Wind Shear

 Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area. (FAA, 2016). Low-Level Wind Shear (LLWS) considered one of the most hazardous weather phenomena due to the proximity of airplanes to the ground when encountering a wind shear.

Pilots need to be well trained to react immediately when encountering a wind shear, as those sudden changes are rapidly affecting the airplane's performance and cause changes to the airplane attitude, including violent updrafts, downdrafts and abrupt horizontal movement.

Low-Level Windshear

The most severe type of wind shear is a Microburst. A microburst is an aggressive degradation in performance that can produce downdrafts of up to 6000 feet per minute in a lifespan of 5 to 15 minutes. The effects of microburst winds consist of performance increasing headwind, followed by a downdraft, and performance decreasing tailwind. Therefore, encountering a microburst during the initial climb after take-off or on a final approach may force the airplane to the ground and result in a terrain impact in a short time.

Microburst Over Phoenix

On August 2^nd, 1985, Delta Airlines flight 191 departed Fort Lauderdale, FL to Los Angeles, CA, stopping in Dallas/Ft. Worth, TX. While approaching to land on runway 17L at DFW, the aircraft passed beneath a rapidly building thunderstorm. The thunderstorm contained a microburst, which in turn became the source of the multiple, horizontally-disposed vortices. (Gorney, 1987.) Due to the inability of the crew to respond successfully, the Lockheed L-1011 (TriStar) hit the ground 6300 feet away from the approach end of the runway. One hundred thirty-seven people were killed, and 28 were injured.

 

Delta Airlines Flight 191

After the crash, the NTSB pointed out the inability of the aircraft to detect microbursts and wind shear hazards, and NASA started the development of an airborne wind shear detector and alert system, which mandated by the FAA to be installed in all of the commercial aircraft. Also, LLWS recovery is one of the special-emphasized areas in airline pilots' initial training and recurrent program.

On Board Windshear Detector and Alert System

References:

U.S. Department of Transportation, Federal Aviation Administration. (2016). Pilot's handbook of aeronautical knowledge. Washington, D.C. Retrieved From https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/pilot_handbook.pdf

Gorney, J. (1987). An analysis of the Delta 191 wind shear accident. National Transportation Safety Board. Retrieved from: https://arc.aiaa.org/doi/abs/10.2514/6.1987-626