Weather Hazards - Wind Shear
Wind shear is a major factor that affects the take-off and landing of an aircraft. In just 39 years after 1956, there were 831 fatalities recorded from air crashes caused by wind shear. This number continued increasing and 700 fatalities resulting from wind shear were recorded between 1970-1985 (FAA, 1988). In fact, for a long time, the old Hong Kong Kai Tak airport was notoriously known as one of the most dangerous airports to land in due to the frequent wind shears that happen in that area due to its geographic location and high mountains nearby. This just shows how hazardous wind shear or low-level wind shears are. You can watch the video below to see just how dangerous this can be.
Wind shear is a result of different weather phenomenon that causes a sudden change in wind direction across a short distance. Wind shear can occur at both high and low altitudes but for this discussion, I will focus on the more dangerous low-level wind shear which mainly affects take-off and landing.
There can be three kinds of wind shear, horizontal, vertical, or a combination of both. While The causes of wind shear can be from natural weather conditions or man-made sources such as the following:
· Microbursts formed by thunderstorms.
· Surface obstructions like buildings and mountains near airports.
When a wind shear (microburst) happens, a strong and increasing headwind will be created over the wings of an oncoming aircraft. This causes a sudden increase in airspeed, which leads to the pilot decreasing their speed and leaving the original calculated flight patch. Once the aircraft passes through the wind shear, the wind becomes a tailwind, reducing the additional lift and increase in speed. This will be the cause of the plane’s loss of airspeed and altitude (FAA, 2008). To correct the loss of airspeed, the pilot reacts by pitching up and increase airspeed. While this may work when the aircraft is at a high enough altitude during landing, most accidents happen when the aircraft doesn’t have ample altitude to recover, causing the aircraft to land short of the runway.
During takeoff, an aircraft is flying extremely close to stall speed. Therefore, if it were to encounter wind shear during taking off, the plane will have to carry out a high-risk maneuver to increase aircraft speed during an extremely short period of time to escape the wind shear.
The best way to solve this problem of wind shear is to be able to identify it in advance to avoid or plan for appropriate measures. For that, pilots need about 10 to 40 seconds of warning ahead of time (NASA, 1992). Therefore, flight crews are now trained to recognize quickly when wind shear is affecting the aircraft. There exists a training program titled “Windshear Training Aid” developed by the Flight Safety Foundation (FAA, 2013). On top of that, there are many various radars to help with detection that are now included in modern commercial aircraft as implicated by the FAA in 1988. These wind shear detection systems by NASA Langley and FAA are placed both on the ground as well as on the aircraft. The systems include onboard microwave radar for airborne detection, infrared, and LIDAR systems for ground detection (NASA, 1992).
References
FAA. (1988). Pilot Windshear Guide [PDF]. Retrieved 10 October 2020, from https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC00-54.pdf.
FAA. (2008). Wind Shear [PDF]. Retrieved 10 October 2020, from https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/56407/FAA%20P-8740-40%20WindShear%5Bhi-res%5D%20branded.pdf.
FAA. (2013). Windshear Training Aid [PDF] (2nd ed.). Retrieved 10 October 2020, from https://flightcrewguide.com/wp-content/uploads/2013/11/Windshear-Training-Aid-Vol-2.pdf.
NASA. (1992). NASA - Making the Skies Safe from Windshear. Nasa.gov. Retrieved 10 October 2020, from https://www.nasa.gov/centers/langley/news/factsheets/Windshear.html.
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