Wind shears are a rapid change in the wind speed and/or direction in either the horizontal or vertical direction. Low-level wind shears are especially dangerous as the pilots have about five to fifteen seconds to react. The rapid changes in the wind causes rapidly changing headwind and tailwind, strong side gusts and affect the lift produced by the wings.
Wind shears are caused by a number of phenomenons including convective weather, gravity waves, terrain and manmade features which change airflow as well as storms. The most prominent of these is the microburst. It is the result of the cool air sinking from the bottom of a thunderstorm cloud and onto the aircraft. As the plane enters the inverted mushroom of air (featured image), the strong headwind increases lift and slows down airspeed momentarily. As the rest of the plane follows, the tail wind increases as well thereby forcing the plane to the ground. At low altitudes, the jet will not have the capacity to dive and increase speed like at higher altitudes.
On August 2,1985 a Lockheed L-1011 TriStar from Delta Air Lines on course to land at Dallas/Fort International Airport encountered a microburst which suddenly forced it down and dipped it to the right. The pilots fought to regain control but struck a field about two kilometres short of the runway and continued into a highway, smashing into a car. The 727 became airborne for a few seconds and landed once again at the north of the airport travelling at 350km/h before ploughing into a water tank, killing 136 people onboard.
WASHINGTON, Nov. 30 /PRNewswire/ — Continental Airlines (NYSE: CAI A&B) today became the first commercial carrier to use the newly certified predictive wind shear detection system in passenger service. Continental’s 737-300 aircraft, operating as flight 1637, departed Washington’s National Airport at 1:40 p.m. with an airborne detection system which provides the cockpit crew notification of up to 90 seconds in advance of wind shear activity.
The technology surpassed previous reactive technology that did not forewarn the pilot about oncoming wind shear but gave information to the pilot on how to deal with the problem once the jet was in the wind shear. The airborne weather radar used in the historic flight was the Bendix RDR-48 weather radar fitted with predictive windshear detection developed by AlliedSignal Commercial Avionics Systems and certified by the Federal Aviation Administration. The radar uses pulse Doppler technology which measures the time elapsed for an emitted radio pulse to be reflected to a receiver therefore providing information on the speed of the aircraft and oncoming objects. The radar alerts the pilot of oncoming danger using a combination of visual and aural cues.
CONTINENTAL AIRLINES IS FIRST TO USE PREDICTIVE WIND SHEAR SYSTEM (1994, November 30). The Free Library. (1994). Retrieved January 20, 2017 from https://www.thefreelibrary.com/CONTINENTAL AIRLINES IS FIRST TO USE PREDICTIVE WIND SHEAR SYSTEM-a015924574
Low-Level Windshear and Its Impacts on Airlines (2005) Journal of Aviation/Aerospace Education and Research Retrieved January 20, 2017 from http://commons.erau.edu/cgi/viewcontent.cgi?article=1530&context=jaaer