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Aviation History
1953
1953 - 0002.PDF
FLIGHT, 2 January 1953 AIRFLOW over MOUNTAINS Reflections on Sometimes-dangerous Phenomena By A. H. YATES ON the evening of January 10th, 1952, a Dakota of Aer Lingus, Saint Kevin, crashed four miles down wind of Snowdon. The report of the public inquiry into the causes has been published, and the findings give the probable cause of the crash as "the encountering of a powerful down-current of air on the lee-side of Snowdon which forced the aircraft down into an area of very great turbulence . . ." The whole question of up-currents and down-currents near mountains is one of great concern in aviation, because of the large number of accidents which have occurred even among the comparatively low hills of Great Britain. Some of these have been attributed to carelessness on the part of pilots who chose an inadequate safety-height and flew into the sides of hills. Many, however, occurred to pilots who had attempted to fly at the approved safety-height but who found excep tional down-currents which dragged their aircraft into the danger area. How much is known about the strength of these currents ? The answer is that the observations and theoretical work of meteorologists and pilots in the last 15 years has completely changed ideas on airflow over mountains, with the result that much new information can now be given. First, let us recall the circumstances of the Saint Kevin crash and, in particular, the weather of January 10th. At nine o'clock that morning the meteorological officer at Liverpool gave the surface wind as "calm" but, only six miles above, the wind speed as revealed by the radar tracking of a balloon was 120 kt. This tremendous wind-shear reveals that it was an exceptional day, and confirmation is seen from the wind-speed records plotted in Fig. 1. It shows the upper air winds determined during the day at the three observation stations nearest to Snowdon. Early in the morn ing a jet .stream at 30,000ft was blowing at about 150 kt from the W.N.W., but abated during the day. By evening the wind at 30,000ft was still about 100 kt and reached 60 kt at 5,000ft. Although the dissipation of this high-speed jet of air must have been accompanied by considerable turbulence it was not such as to worry pilots on the Dublin-Northolt route except over the Welsh hills. Several pilots reported "quite rough turbulence" over the hills near Harlech on the Welsh coast, and the last words from the pilot of the Saint Kevin were "You'll find it pretty rough over the hills tonight. We were at four-five and went up to six-five. It seems to be right through." The temperature lapse-rate was normal, without inversions or isothermal layers and, although the night was cloudy, little icing was found. The Saint Kevin left Northolt at 1725 hr G.M.T. for Dublin (Fig. 2) and, after flying along the Airway to Daventry, turned into the 60 kt headwind on course for Dublin. The course should have carried it some 15 miles south of Snowdon to reach the coast at the Nevin radio beacon but, owing to un explained errors in navigation, the air craft followed a track which took it directly Fig. 1 (left). Wind- speeds on January 10th, 1952. Figs. 3 and 4 (right). Aerodynamic flow over a ridge; and flow of stable air over the same ridge (wet adiabatic lapse- rate, wind 20 m'sec). DUBLIN ^^5^ SNOW DON NEV»N>C Fig. 2. Route of the "Saint Kevin" from Northolt to the location of the accident near Snowdon. towards Snowdon. An error in estimating ground speed was also made with the result that the captain thought he was progressing faster than he was. At 1854, when a reconstruction of the flight suggests that he had just reached the first Welsh hills, west of Oswestry, he sought and received permission to climb from his quad- rantal height of 4,500ft to the next one at 6,500ft. No reason was given, but the fact that the request came on his arrival over the first hills suggests that rough air was the cause. After flying at 6,500ft for about 10 minutes the captain reported, erroneously, that he had reached the sea and requested a descent clearance (presumably to 4,500ft again, the lowest height permitted over the sea). By this time the Saint Kevin was probably about ten miles downwind of Snowdon, and it is suggested in the report that the aircraft, descending in air influenced by the mountains upwind, got out of control and crashed. Standing Waves : Theory.—For an understanding of the disturbances in the atmosphere which can be caused by mountains beneath, we must turn to the evidence which has been collected in the last few years on atmospheric waves. The theoretical work has been well summarized by Scorer,1 who points out that the "aerodynamic" airflow of a uniform stream across a long ridge, which occurs when the air has no static stability, is similar to that obtained in a wind tunnel at the appropriate Reynolds' Number. When account is taken of a typical, stable atmospheric lapse-rate the flow is very different and the disturbance reaches much greater heights (Figs. 3 and 4). When, in addition, the speed of the wind approaching the hill increases with height, then Scorer has shown that a series of waves will form in the lee of the hill and that their influence will extend far downstream and to great heights. In the par ticular example which Scorer calculated (Fig. 5), a ridge 1,500ft high was producing down-currents of 1,000ft/min at 60 80 100 120 WIND-SPEED (Kt) o 10 20 -~*-rrT7777777TTTTT-TT> o 10 20 Km Km
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