Understanding the wind shear phenomenon

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.

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 727-235 aircraft was on a scheduled domestic passenger flight from Miami to San Diego, with enroute stops in New Orleans and Las Vegas. The aircraft crashed in the New Orleans suburb of Kenner after being forced down by a microburst shortly after takeoff. All 145 on board, as well as eight people on the ground, were killed.

The NTSB determined that the probable cause of the accident was the aircraft's encounter with microburst-induced wind shear during the liftoff. This caused a downdraft and a decreasing headwind, the effects of which the pilot would have had difficulty recognising and reacting in time for the aircraft's descent to be stopped before its impact with trees. Contributing to the accident was the limited capability of the current wind shear detection technology.

On July 2, 1994, USAir Flight 1016 operated by a McDonnell Douglas DC-9-31, was on a scheduled flight in the southeastern US, between Columbia, South Carolina and Charlotte, North Carolina. The flight encountered heavy thunderstorms and microburst-induced wind shear while attempting to land at the Charlotte/Douglas International Airport and crashed into heavy trees and a private residence near the airport. The crash and ensuing fire caused 37 fatalities and seriously injured 20 others.

After a lengthy investigation, the NTSB concluded that a microburst generated by the thunderstorm over the airport at the time of the crash was the probable cause of the accident.

Two other causal factors listed by the NTSB were the flight crew's decision to continue an approach to an area where a microburst was likely and the lack of timely weather information by air traffic control to the crew of Flight 1016.

On August 2, 1985, Delta Air Lines Flight 191 operated by Lockheed L100-1 aircraft on a scheduled domestic service from Fort Lauderdale, Florida to Los Angeles, California, with an intermediate stop at Dallas/Fort Worth (DFW) International Airport crashed. During its landing approach into DFW, the aircraft encountered a microburst, hitting the ground just over one mile short of the runway. The aircraft struck a car, collided with two water tanks and disintegrated. Out of the 163 occupants on board, 136 people died and 25 others were injured in the accident.

The NTSB determined that the crash resulted from the flight crew's decision to fly through a thunderstorm, the lack of procedures or training to avoid or escape microbursts and the lack of hazard information on wind shear.

Wind shear can be detected using Terminal Doppler Weather Radar and aircraft can be fitted with airborne wind shear detection and alert systems.

A Doppler weather radar is a type of radar that detects precipitation, estimates its type, and calculates its movement.

The installation of high-resolution Terminal Doppler Weather Radar stations at many US airports that are commonly affected by wind shear has further aided the ability of pilots and ground controllers to avoid wind shear conditions.

Following the 1985 crash of Delta Air Lines Flight 191, in 1988 the US Federal Aviation Administration mandated that all commercial aircraft have airborne wind shear detection and alert systems by 1993.

Today, commercial jet aircraft are fitted with an airborne wind shear detection and alert system that detects wind shear and provides pilots with visual and aural alerts of a wind shear condition.

Several years ago, a wind shear detection system was installed at Piarco International Airport.