Windshear refers to wind speedvectorOr the change of its component in a vertical direction or a horizontal direction.Wind shear is a vector value, which reflects the changes of wind speed and direction between the two points studied.stayaeronautical meteorologyMedium and low level wind shear usually refers to the wind shearwind shear。[1]
The formation of low-level wind shear requires certain weather background and environmental conditions.thunderstorm、Cumulonimbus cloud、TornadoWhen there is strong convection, strong vertical wind shear can be formed;The strong downburst can form strong horizontal wind shear when it reaches the ground and spreads around;The difference of meteorological elements on both sides of the front is large, and it is easy to generate strong wind shear.
classification
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（1） Classification of windshear
1. Vertical wind shear: refers to the change of wind speed and/or direction between two points in the vertical direction.
2. Horizontal wind shear: the change of wind speed and/or direction between two points in the horizontal direction only.
3. Shear of vertical air flow: refers to the change of upward or downward air flow (vertical wind) between two points in the horizontal direction.
（2） Classification of low-level windshear
According to different situations of aircraft motion relative to wind vector, wind shear is divided into:
1. Downwind shear: downwind increases or downwind decreases along the flight direction of the aircraft, and the aircraft enters the calm or downwind area from the upwind area.The downwind shear reduces the airspeed of the aircraft, reduces the lift, and makes the aircraft sink. It is a dangerous low-level wind shear.
2. Upwind shear: the upwind increases or decreases along the flight direction of the aircraft, and the aircraft enters the calm or upwind area from the downwind area.Upwind shear increases the airspeed and lift of the aircraft, and the aircraft rises. Its flight hazard is lighter than downwind shear.
3. Crosswind shear: The aircraft moves from a crosswind or no crosswind state to a significantly different crosswind state.The crosswind shear can make the aircraft slip, roll or yaw.
4. Vertical wind shear: The situation that an aircraft enters a strong updraft area from an area without obvious updraft.
3 Strength
1. Vertical shear of wind
international civil aviation organization （ICAO）The recommended strength standards are shown in Table 1.The vertical thickness of the air layer is 30 meters, the wind data is taken as the average value of about 2 minutes, and the vertical shear value of the wind speed is 0.1
S or above will pose a threat to jet transport aircraft.
Table 1 Vertical Shear Strength Standard of Wind
Grade
Numerical standard
Impact on flight
（m/s）/30m
light
0～2.0
0～0.07
Slight change of aircraft track and airspeed
moderate
2.1～4.0
0.08～0.13
It is difficult to control the aircraft
strong
4.1～6.0
0.14～0.20
It's very difficult to control the aircraft
serious
>6.0
>0.20
Loss of control of aircraft, causing serious harm
2. Horizontal shear of wind
The horizontal wind shear value is
It can be used as the strength standard that can cause damage to aircraft.
3. Vertical airflow shear
The shear intensity of vertical wind is mainly determined by the size of the vertical wind itself within the same space distance. The strongest downdraft is the strong downdraft, which is the downdraft speed and arrival areaDivergenceValue.
According to the suggestions of Fujita and Byers (1978), a numerical standard called downwash flow is proposed, which is determined from the downdraft velocity and the divergence value of the arrival area.[1]
Table 2 Numerical standards for downdraft and downdraft
Downdraft
Downwash
Descending speed above 91m height
<3.6m/s
≥3.6m/s
Divergence value within 800m diameter
<144km/h
≥144km/h
Impact on aircraft takeoff and landing
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Low level windshear has a great impact on aircraft takeoff and landing, and may even cause accidents in serious cases. The extent of this impact depends on the strength of windshear and the height of the aircraft.The main effects of low-level windshear on aircraft takeoff and landing include: changing the aircrafttracks；Affect aircraft stability and operability;The accuracy of some instruments is affected.[2]
1. Influence of downwind shear on land
When the aircraft enters the downwind shear zone during landing (for example, from strong headwind to weak headwind, or from headwind to calm or downwind), the downwind shear reduces the airspeed, lift, and aircraft sink.At this time, the corrective action is to increase the speed of the aircraft with the fuel filler rod, reduce the descent rate, return to the glide line, and then stabilize the rod to retract the throttle to re-establish the glide attitude.However, if the altitude of downwind shear is very low, the pilot cannot correct it in time, which will cause large deviation.
2. Influence of upwind shear on land
When the aircraft lands and glides into the upwind shear zone (for example, from a strong downwind to a weak downwind, or from a downwind to a calm or upwind), the downwind shear suddenly increases the airspeed, lift and lift of the aircraft.The pilot's corrective action is to retract the throttle and release the lever to slow down the aircraft and increase the descent rate. After returning to the glide line, the refueling valve is equipped with the lever to make the aircraft reestablish the glide attitude.
3. Influence of crosswind shear on land
The crosswind shear will make the aircraft skidding, rolling or deflecting during landing, which will not align with the runway and cause lateral deviation.
4. Influence of vertical wind shear on land
When the aircraft encounters the updraft during landing, the lift of the aircraft will change, which will change the descent rate.Vertical wind does great harm to aircraft landing, and aircraft often encounter serious downdraft when approaching and landing under thunderstorm clouds. In this case, the pilot can only go around.
