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Success Story: The Wind-Shear Phenomenon

Wind shear is a weather phenomenon where a sudden and large increase in an airplane's headwind is quickly followed by a sudden and large increase in the aircraft's tailwind. A microburst is one form of wind shear that can generate hazardous low-altitude conditions for aviation. The vast majority of wind shears are in fact microbursts, which is why the terms wind shear and microbursts are often used interchangeably.

A microburst is formed when a column of air at high altitude cools quickly due to evaporation of ice, snow, or rain. The cooling air becomes denser than the surrounding atmosphere and falls rapidly to the ground. When it nears the ground, this downward air spreads rapidly in all directions away from the descending core (see figure 1). Many pilots have encountered wind-speed changes in excess of 40 meters/s (80 knots) over a 4-km distance.

An aircraft flying through a microburst may experience extremely hazardous airspeed fluctuations. As the aircraft enters the downburst outdraft, it initially encounters an increased headwind (see figure 2). This headwind increases the aircraft's lift, and thus its airspeed and altitude. A pilot unaware that these increases are due to a wind shear will most likely react by reducing engine power to correct the aircraft's approach angle. The aircraft then passes into the vertically descending microburst core, where it encounters an abrupt change from headwinds to downwinds, resulting in the loss of lift and altitude. Immediately thereafter, the aircraft crosses into a region of tailwinds. This wind change reduces the aircraft's relative speed and further decreases lift, which causes the aircraft to lose more altitude.

Because the aircraft is now flying on reduced power, it's vulnerable to sudden losses in airspeed and altitude. The pilot may be able to escape the microburst by adding more power to the engines. Yet if the engines can't respond fast enough or if the wind shear is too strong, the aircraft may crash.

Microbursts are extremely difficult to detect. In the NASA AWDAP research program, some measured less than 2.5 miles in diameter and lasted only 5 to 10 minutes.

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