Aviation Weather Hazards - Low Level Wind Shear

 Aviation Weather Hazards - LLWS 

    Low level wind shear, or LLWS, is defined by the United States Government as "a wind shear of 10 knots or more per 100 feet in a layer more than 200 feet think which occurs within 2,000 feet of the surface" (US Department of Commerce, 2016). In short, it is a rapid change in speed or direction of wind within close proximity to the ground. The hazard that this poses to aircraft is most pronounced when taking off or landing. Wind shear is specifically hazardous to aircraft because of the effect that it has on the performance of an airplane. Sudden losses in airspeed will have a detrimental affect on an airplane's lift and, in the most severe cases, can lead to collision with the ground. Down drafts and up drafts will alter an airplane's approach path and will make it difficult to maneuver appropriately. On takeoff, a sudden shear that leads to a tailwind can increase takeoff distances and make it impossible to safely clear obstacles. 


Four Causes of LLWS

    According to the FAA Safety team, there are four main causes of LLWS: Frontal activity, thunderstorms, temperature inversions, and surface obstructions (FAA P-8740-40 Windshear 2008). Wind shear associated with frontal activity will be present when there are steep pressure gradients and/or strong differences in temperature between the two air masses. Strong winds, turbulence, precipitation, and cloudiness will only add to the dangers associated with frontal activity. 

    Thunderstorms are by nature very convective weather phenomenon and as such will have strong updrafts and downdrafts associated with them. The downdrafts that are produced by a thunderstorm will hit the ground and dissipate outward, enabling a strong horizontal flow near the ground. As an airplane flies through this kind of weather pattern, they will initially gain a strong headwind boosting aircraft performance which is then followed by a downdraft and a subsequent tailwind. This change in direction can lead to conditions that most aircraft are unable to fly through without sufficient altitude for recovery. See below a diagram of this particular weather pattern. 





    Temperature inversions, meaning a cool and stable air mass close to the ground with a warm airmass above it, may also be ripe conditions for LLWS. The stable airmass underneath may have little to no wind or convective activity, while the layer of warmer air that sits above it may have different weather characteristics entirely. Climbing through into the warmer air mass above will present a rapid change in wind direction, speed, or both. Additionally, "as the inversion dissipates, the shear plane and gusty winds move closer to the ground" creating a challenging condition for takeoff and landing (FAA P-8740-40 Windshear 2008). 
    
    Moreover, obstacles like trees, structures, hangers, buildings, and local topography may bring about LLWS hazards. In my home airport in Sisters, Oregon, there are trees 50-100 feet tall that line the entire airport runway. When there are strong winds that are not aligned with the runway centerline, a strong wind shear condition will often persist just as you are crossing the runway threshold. Above the trees, there might be 20-30 knots of wind while just below the trees, there may be less than 10 knots. This rapid change in wind speed as an airplane reaches within 100 feet of the ground may cause a serious loss of lift or even complete stall of an aircraft that is flying too slowly. Couple that with the large hill on the approach end of runway 20 and a runway landing distance of less than 3500 feet, and you have a challenging approach and landing. 



Below, a picture of the airport at which I work as a flight instructor. The trees line each side of the runway, and a large hill lies near the approach end of the opposing runway.

    





FAA. (2008). FAA P-8740-40 Windshear . FAASafety.gov. Retrieved January 21, 2022, from https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/56407/FAA%20P-8740-40%20WindShear[hi-res]%20branded.pdf

US Department of Commerce, N. O. A. A. (2016, September 22). Safety_llws. safety_llws. Retrieved January 21, 2022, from https://www.weather.gov/zme/safety_llws













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