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Aviation History
1953
1953 - 0003.PDF
15,000ft above the ridge while, in the 20 kt wind at twice the height of the hill, there were periodic up-and-down currents in the lee of the hill of about 50oft/min. The wave length was about four miles. Standing Waves : Experiment.—The general pattern of the flow as calculated by Scorer agrees well with observa tions. These are of several kinds, but the most valuable have been the reports of glider pilots. A 50oft/min wave up- current is an embarrassment to an airline pilot, but is very welcome to a sailplane pilot, who explores it to find its extent and strength and then climbs in it as far as he can or dares. In the waves in the lee of the Californian High Sierras climbs have been repeatedly made into the stratosphere— the highest to almost 45,000ft. On one occasion the pilot of a P-38 (Lightning), finding the airport obscured by a dust storm, used his knowledge of the waves to fly for ij hr with both airscrews feathered. During this period he climbed 15,000ft to 30,000ft—an eloquent testimony to the strength of the up-current. A brilliant description of a sailplane wave flight has been given by Kuettner 2. He first climbed to 36,000ft and then flew 400 miles downwind in four hours, using the up-currents of several other waves of the sequence. The wind speeds existing on this day (March 19th, 1952) are shown in Fig. 6 and the wind-shear, which Scorer shows to be a decisive factor, was notable. Similar flights in the lee of high mountains have been made in Germany, France, Spain, Argentina and in many other countries. The waves are no less real when they are produced by winds across the much smaller hills of Great Britain. Gliders have frequently reached 13,000ft to 15,000ft in England—at least five times the height of the hills producing the waves. Powered aircraft, which have less difficulty in being at the right place at the right time, have explored waves at heights of 25,000ft above Yorkshire 3 and at 32,000ft above Hamp shire where the hills are scarcely 1,000ft high4. If the humidity conditions are suitable the wave crests are marked by lenticular clouds. Recent observations by Ludlam 5 have shown that cirrus clouds at 30,000ft over the Cotswolds are caused by vertical displacements due to the small hills below. In Alaska, "mother-of-pearl" clouds estimated to lie at a height of 95,000ft are also believed to be orographic (i.e. to be produced by the influence of the hills below). This evidence of the extensive influence of a hill is in contrast to the view frequently found in textbooks that the effect of a hill is confined to about three times its height. Further, the influence of the hill downstream is considerable : gliders have soared in ten successive waves downstream from the Pennines to the Lincolnshire coast, 70 miles away. Pilots of Wellingtons have described 6 how smooth, rising air currents of 75oft/min for 10-15 min have been encoun tered at 13,000ft near Newcastle, some 40 miles downwind of the Pennines. The strength of the up-current has rarely exceeded 75oft/min in England, although Kuettner in California found up- and down-currents of over 2,oooft/min. Never theless the pilot of an aeroplane flying parallel to the wave crest and trimmed for level flight may well be disconcerted to find himself with a steady rate of climb or descent of even 75oft/min. A surprising characteristic of this "wave air" is that it is often very smooth, and, as a result, a pilot trimmed to cross hills with 1,000ft clearance might descend 0 2 4 6 8 10 DISTANCE (n.m. downwind) Fig. 6 (left). Wind-speeds during Kuettner's Californian flight. 1 Fig. 7. (above). Sect/on downwind of Snowdon (vertical scale exaggerated); supposed conditions leading to "Saint Kevin" accident. in one minute to a dangerous height without receiving any warning from the turbulence of the air. The turbulence in other parts of the wave system may, however, be very severe. Kuettner describes how "the towplane banked vertically . . . disappeared completely from sight . . ." and how "the nylon tow-rope coiled towards the sailplane like a lassoo." This extreme turbulence has been found in several well- known systems of standing waves. Comments on the "Saint Kevin" Crash.—In the light of this accumulation of knowledge it is possible to guess the fate of the Saint Kevin. The last message indicated that the captain began to let down from 6,500ft in the belief that he was over the Irish Sea. In fact, he was about ten miles downwind of Snowdon in meteorological conditions favour ing the setting up of powerful lee waves. The lower quad- rantal height of 4,500ft clears Snowdon by less than 1,000ft and, if reaching this height was coincident with a severe downcurrent from the lee wave, the Saint Kevin may easily have been drawn into the eddy system associated with the mountain (Fig. 7). The pilots, thinking that they were now over the sea, would not be expecting violent turbulence; and it appears that they lost control of the aircraft which then shed part of a wing just before diving into the ground 1,200ft above sea level. Among the recommendations of the assessors after the inquiry were (a) "The safety-height for stages of an air route which cross mountain ranges should be related to the meteor ological as well as orographic data. This means that, on occasions when the meteorological forecast indicates that strong winds will be encountered at the approaches to and over the range, the safety-height (which is usually 1,000ft above the contours) should be increased. . . ."; (b) "Investi gations should be made of the vertical currents in airstreams of high velocity and differing degrees of stability crossing mountain ranges so that the resulting data may be applied to the establishment of safety heights on regular air routes crossing such ranges." Further Action.—A safety-height of 1,000ft above the mountain tops, which was until recently accepted even for much higher ground, is clearly inadequate in the light of the present knowledge of airflow over mountains. It is also to be hoped that pilots will inform themselves as much as possible of the results of the researches outlined above. There is no information yet of the intentions of the Government regard ing recommendation (b) above, but it is noteworthy that the U.S. Navy (!) have sponsored the considerable research in America into the behaviour of mountain-lee waves and have decided to use gliders as their tools. These are fitted with oxygen and radio and, because of their great strength, ability to fly at low speeds and accurately known perform ance, are able to collect invaluable data. Great Britain is well to the fore in the relevant theoretical work, and sailplane pilots have made hundreds of flights in these waves, but without any Government encouragement or financial support. Surely, here is an opportunity to help skilled amateurs to collect further data on this lesser-known but important aspect of air safety. REFERENCES. 1. Airflow Over Mountains, Science Progress, No. 159, July, 1952. 2. The Sailplane and the Glider, November, 1952. 3. Mallett, Gliding, Winter, 1951-2. 4. Austin, Weather, February, 1952. 5. Weather, October, 1952. 6. The Meteorological Magazine, May and November 1952.
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