Bogeyman in Thin Air

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smhusain_1
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Bogeyman in Thin Air

Post by smhusain_1 »

If in doubt, plead “Act of God". Failing that, blame the pilot. Combine the two, and you have the perfect let-out: weather. When the introduction of jets brought hopes of flying above the weather, jetstream became the dirtiest word in the air. This high velocity wind circling the earth along a narrow path some 30,000 feet up, offered extra speed and fuel-saving free rides to aircraft, but it was soon to be classified as an associate of murderous downdrafts. Then there was turbulence. Then came something even more mysterious and sinister,clear air turbulence, the sneaky CAT. Weather radars could spot thunderstorm cells, and warn about probable turbulence ahead, but CAT had nothing to do with stormy weather, and would strike out of clear blue skies.

Needless to say, all these phenomena had always been up there, but we encountered them only when flying became more adventurous in terms of speed, height and the streamlined structure of the jets. Once recognized and understood, new flying techniques could deal with them. So, apart of obvious mistakes or negligent oversight, all was well, almost--until something else was found lurking not up there but down here. near the ground. It was not even a case of now you see it and now you don't because you never saw it. Like Jack the Ripper, it was there, it had to be here, because countless deaths testified to the presence of a killer. It was selective in choosing its targets, and unless it just vanished in thin air every time, it had to have an ephemeral life, for it might slam one aircraft on the ground without so much as touching the two others that sandwiched the victim with barely a minute separation. They called it vicious windshear.They called it ferocious downdraft. Many aviators were inclined to pooh-pooh it for they had dealt with successfully throughout their working lives. But the goalkeepers of safety were worried.

So what are those too severe adverse winds? First there is a downburst. Hitting the ground, it fans like an inverted mushroom, and creates horizontal winds in all directions. On approaching a microburst during take-off the pilot flies into a headwind of up to eighty knots that rapidly increases his indicated airspeed; following his instinct or Flight Director, he pulls up to try to maintain climb out airspeed, but barely a mile away, the aircraft is slammed down by the vertical wind-shaft forcing it to descend. Losing indicated airspeed, the nose of the aircraft is now lowered in order to try to maintain climb out speed of at least V2 + 10. To complete the dramatic scenario, al within a minute or so, the wind changes to the opposite direction: the aircraft is in a nose-down attitude, and the sudden strong tailwind gives a further reduction in indicated airspeed even to the point of stalling. As this happens, the stickshaker operates cueing the pilot to push the control column further forward in a desperate attempt to increase airspeed which drives the aircraft into the ground. During approach the problem is less serious.

Excerpts from: Final Call by Stephen Barlay, Sinclair & Stevenson Ltd., London UK 1990