Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level . Initially , this outflow causes a headwind that increases airspeed , which normally causes a pilot to reduce engine power if they are unaware of the wind shear . As the aircraft passes into the region of the downdraft , the localized headwind diminishes , reducing the aircraft 's airspeed and increasing its sink rate . Then , when the aircraft passes through the other side of the downdraft , the headwind becomes a tailwind , reducing lift generated by the wings , and leaving the aircraft in a low-power , low-speed descent . This can lead to an accident if the aircraft is too low to effect a recovery before ground contact . As the result of the accidents in the 1970s and 1980s , most notably following the 1985 crash of Delta Air Lines Flight 191 , in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board wind shear detection systems by 1993 . Between 1964 and 1985 , wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries . Since 1995 , the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years , due to the mandated on-board detection as well as the addition of Doppler weather radar units on the ground ( NEXRAD ) . The installation of high-resolution Terminal Doppler Weather Radar stations at many U.S. airports that are commonly affected by wind shear has further aided the ability of pilots and ground controllers to avoid wind shear conditions
Delta Air Lines Flight 191
Delta Air Lines Flight 191
Wind shear