WINDSHEAR AND MICROBURSTS

The dangers of the wayward wind on flying

Windshear is a marked change in the direction or velocity of wind, brought by the mixing of air streams, and often presents pilots with handling difficulties close to the ground. Low level windshear may be encountered horizontally on takeoff or when flying level and vertically when climbing or descending. A common occurrence in the vicinity of a storm cell, windshear is also observed with weather fronts, temperature inversions near the ground, and high terrain that promotes the shearing of air.

Windshear manifests as an abrupt change in airspeed. An increase in headwind or a decrease in tailwind enhances lift and is an energy gain. Similarly, a decrease in headwind or an increase in tailwind degrades lift, and a resulting sink ensues. To the unsuspecting flier, this may cause alarm and a momentary loss of control. The aircraft risks entering a state of upset or deviating significantly from the desired flight path. Hence, pilots must be trained to recognize and apply procedures quickly and positively without hesitation to recover from the debilitating effects of a windshear and get re-established on the flight path.

Pilots train to expect an imminent windshear from meteorological forecasts and pilot reports on weather close to aerodromes; recognize an active windshear from onboard warning systems and external cues. Cues include strong gusting sensed by a discernible fluctuation on the airspeed indicator, smoke plumes shearing in different directions with height, lenticular clouds signifying the presence of mountain waves, roll clouds ahead of a storm belt, and an unresting windsock.

A microburst is characterized by a temporary but strong downdraught, often associated with convective clouds. There are several reported accidents where microbursts have been attributed to airplanes experiencing a lethal loss in altitude at their most vulnerable phases (approach and climb out). Flying through the periphery of the microburst also presents difficulties in lining up with the runway on an approach. Pilots flying in areas of convective activity must treat the presence of precipitation that evaporates on reaching the ground or areas of dust raised by winds as an active indication of a microburst.

With windshear in the proximity of a microburst, the perceived increase in energy from the headwind (ballooning above the glide path) is deceiving and even disastrous to counter, as the aircraft soon transitions into the downdraught and further, into the tailwind, leading to a progressively receding energy state (sinking below the glide path). A successful recovery will depend upon the energy reserves of the aircraft and the height above ground.

Should the pilot inadvertently enter a microburst, engine power must be swiftly maximized, and a pitch attitude consistent with a go around flown to climb away safely. However, the best defense in the arsenal of a pilot would be to exercise good airmanship and avoid potential flight into a microburst, which would mean delaying the takeoff or abandoning the approach until the weather at the airport improves.