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Aviation History
1977
1977 - 0063.PDF
FLIGHT International, 8 January 1977 mm$fw:' •YSAT^ 57 Encounter with a giant By DAVID BROWN WAVE-SOARING provides the glider pilot with a stepladder to heights that could otherwise only be scaled inside the turbulent depths of a cumulo-nimbus cloud. The world absolute altitude record of over 46,000ft was attained in American wave, and even the modest mountains of Scotland have produced height gains of 29,000ft. But the smooth lift and corresponding sink of a wave system can be a real hazard to the unsuspecting power pilot . . . One summer evening the sun was glinting on a smooth sea as I climbed away from Blackpool Airport for an aerobatic detail in a Chipmunk. A light south-westerly wind was enough to fill the sails of the yachts clustered in the Bibble Estuary. The clear blue sky was peaceful, marred only by scattered patches of stratus at about 5,000ft. The climb-out northwards along the coast was uneventful until, on passing through 2,500ft abeam Blackpool Tower, the aircraft hit a patch of tur bulence under cloud. The motion was sharp-edged and random, just fitting the characteristic "cobblestone" des cription of clear-air turbulence, and was so unlike normal convective tur bulence that I began to suspect at once that this was some sort of rotor flow. But after a few seconds of jolt ing flight the air smoothed com pletely once again. As the Chipmunk passed out iato the sunshine 1 scanned the instruments: airspeed 70kt; engine temperature and oil pressure normal; vertical speed indi cator zero ... I performed a rapid double-take, but zero it stayed, and the altimeter confirmed the situation. The Chipmunk, suspended in space in a climbing attitude at full power, was not gaining an inch of height. Suspicion was hardening into cer tainty: I was in the descending air of a mountain wave system, even though the nearest hills were over 40 miles upwind in North Wales. I turned downwind, tracking across the gap between two clouds but barely maintaining altitude. Ap proaching the upwind side of the cloud, I turned to parallel its edge. The VSI pointer reluctantly left the zero mark, inched up to a climb rate of 200ft/min and then, as smoothly as the sweep hand of a clock, swung to 800ft/min. The Chipmunk was shooting up like a champagne cork past the thin layer of cloud. As the altimeter passed 7,500ft the rate of climb peaked at l,000ft/min, almost double the still-air rate of climb at this altitude. Level at 8,500ft and throttled back to cruise power, the aircraft was still being pushed up ward at 400ft/min. At this height visibility was excellent and, as I turned, an awe- inspiring panorama was spread out beyond the cowling. The whole of the Lancashire coast was visible, with the wave system's characteristic len ticular clouds forming at different heights. An irregular pattern of small lenticulars at high altitude stretched westwards across the Irish Sea. Over the Lake District a stack of saucer like clouds hung over more extensive cover, and yet more saucers were visible over the Pennines. Despite the apparent peace of the scene, one sobering thought lingered on. Had I been a few miles further east over the rising ground and un aware of my position, the torrent of air descending in the down-going side of the wave could easily have pushed the slow-climbing Chipmunk down into the hills. I had occasion to remember this lesson on a subsequent sortie when, flying IMC in the same area, the air craft insisted on climbing even when throttled right back and trimmed for the glide. This rather baffling behaviour could only be explained by the presence of wave lift. This theory was confirmed the same day by the appearance of lenticular clouds through a break in the extensive cloud cover. This account has a moral for the light-aircraft pilot. Wave action is not just confined to the area over and immediately downwind of high ground. The air many miles downwind often rises and falls quickly enough to have a serious effect on the flight- path of a light aircraft. Our hilltops are carpeted well enough with heather. Don't be the one to litter them with crumpled aluminium.
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