Wind shear is a sudden change in wind speed and direction over a short distance in the atmosphere. It can occur at different levels of the atmosphere, but it is most dangerous at low levels.
Wind shear has caused numerous aircraft accidents resulting in the loss of hundreds of lives.
It is defined as a variation of wind velocity over either horizontal or vertical distances caused by microbursts during a thunderstorm.
Microbursts are a dangerous weather phenomenon that occurs when a powerful column of air sinks within a thunderstorm and becomes an extremely powerful gust of air that, once hitting the surface, spreads out in all directions.
As an aircraft is coming for landing, the pilots decrease the airspeed to achieve the required landing speed. When a microburst hits, the pilots will see a sudden increase in airspeed, caused by the force of the headwind created by the microburst. A pilot who is inexperienced with microbursts would try to decrease the speed. The plane would then travel through the microburst, and fly into the tailwind, causing a sudden decrease in air flowing across the wings. The decrease in airflow over the wings of the aircraft causes a drop in the amount of lift the wings produce and can cause the aircraft to stall. If the aircraft is at a low altitude shortly after takeoff or during landing, it will not have sufficient altitude to recover.
[caption id="attachment_1141203" align="alignnone" width="905"] Diagram showing a microburst -[/caption]
Low-level wind shear can affect aircraft airspeed during takeoff and landing in disastrous ways. Airline pilots are trained to avoid all microbursts that can cause wind shear.
Wind shear has been responsible for several deadly accidents worldwide. In the US several fatal accidents occurred including the Eastern Air Lines Flight 66, Pan Am Flight 759, Delta Air Lines Flight 191 and USAir Flight 1016 accidents.
On June 24, 1975, Eastern Air Lines Flight 66, operated by a Boeing 727-225 aircraft was on a scheduled flight from New Orleans to New York City. While on approach into New York's John F Kennedy International Airport, the aircraft crashed killing 113 of the 124 people on board. The National Transportation Safety Board (NTSB) published its final report on March 12, 1976, determining the following probable cause of the accident:
"The NTSB determines that the probable cause of this accident was the aircraft's encounter with adverse winds associated with a very strong thunderstorm located astride the ILS localiser course, which resulted in a high descent rate into the non-frangible approach light towers. The flight crew's delayed recognition and correction of the high descent rate were probably associated with their reliance upon visual cues rather than on flight instrument reference. However, the adverse winds might have been too severe for a successful approach and landing, even if they had relied upon and responded rapidly to the indications of the flight instruments."
On July 9 1982, Pan Am Flight 759 operated by a Boeing 727-2
