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Thursday, December 14, 2000

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Crop dusters call it 'bad air'

WHATEVER THE terminology, abrupt changes in wind velocity and/or direction have been responsible for a bunch of jet transport crashes. And several near-misses that must have caused the cockpit crew to take up another line of work.

The recent Singapore-bound Indian Airlines Airbus-300 flight from Chennai caught in one of the `worst and unprecedented natural calamity' sent shock waves not only among passengers, but also among ``cockpit managers''.

Dr.N. Jayanthi, Director, Meteorological Office, Chennai Airport, explains: a wind shear is nothing but a change in wind speed and or wind direction between two points in the atmosphere. Such changes can occur due to changes in the horizontal component or vertical component of wind. The wind shear arises mainly due to the variation in atmospheric air motion ranging from small scale eddies and gustiness to the large scale flow of one airmass layer past an adjacent layer. Since uniform wind speed in all directions and at all levels is not possible at any time, wind shear is bound to be present always in the atmosphere, perhaps in all levels of the atmosphere.

Just yards inside the wall, the wind can be calm, but at and outside the wall, it can be blowing 200 knots! Now, that is wind shear. There are many situations where the wind can change in seconds, dramatically so. It can change during climb or descent or with just horizontal flight. If you fly through the division between the winds, the effect on the airplane can be quite dramatic, equal to the change in wind.

A change in wind direction can be just as catastrophic as a change in speed. A change in direction from a direct headwind to a direct crosswind for example will have the same effect of an airspeed as a drop in windspeed to zero.

According to a global weather update website - ``From the Ground up''- clear-air turbulence is closely associated with the jet stream when flying at high altitude. This occurs in clear air with no cloud form to warn of its existence, and can be quite violent. It is caused by wind shear, a sudden ``tearing or shearing'' effect encountered along the edge of a zone in which there is a violent change in wind speed or direction, such as when entering or leaving a jet stream. Wind shear will also be experienced when encountering a frontal surface and when climbing or descending through a temperature inversion.Thunder cloud drafts may be strong enough to displace an aircraft up or down vertically as much as 2,000 to 6,000 ft. Gust loads can be severe enough to stall an aircraft flying at rough air (maneuvering) speed or cripple it at design cruising speed.

In the early days of the jet age, there were several incidents and accidents that initially were a mystery. Contrary to popular belief, not all of them occurred at high altitudes.

The Singapore bound flight incident is typical of a number of such incidents, and several did not make the recovery at all, with the loss of the aircraft and all aboard.Of course, the culprit in all these events was ``wind shear''. Just how fast can the wind change? Have you ever felt the sudden outreach of cold air from an approaching thunderstorm? The microburst on the ground at Andrews Air Force base, near Washington DC occurred just a few minutes after Air Force One landed with the then President-Ronald Reagan on board, The wind increased by 98 knots in less than two minutes, dropped off about as quickly, then peaked again at over 60 knots as the cell passed overhead! Both bursts were in operation within two miles of each other!

T.S.Shanker

in Chennai

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