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Aviation History
1954
1954 - 0881.PDF
2 April 1954 399 < .£ ^" O RAT E ^•v.'W ATE D (kt ) Q u r £ so >^v kj ^ ^ •^ A. V A r \ V A \r V 'V 5 ^ / ^ 'L r 1M r J ^ i, V* W w TIME (min) Fig. 2. Sailplanes were used extensively in wave research. This six- minute flight record shows high-level turbulence encountered on one fight: note the enormous changes in rate of climb. howling. Several times you hang in your belt without the slightest idea of attitude. You have not encountered anything like this before. You recall a thunderstorm flight which scared you to death but the turbulence was nowhere near this bad. Suddenly you drop out of the cloud base and the view excites you: everything seems to have changed. X-Mountain looks down on you like a big barrier, the clouds sweeping down its slopes with visible speed and dissipating just in front of you. You are about ready to turn back when your plane is lifted with enormous power. In heavy vertical gusts your rate of climb jumps to 1,000ft/min, later to 2,000ft/min. The leading edge of the cumulus line is now just above you. To avoid being pulled back into the roll cloud you push the nose down. Apparently you now have a good ground speed and the ship is climbing fast just in front of the cloud line which looks like a long railroad train. Suddenly the gusts die out. The air becomes smooth as glass. But your rate of climb is now 2,500ft/min. You are stunned by the fact that such extreme degrees of smoothness and turbulence can co-exist so closely in the atmosphere. Looking back after a few minutes you notice that you are already higher than the top of the cloud. You are now flying at a safe level. That should be enough finally to cross X-Mountain and the cap cloud. Your altitude is 3,000ft over X-Mountain and probably 2,000ft over the cap cloud. There is no roll cloud line ahead now and you have reason to believe that you are out of trouble. The foot of X-Mountain lies just below you. The trailing edge of the cap cloud is only one mile ahead. The cloud mass pouring down the mountain slope and dissipating is a fascinating spectacle. The upwind edge of the high lenticular cloud is directly overhead, maybe between 30,000 and 40,000ft. The ship makes good headway now but the up-draught is tapering off and you need more power to keep altitude and ground speed. High as you are above the low-level clouds you feel almost— but not quite—safe. This completely smooth air has proved treacherous before and you are not sure what it has in store for you this time. The crestline of the mountain is not yet passed and ground speed seems to drop again. After another minute the low clouds look nearer. There has been no indication of what your altimeter and rate of climb now reveal: you are falling again at l,000ft/min, and full throttle does not help. You feel if you can go another mile upwind you should be through. But once more there is this unfortunate combination of a jetlike headwind and a strong downdraught. You have been running through several consecutive up and down draught areas. This is indeed the pattern of an atmospheric wave. In another minute you will know if you can pass X-Mountain. The cloud waterfall is directly beneath the plane now. But in front of you the cap cloud climbs fast. The air is still quite smooth, but now you are falling at about 3,000ft/min. Three thousand feet per minute? That means you will crash into the mountain within another minute. What does your altimeter show? A thousand feet above the highest peak of X-Mountain. But now you can see a mountain peak through the cap cloud. That is cer tainly not 1,000ft below you. It is just about your present height. Altimeter wrong? Only a quick decision will save you. Turn back. While you bank in a steep left turn the air becomes hazy. A glance at the instrument panel and the mountains shows that you are falling at almost 4,000ft/min into the lower end of the cloud waterfall. Suddenly a terrific gust banks the airplane into a steep right turn towards the mountain. For a moment you see the rocks of the mountain rapidly coming nearer. Then you succeed in manoeuvring the plane away from the stone wall. You are right in the foot of the cloud waterfall which looked so smoorh from above and the airplane shoots with an enormous tailwind 1,500ft over the valley floor. As the heavy gusts diminish you look back on the towering mountain range and the cap cloud which only a few minutes ago lay under your feet. In a matter of moments you have passed under the two roll clouds and the nightmare is over. You decide to do what you should have done in the first place: change your flight course, flying around X-Mountain and a full-scale "Mountain Wave." Wave Investigations. In order to investigate this type of airflow, the "Mountain Wave Project" was commenced, sponsored jointly by the Geophysics Research Directorate of the Air Force Research Centre, Cambridge (Massachusetts), and the U.S. Office of Naval Research. It was conducted by the University of California Soar ing Association, together with a number of Government and private organizations, and the field tests were made during 1951-52 in the Sierra Nevada mountain range in California. Conditions of temperature, pressure and wind were investigated up to a record height of 44,500ft by the use of specially instru mented sailplanes, which were tracked by radar, Raydist, and cinetheodolites. Time-lapse cameras took motion pictures of the associated cloud structures from the ground, and meteorological stations were established on both sides of the mountain range up to an elevation of 9,000ft. It was found that the phenomenon of the mountain wave is essentially the same as the flow of water over a barrier which forms rapids and waves downstream, but with added comp'ica- tions due to atmospheric variables such as temperature, humidity and wind. The troposphere shown in Fig. 1 consists of two layers separated by a temperature inversion on top of the cap cloud. At least two processes work simultaneously: (1) a "spill-over" of the lower layer which shoots down the mountain slope with increasing speed after passing the crest, and (2) an internal lee-wave in the upper layer which forms in the wake of the mountain barrier. The following conditions favour the formation of waves: — (a) Wind flow perpendicular to the mountain range line and with a speed of more than 25 kt at mountain top level. (b) A wind profile showing a strong consistent flow extending several thousand feet above the mountain tops, or showing an increase in speed with altitude. (c) An inversion or stable layer somewhere between the mountain tops and the 600 millibar level. The down-draughts to the lee of the rotor, and the up-draughts below it, can carry a plane into the rotor cloud while a pilot is attempting to pass above or below this cloud. If the aircraft approaches the crest of the mountains from the downwind side with insufficient height, it will be practically impossible to climb through the air currents near the mountain slope. These condi tions plus the fact that the peaks are hidden most of the time make it probable that an aircraft fighting strong headwinds at minimum clearance altitude would fly into the mountain peaks. As the barometric pressure is considerably disturbed in the mountain wave, altimeter errors are associated with the wave con ditions. The maximum total error possible (giving a high reading) has been estimated to be about 1,000ft, but errors as much as 2,500ft near the mountain peaks have been claimed by pilots. On some occasions, when meteorological conditions are favour able for the creation of a mountain wave, the lack of moisture in the atmosphere can prevent the formation of clouds. The main danger of this cloudless or "dry" wave is that it lacks the warning features provided in most waves by recognizable clouds. More serious still is the case where the wave flow is completely obscured by a thick overcast with a low ceiling. The following rules of flight are suggested to pilots for flights over mountain ranges when wave conditions exist: (a) If possible, fly around the wave area. If not, fly at least 50 per cent higher than the height of the mountain range, (b) Do not fly high-speed aircraft into the wave; particularly, do not fly downwind. Struc tural damage may result, (c) Avoid the rotor (roll) cloud, (d) Avoid the cap cloud (foehnwall)* area with its strong down- draughts, (e) Avoid high lenticu'ar clouds if the edges are very ragged and irregular, particularly if flying high, (f) If flying against the wind, up-draughts areas, especially the one upwind of the rotor clouds, may be used as an aid in gaining the altitude necessary to pass through the down-draught areas and cross the mountain range, (g) Do not p'ace too much confidence in pressure alti meter readings near the mountain peaks, (h) Avoid penetrating a strong mountain wave on instrument flight. *"Foehn" is a meteorological term for the air current descending from a mountain range. E
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