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A Downburst is created by an area of significantly rain-cooled, descending air that, after hitting ground level, spreads out in all directions producing strong winds. A Downburst affecting an area 4 km in diameter or less is commonly refered to as a Microburst (this term was defined by severe weather expert Tetsuya Theodore Fujita).
As a Cumulonimbus (Cb) cloud becomes mature, strong downdrafts occur, usually associated with precipitation in the form of Rain, Hail, or Virga. These downdrafts can be very powerful, with vertical winds of 6,000 ft per minute. When a strong downdraft, referred to as a downburst or microburst, hits the surface, the wind diverts horizontally outwards. Downdrafts ahead of a cumulonimbus cloud push warm surface air upwards, a little like a cold frontal system, often creating a wall of cloud commonly referred to as a Gust Front.
Downbursts are a particular hazard to aircraft at low level, especially on take-off or landing. An aircraft approaching a downburst will first encounter a strong headwind, which will lead to an increase in indicated airspeed. When trying to fly a set airspeed on approach, a pilot might therefore be tempted to reduce power. This would be very dangerous because, as the aircraft passes thorough the downburst, the wind becomes a tailwind and the indicated airspeed and lift drops. The significant downward force of air in the downburst may be enough to force the aircraft into the ground or at least cause it to lose a significant amount of height. The subsequent loss of performance, as the aircraft encounters tailwinds, may cause further loss of height and be enough to cause the aircraft to stall. Once caught in a downburst, escape is only possible by flying straight ahead; whichever way an aircraft turns, it will encounter the tail winds and the associated performance impact. If the aircraft is in a turn at that point then the stalling speed will be higher, possibly making the situation worse.
Detecting a downburst is not easy. The effects are usually localised and, if the precipitation evaporates before reaching the ground (Virga), may not necessarily be associated with heavy rain or hail.
Many airports, which experience regular severe thunderstorms, have systems in place to detect wind shear, often comprising anemometers in a network around the airport. In the USA, this system is known as low level wind shear alerting system (Low Level Wind Shear). A limitation of such systems is, of course, that it only detects wind shear at ground level. Hong Kong airport has a sophisticated system for detecting wind shear which combines a network of anemometers with Doppler weather radar and a Light Detection And Ranging (LIDAR) (LIght Detection And Ranging) wind shear warning system which can detect the movement of much smaller particles, for example dust particles, than can a conventional weather radar and therefore can more effectively detect wind shear in dry air. This is particularly important at Hong Kong where wind shear is caused by terrain effects as well as weather.
Many modern aircraft, such as the B777 Series, have predictive wind shear (PWS) warning systems which collect wind velocity data gathered by the weather radar to identify the existence of wind shear. These systems have a short range and are dependant on the radar picking up velocity data from water and ice particles ahead of the aircraft. In moist conditions, they are effective, providing the pilot with an opportunity to abort a take-off or to carry out a missed approach before encountering the shear zone; however, they do not work in dry conditions.
Thorough weather briefings, contingency planning , appropriate use of the weather radar, listening to Automatic Terminal Information Service (ATIS) at regular intervals, access to up to date actual weather conditions, warnings and forecasts, asking for reports from other pilots, as well as looking for the visual clues (cumulonimbus clouds, mammatus clouds, gust fronts, heavy precipitation, lightning, etc), and familiarity with local weather phenomena (at certain times of the year, some airports have predictable thunderstorm activity which can be avoided by careful scheduling of flights), all help to provide the flight crew with the best chance of avoiding downbursts and making the right decisions to safeguard the safety of the flight.
Accidents & Incidents
- DC93, vicinity Charlotte NC USA, 1994: On 2 July 1994, a DC-9 operated by US Air, collided with trees and a house shortly after attempting a missed approach at Charlotte Airport, USA, in a severe thunderstorm. 37 passengers were killed.
- B734, Brisbane Australia, 2001: On 18th January 2001, a Boeing737-400 encountered a Microburst while conducting a go-around at Brisbane Airport Australia.
- B744, Sydney Australia, 2007: On 15 April 2007, a Boeing 747-400 operated by Qantas AW on a scheduled passenger flight from Singapore to Sydney was in the final stages of a daylight approach to land on Runway 16R at Sydney when at about 100 ft agl it encountered a significant and rapid change in wind conditions. The aircraft touched down heavily and the windshear warning sounded in the cockpit. The crew carried out the prescribed windshear escape manoeuvre and subsequently made a second uneventful approach and landing.