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Aviation History
1948
1948 - 0542.PDF
45° FLIGHT APRIL 22ND, 1948 VISUAL AIDS FOR LOW VISIBILITY CONDITIONS IN a Paper delivered before the RoyalAeronautical Society on April 15th,Mr. E. S. Calvert, B.Sc, A.R.C.Sc.I., who is head, of the Illu-• minations Sections, Electrical Engineer- ing Department, Royal Aircraft Estab-lishment, described some of the work which has been done at Farnboroughwith the object of establishing some general principles for the guidance of theMinistry of Civil Aviation Airfield Light- ing Committee, this being the bodycharged with the task of deciding what visual aids shall be installed at the civilairports of this country. The lecturer pointed out that the work is far fromcompletion and from the scientific point of view his account must necessarilyleave some loose ends. However, he felt that the method of attack and the under-lying ideas might prove useful to other workers in this particular field. Visual aids consist of day markingsand lights laid out on the ground in pat- terns which are intended to assist thepilot in making those judgments of height, distance and direction which arenecessary in order that he may approach the runway, land on it and taxi to theunloading point. Any discussion of visual aids, therefore, involves the ques-tion of how the pilot makes these judg- ments Everyone is so accustomed tomaking visual judgments from childhood onwards that it has become instinctiveand subconscious and it is rare to find anyone who possesses sufficient insightinto his own mental processes, to enable him to describe convincingly and intellig-ibly how he makes them. The lecturer was convinced that a systematic studyof this problem would pay handsome dividends in increased safety. Degrees of Visibility In good and moderate visibility, thebest daylight approach aids are white or yellow markings on the runway- As thevisibility gets worse, a point is reached at which the runway ceases to be visibleat a distance sufficient to enable a visual approach to be made. For a givenmeteorological visibility* it is not possible to say at exactly what distancethe runway will be seen as this depends on such factors as the reflectivities of therunway and its surroundings, the direc- tion in which the sun is shining, thetime of day and so on; but, as a rough guide, it may be taken that, in thiscountry, the runway is visible for about a mile when the meteorological visibilityis three miles. In this visibility, a range of one mile in daylight can be obtainedfrom lights having an intensity of 5,000 candles. In visibilities worse than threemiles, lights of this intensity will be seen at greater distances than the runway:lights of more than 5,000 candles could therefore constitute a useful daylight aid. The Royal Air Force discovered this in1942 by noticing that they could easily find airfields where synthetic day / • Strictly speaking, the meteorological visibility is thedistance at which a very large dark object can best be seen in daylight. In drawing range diagrams, this is conven-tionally taken as the distance over which the atmospheric transmission is 2 per cent. night training was in progress when con-ditions were such that they had diffi- culty in finding their own airfields, thereason being that a sodium flare path was used as part of the day/night equip-ment. As a result of this discovery, the use of sodium flare paths spreadrapidly throughout Bomber Command. This was the origin of the well-knownSodium Flare Path Type F which, since the flares gave an intensity of about 8,000candles, may be regarded as the first high-intensity landing aid to be used indaylight in this country. Because of their yellow colour andtheir large size, i.e., about 2ft by ift, these sodium flares are very distinctive,particularly when seen against* a back- ground of bluish haze or morning mist.Their light output per watt is very high, about 70 lumens per watt as against 6lumens per watt for yellow light obtained by filtering an incandescentsource. This high efficiency means that wide angles of the order of 120 deg hori-zontally by 30 deg vertically can be covered without using more than 150watts per fitting and this, in turn, means that the flares are suitable for use asshort-range aids to navigation, that is, for marking fixed points. They are nowcommonly used by the R.A.F. as day- light markers and they are very effective,particularly in the dull, misty weather which is so common in these islands. Candle-power and Range A light of 8,000 candles has a day-light range of one mile in a meteorologi- cal visibility of two miles, and this maytherefore be regarded as about the limit- ing condition in which visual approachescan be made on the sodium flare path. If the intensity is pushed up to100,000 candles, then the limiting condi- tion can be lowered to a meteorologicalvisibility of about 0.85 miles. Beyond this point, enormous increases in in-tensity are required to effect much im- provement, and it becomes clear that ananswer to the problem of daylight landing in really bad weather can never be foundby increasing the intensity of the run- way lights even if dazzle could be pre-vented. The obvious all-weather solution istherefore to lay down a pattern of high- intensity lights in the approach zone andto feed the aircraft into this pattern by means of a radio approach aid. Theradio aid is then relied upon to bring the aircraft down to a height of about 200ftin the worst conditions in which the visual aids are usable, and the problemis to make the visual aids link up smoothly with the radio aid at about thispoint. Instruments and Lights In really bad visibility, the techniqueof making the final approach on a con- tinuous string of lights is quite different,first, because the runway lights are in- visible for most of the approach and,second, because the horizon is ill-defined or non-existent. It was widely supposedthat if a pilot were able to see three or A Discussion of the Fundamental Re- quirements four lights ahead of him, he would some-how be able to complete his approach. On this supposition, pilots who wereskilful enough to make an accurate initial approach by radio down to 200ftshould have been able to land on the lights in meteorological visibilities of theorder of 200 yards by day and 100 yards by night, provided that the aircraft hada reasonably good downward view. A f^wv very experienced pilots have done thi>iunder test conditions but the. report? available to the lecturer indicated thatthese landings were attended by far more risk than could be accepted under opera-tional conditions. Even in better visibilities, many pilot?found difficulty in changing over from in- struments to lights and complained thatthe lights seemed to play tricks. For in- stance, on first picking up the lights, thepilot, in correcting a lateral error, might find that the lights seemed to slip side-ways away from him or even appeared to swing from side to side. Also, since thelight sources were small enough to appear as points, there was nothing except dif-ferences in intensity to indicate which lights were far off and which were near,and even this indication was faulty be- cause of the narrow divergence of thebeams. Since the lights had no appreciablesize, no use was made of one of the chief mental mechanisms which everyone usesin order to find his way about the world, namely, observation of the texture of theforeground, or to put it another way, the observation at short range of objectsthe real size and shape of which are known. The result was that the psycho-logical obstacles to quick interpretation were increased, an effect which is particu-larly dangerous to a pilot suffering from fatigue, worry or other distractions. Finally, since this pattern gives no in-dication of whether or not the aircraft is over-shooting or under-shooting untilthe runway lights come into view, the meteorological visibility at which thispattern ceases to be safe for operational use is about 900 yards in daylight or about300 yards at night. This applies more or ; less to all systems consisting merelyJj#single or double lines parallel to the$.«^ tended centre-line of the runway. Inconspicuous Runways On a modern airport, the contrast :between the runway and the rest of the : surface is often small and the surfaceitself may have little texture. Day-time mist first reduces what contrast intexture there is and, as it gets worse, finally blots out the whole horizon andall the vanishing points associated with it, so that the pilot is left only with thelights for making his visual judgment. It follows that the approach light pat-tern must, as a first necessity, supply the equivalent of a horizon and it also M-lows that this can be done by bar? oi lights running transversely across theextended centre-line of the runway. It is also clear that patterns, the directionalindications of which are derived from vanishing points, e.g., parallel lines,
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