical radiator system, and the beam itself is only one degree in width for "on course" signals, which are made apparent by visual and aural means as in the case of the Lorenz re ceiver. When "off course," strong dots or dashes are heard, so that, at a distance from the beacon, there is little chance of wandering inside the equi-signal zone of the beam. The usual marker beacons are provided at the edge of the aerodrome and at a distance of 5 kilometres. Refinements For the glide-path definition an entirely separate trans mitter is used ; this operates into a horizontal aerial system and gives, substantially, a flat line of descent. These transmissions are picked up in a separate receiver aerial and circuit in the aircraft and are made visible through a vertically moving needle. Quite a wide tolerance is given in the movement of the needle in relation to the path to be followed, so that see-sawing down the beam is pre vented. This landing beam flattens out sufficiently near the ground to permit of flying on without risk ot " balloon ing " provided that the speed is controlled within reason able limits. One advantage of this new system is that the two most interesting components, the landing beam beacon and the associated aircraft receiver, can be added to an existing blind-approach system of Lorenz or Telefunken type, such as now found at most of the major European airports. Thus no capital invested in existing plant need be thrown away, and, in effect, a refinement can be added. Similarly, if this new system is installed in its entirety it can still bo used for directional guidance by aircraft fitted with the present type of V.H.F. receiver. There seems little doubt that, ultimately, the fully auto matic blind landing will become a normal event. By the time that the combined auto-pilot and radio system is per fected, an added safety factor may be given in aircraft characteristics where these affect normal approach and landing methods. Certainly there is something to be said for the theory that when flying is easier it will become safer; merely to mechanise everything at the present stage is rather begging the question of safety. The science of radio can always be called in to give assistance, but it must not be made entirely responsible for the solution of all its difficulties. Rather it is to be hoped that continual collaboration between air transport, aircraft design, and radio engineering will result in the real safety which is their individual aim. Due to circumstances beyond our control, the article on Ah Traffic Problems, announced as a feature of this \ I number, is unavoidably held over until a future issue. •
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