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Aviation History
1954
1954 - 0880.PDF
398 FLIGHT WAVE FLYING-THE HARD WAY A Comprehensive Report on the Menace of the Mountain Wave WHY do aircraft sometimes fly into mountains? The correlation between the periodical accidents of this type and standing-wave conditions in the atmosphere has been suggested and discussed many times. A review of existing knowledge, with particular reference to a Dakota accident near Snowdon, was given by A. H. Yates in an article, "Airflow Over Mountains," in Flight of January 2nd, 1953. A summary of an American report* recently received has provided added detail to our picture of wave conditions, and it includes a particularly vivid account of what a pilot experi ences when flying through—or attempting to fly through— a typical "mountain wave." Many readers will already know that a mountain wave is a disturbance of the atmosphere set up by mountain barriers, and possessing a wave-like airflow in which severe turbulence, vertical currents and altimeter errors combine to form dangerous flight conditions. To expand this picture, we print below the main facts from the report: first, in slightly abridged form, the account of a typical flight in wave conditions (based on a large amount of actual flight experience), and secondly a summary of the methods and conclusions of the extensive research programme undertaken. A Typical Wave Flight. Let us suppose [the account begins] you are fighting strong headwinds at 10,000ft altitude in a moderate-speed aircraft. Two hundred miles ahead is X-Mountain. There are not many clouds around and visibility is very good. The air is smooth up here although in the lower layers it is quite turbulent. You are flying with a slight drift correction. Some time ago you spotted a long white cirrus band over the horizon far ahead. At about 100 miles from X-Mountain you notice that this cloud bank seems to extend just along the mountain range although at a much higher level than the peaks of the range. Apparently it does not move despite strong upper winds. The summits of X-Mountain are covered by a white cloud blanket. Every minute you can see more details. The high cirrus cloud ahead consists of a few parallel banks extending from right to left, normal to the wind. As you approach this cloud it does not look as white and harmless as it looked from 200 miles away. There are dark, dense parts in it and you would not dare to guess how high it is. You would not even call it a cirrus cloud any more. It looks more like a big altocumulus cloud. You can *Flight Aspects of the Mountain Wave: a Summary of Air Force Surveys in Geophysics, No. 35, by J. Kuettner and C. F. Jenkins. (Published by the Flight Safety Foundation, New York.) Fig. 1. A cross-section through a typical mountain wave. Main dangers to pilots occur in the "rotor" or "roll" cloud, and in the main down- draught zone. Up-draughts and down-draughts can reach S,000ftjmin. 90 80- 70 MILES O <r=T> Main down-draught M^ Main up-draught fv73 Turbulent layer [\\\j Mountain ^M^>^^ 6 rnrrrm lO 12 14 16 18 20 Lenticular!: cloud Roll cloud "Mother of pearl" cloud Cap cloud see that this cloud is composed of a number of layers staggered vertically like pancakes. The leading (upwind) edge appears quite sharp and seems to follow every bend in the long mountain range. Farmer upwind blue sky extends over the flat cloud blanket ("cap cloud") which covers the mountain tops. The high cloud extends only downwind of the mountain. It is a "lee cloud." You are now 50 miles from X-Mountain. Climbing slowly you should be able to pass below the high altocumulus cloud and then above the cap cloud and X-Mountain. There is a wide gap of blue sky between these two cloud layers, and except for some long lines of cumulus under the high cloud bank, you do not expect any clouds on your flight path and head directly into this gap. Apparently there are two of these cumulus lines extending from right to left just this side of the mountain range. They look so harmless that you really do not worry about penetrating them. The row nearer you consists only of some broken ragged cloud pieces (fractc-cumulus). They seem to be just about as high as the mountain tops and the cap cloud. The cloud line nearer the mountain range looks much more dense and builds up higher than the cap cloud over the mountains, although it is certainly not comparable in depth to the big shower clouds you had to penetrate sometimes. There is here no indication of precipitation. As you cannot estimate how much space exists between X- Mountain and the cumulus lines, you have to decide now if you want to pass beneath, above, or through the cumulus clouds. You decide to continue your flight towards the cloud gap by climbing steadily. You will probably pass through the first tiny cumulus line which is now only a few miles ahead. Fixing your eyes on some of the cloud fragments you notice that they show strong rolling motion. You remember having heard of a "roll cloud" and anticipate some turbulence. Upon contact with the first cloud pieces, your ship banks steeply and you are thrown against the ceiling of your cockpit. You have your hands full to regain control of the plane and you do not find any time to watch your altimeter or rate of climb indicator. Never theless, you feel that the ship is climbing and descending rapidly in what you would call severe turbulence. This dance lasts only one or two minutes then suddenly the air is smooth again and you have a good rate of climb. You have passed the first roll cloud and have time now to fasten your shoulder straps and to think your situation over. Looking upwards you notice that the high cloud is now huge and compact, completely shading the country side. Your decision to climb over the cumulus lines seems justified by your first experience. The rate of climb is unusually good after passing the first roll cloud. You can already see over the next cloudline, which seems to be 5 to 10 miles ahead, and you should have plenty of height to clear even the highest cloud tops of this roll cloud. With the air quite smooth you are confident that you are out of trouble by now. Looking down at the valley floor you notice that jet-like dust streaks indicate strong surface winds. Your progress is slow. Apparently the upper winds are very strong. Now you are high enough to look down on the next roll cloud. The cap cloud over X-Mountain ahead is snow-white in the brilliant sun. It seems to pour down the mountain slope like a cloud waterfall. Farther upwind the cap cloud merges with the horizon and it is hard to estimate whether you are higher than this cloud layer or not. The huge altocumulus cloud above is even darker now. The leading (upwind) edges of the different pancake layers are staggered toward the wind. The highest one is still far ahead and shows a brilliant white rim. Now you can see the profiles of the staggered layers. They are lens-shaped and you remember having heard meteorologists say that so-called "lenti cular clouds" occur frequently over mountains. Something unexpected must have happened suddenly. The roll cloud ahead has started to build up quickly in front of you. Looking downwards you notice that the plane does not seem to be making any headway. Now the first cloud drifts by under the plane. If the cumulus continues to rise that way you are not sure that you can make it. A glance at the rate of climb indicator reveals what is going on: the ship is falling at over 2,000ft/min in completely smooth air. What you need now is ground speed. With the nose down and full power, clouds seem to shoot by underneath the plane but the ground still does not show noticeable movement. The rate of descent is now 2,500ft/min. A big cumulus turret builds up ahead and engulfs the plane. You have fallen into the roll cloud. What follows is no longer controlled instrument flight. Heavy gusts make all the instruments dance. The speed drops down, then shoots up, the r.p.m.s are changing rapidly and the engine is
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