Wind shear can be divided into horizontal shear of horizontal wind, vertical shear of horizontal wind and shear of vertical wind according to wind direction.
The existence of vertical wind shear will cause damage to bridges, high-rise buildings, aviation flight, etc.The windshear at low altitude is an aircrafttake offandlandingAn important risk factor of the stage is called "invisible killer".
Weather background producing windshear.There are three types of weather background that can produce low-level wind shear with certain influence.
a. Severe convective weather。Usuallythunderstorm、Cumulonimbus cloudWait for the weather.Under the influence of such weather conditions, strong wind shear can be generated in a certain space.Especially inThunderstorm cloudThe strong downdraft area and the front gust front area of cumulonimbus are more serious.The extremely strong downdraft is called micro downdraft, which is the most harmful to flight.It is a comprehensive wind shear area characterized by vertical wind.
c. Radiation inversionTypeLow-level jetWeather.Autumn and winterA clear skyAt night, due to the intenseGround radiationThe existence of a low altitude inversion layer formed by cooling. There is momentum accumulation on the inversion layer, and the high wind speed forms a jet streamInversion layerThe wind speed below is low, and the wind speed near the ground is oftenCalm windTherefore, reverse temperature wind shear occurs.This kind of wind shear is usually less intense, but it is easy to be ignored, and it will also be dangerous if it is handled improperly.
Geographical and environmental factors
Wind shear caused by geographical and environmental factors.The geographical and environmental factors here mainly refer to mountain terrain, land and water interface, tall buildings, large forests and other natural and human factors.These factors can also cause windshear.The wind shear condition is related to the prevailing wind condition (direction and size) at that time, as well as the size and complexity of mountain terrain, windward and leeward positions, the size of water surface, the distance from the airport to the water surface, and the size and shape of buildings.Generally, the height difference in mountainous areas is large, the water area is large, and the buildings are tall, which is not only prone to wind shear, but also has a large intensity.
Classification
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According to the wind direction
wind shear
In order to ensure safety, the international aviation, aerospace and meteorological communities have actively carried out research on low-level wind shear.Windshear is often divided into the following types:
② Horizontal windyVertical shear(also known aswind shear ）Is the wind direction and/or wind speed atVertical distanceChanges on;
③ The shear of vertical wind is the change of vertical wind (ascending and descending airflow) in horizontal or track direction.Downwash airflow is a form of vertical wind shear, which presents a strong downdraft.The downwash flow with small scope and great intensity is called micro downwash flow.
By height
Wind shear can occur at high altitude or low altitude.Low level wind shear occurs below 600m.
Strength standard
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The intensity of low-level wind shear is directly related to flight safety.It indicates the degree of harm to flight. At present, the following three types are introduced and used.
Vertical shear intensity standard of horizontal wind
Horizontal shear intensity standard of horizontal wind
wind shear
There is no uniform standard for this item.butU.S.AAHorizontal windshearStandard value of intensity alarm.The system has six wind measuring stations at the airport level, namely the central station and five outer stations.The average distance between the outer stations and the central station is about 3 km.The system stipulates that when the difference between the wind vector and the central station is more than 7.7 m/s every minute, the system will send an alarm signal. Based on this, it can be calculated that 2.6 m/s/km can be used as the horizontal shear strength standard of the horizontal wind that can pose a hazard to flight.
Shear intensity standard of vertical wind
The shear strength of vertical wind is mainly determined by the size change of vertical wind itself within the same space distance.The strong downdraft is the most harmful to flight safety.According to the suggestion of famous meteorologists Fujita and Kors, a standard called downwash flow value is proposed.It is determined by the downward flow velocity and the divergence value when reaching the ground.Later, the downwash with a diameter of less than 4 km, which is the most harmful, is called micro downwash.
Hazard analysis
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principle
Schematic diagram of windshear caused by slightly downbeat airflow
The biggest threat to aircraft takeoff and landing safety isLow level windshear, that is, the windshear that occurs during the landing approach or takeoff climb phase.It can not only make the aircraft track deviate, but also may make the aircraft lose stability.If the driver misjudges and mishandles, serious consequences will often occur.As a result, there have been many accidents of aircraft destruction and human death in the world.Wind shear also seriously affects the stability of rocket flight, and the environmental constraints during rocket design and launch include wind shear.Windshear is mainly caused byfrontal surface(the interface between cold and warm air)Inversion layer、thunder, complex terrain and ground friction effect.
Low level windshearharmSex is caused by the inherent characteristics of windshear.The micro downwash airflow is the most harmful, which is a comprehensive wind shear area characterized by vertical wind shear.Because in the horizontal directionVertical movementThe air flow ofvelocity gradientThat is to say, the wind speed of vertical movement will suddenly increase, which will produce a particularly strong downdraft, called micro downdraft.This strong downdraft exists in a limited area, and turns parallel to the ground after hitting the ground to become a horizontal wind. The wind direction diverges around the impact point, so in a larger areaHorizontal windshear。If an aircraft enters this area during takeoff and landing, it may cause a crash.For example, when the aircraft lands, the glide path just passes through the micro downwash airflow, then the aircraft will suddenly descend abnormally, deviate from the original glide path, and may be too low to cause danger.When the aircraft flies out of the slightly downwash airflow, it enters the downwind airflow again, which makes the aircraftrelative velocitySudden lowering. Since the aircraft is always decelerating during landing, we know that the aircraftFlight speedIt must be greater than the minimum speed to avoid stalling. Sudden deceleration is likely to cause the aircraft to stall,Flight attitudeUncontrollable, and at such a low altitude and speed, it is impossible to leave it to the pilotSpace and timeTo restore control, resulting in a flight accident.
Severe low-level wind shear, often occurring inLow-level jetThat is, the long and narrow strong wind area poses a great threat to flight safety.This wind shear airflow often rushes down rapidly from high altitude, like a giant downpour faucet. When the aircraft enters the area, it first encounters strong headwind, then strongDowndraft, followed by a strong downwind, the plane is like a leaf thrown up and down in the gale and loses control, so it is very easy to have serious falling incidents.
characteristic
The existence of strong vertical wind shear will cause strong damage to bridges, high-rise buildings, aviation flight, etc., which can cause bridge building collapse, aircraft crash and other malignant accidents, and have a serious impact on human life.Low level windshear poses the greatest threat to aircraft takeoff and landing safety.
During the 16 years from 1970 to 1985, there were at least 28 flight accidents related to low altitude windshear in international scheduled and non scheduled flights and some mission flights, according to incomplete statistics, most of which occurred in the takeoff and landing stages at flight heights below 300 meters, with landing being the most frequent.Through the analysis of these 28 flight accidents, it can be found that low altitude windshear flight accidents have the following characteristics:
1. Windshear flight accidents occur at takeoff and landing stages when the flight altitude is less than 300m, especially landing.
2. The proportion of windshear flight accidents of modern medium and large jet transports is large.
3. Windshear flight accidents are closely related to thunderstorm weather conditions.
4. The occurrence time and season of windshear flight accidents are irregular.
Research progress
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In 1985, 137 people died when a plane crashed at Dallas Fox Airport in the United States.Since then, wind shear has been studied as an international topic.
Because the wind shear phenomenon has the characteristics of short time, small scale and large intensity, it has brought a series of difficulties such as difficult detection, prediction, navigation control and flight, which is not easy to solveAviation meteorologyDifficulties.Therefore, the best way to deal with wind shear is to avoid it.Because some strong wind shear is irresistible to the performance of existing aircraft.At present, the main ways to mitigate and avoid windshear hazards are to train pilots and set flight operation procedures for windshear, install windshear detection and alarm systems at airports, and install airborne windshear detection, alarm and avoidance systems.Since 1985, all aircraft in the United States have been installed with windshear detectors.Installation began in Canada in the 1990s.
Judgment and prevention
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Judgment method
Visual discrimination
1. Dust clouds of thunderstorm cold outflow airflow.The dust cloud caused by the strong airflow at the front edge of the cold outflow of thunderstorms moves with the airflow, usually followed by strong wind shear.
2. Rain streamers hanging from thunderstorm clouds are an important sign of strong downwash airflow. The lower the height of rain streamers, the larger the individual shape, and the darker the color, indicating that the downblow of wind shear is stronger.
3. Roller clouds.In a cold front thunderstorm, the strong cold outflow flow will have a vortex motion structure, accompanied by low-level roller clouds.The appearance of such clouds indicates the existence of strong low-level wind shear.
Cabin instrument discrimination
1. Airspeedometer
The airspeed indicator is one of the most sensitive instruments when the aircraft encounters windshear. Once there is an abnormal indication, you should be alert to the danger of windshear.Boeing stipulates that if the airspeed indicator suddenly changes 15-20 knots, it shall be regarded as windshear and no approach landing is allowed.This large fluctuation of indicated airspeed often leads to errors in judgment and manipulation.When on the ground, the takeoff shall be aborted.
2. Altimeter
The normal glide height indicated by the altimeter is an important data for aircraft approach and landing.When the altimeter deviates greatly from the normal value in a short time during the glide, measures must be taken immediately to go around.
3. Lifting speed meter
The ascending and descending speedometer is closely related to the altimeter, and the response is obvious when encountering wind shear.Boeing suggests that the descending speed should be changed to 500Ft/m in a short time, which means that strong wind shear is encountered.The driver shall take corresponding measures.
4. Pitch attitude indicator
Pitch angle is an important parameter that pilots master during takeoff and landing.When encountering windshear, the pitch angle indication changes rapidly. When the change suddenly exceeds 5 degrees, it is considered that windshear is encountered and the pilot should abort the approach.
Preventive measures
Wind shear encountered in flight is a very difficult and complex problem.It can respond to the pilot, take measures, and control the flight path until it is corrected in a very short time. In order to respond quickly and accurately, the pilot should:
1. Carefully understand the weather forecast, and be psychologically prepared for the possible location, height and intensity of windshear.
2. Pay attention to listening to the ground weather report and the report of other aircraft during takeoff and approach, understand the existence and nature of windshear, and make a correct decision on whether your aircraft can pass the risk assessment of windshear.Generally, avoidance, waiting, alternate landing and other measures shall be taken.
3. Strengthen crew cooperation and make full use of cockpit resources.When flying in complex weather, the captain shall organize the crew to take responsibilities.Various passwords shall be clear and in place during takeoff and approach.The crew should constantly scan the instrument, pay close attention to whether there is any abnormality, and be familiar with the runway environment, wind direction and speed, go around procedures, etc.Once there is any abnormal situation, it can be found in time and countermeasures can be taken immediately.
4. Do not consciously try to cross areas with severe wind shear or strong downdraft, especially in mountainous areas, low altitude, or when the engine fails.
5. Keep a distance from the strong downblow airflow area of thunderstorm.The outflow of thunderstorms can exceed 20-30 kilometers before thunderstorms.Don't fly this area by chance.
6. In case of windshear in the final approach phase, as long as the stable landing profile cannot be reconstructed, relevant procedures shall be taken immediately to get out of the shear zone for missed approach, join in waiting or land at the alternate aerodrome.We firmly oppose blind and reckless action.
7. When the aircraft encounters windshear, after the departure procedure is completed, the area, altitude, airspeed change range, etc. where the windshear occurs shall be reported to the flight control department immediately, so as to avoid other aircraft being accidentally involved.
