What is the maximum wind force that affects the normal flight of helicopters?

About level 4! ! ! ! Meteorological problems that have great influence on aviation include clouds, fog, precipitation, smoke, haze, sandstorm and floating dust, which will reduce visibility. When the horizontal and oblique visibility of the airport drops below the critical value, resulting in line-of-sight obstacles, it will be difficult for the aircraft to take off and land. When the horizontal visibility is less than1500m, the runway sight distance should be observed at the airport with instrument landing facilities. Although the aircraft can land in low visibility in airports with instrument landing system, it is difficult for the aircraft to land in larger airports in the world when the runway sight distance is less than 400 meters and the judgment height is less than 30 meters. At present, there is no effective instrument to observe squint visibility, which can only be inferred from horizontal visibility. Atmospheric turbulence will cause the aircraft to bump instantly or for a long time during flight. When the turbulence is comparable to that of an airplane, the turbulence is serious. The response of aircraft to turbulence is related to flight speed, flight attitude and wing load. Strong turbulence will make the aircraft out of control, and even the fuselage structure will be deformed or broken due to overload. Clear-sky turbulence, low-level wind shear and topographic waves have great influence on flight. Clear sky turbulence is a small-scale atmospheric turbulence phenomenon, which occurs at an altitude of more than 5000 meters. It often occurs in the place where the wind shear is the largest near the center of the maximum wind speed in the rapids, and its vertical thickness is only a few hundred meters to more than a thousand meters. Clear-sky turbulence will cause continuous aircraft turbulence, which is difficult for pilots to find in advance because it is not accompanied by visible weather phenomena. It has a great influence on flight. The physical mechanism of clear-sky turbulence is not very clear, and there is no practical prediction method. Some people have studied the airborne instruments that use infrared or laser to detect the clear sky turbulence in front of the route, but they are still in the experimental stage. Low-altitude wind shear is wind shear that occurs below a height of several hundred meters. Because it affects the airspeed, changes the lift and suddenly changes the flying height, it will often cause serious flight accidents for large aircraft that have reduced their altitude and are slowing down. Thunderstorm, low-level jet and frontal activity are the main weather conditions for the formation of low-level wind shear. The strong seeding airflow produced by thunderstorm or convection cell, accompanied by strong wind shear, is very small in time and space scale, so it is difficult to detect and predict. Terrain wave is a wave-like vertical movement formed by the influence of topography when the airflow passes through the mountainous area. When the airflow is strong, the vertical movement is also strong. According to the vertical distribution of airflow and wind, Fulchtgott divides topographic waves into four types: laminar flow, steady vortex flow, wavy flow and rolling flow. The vertical airflow in the terrain wave will make the flying height of the plane suddenly drop, which will seriously cause a mountain collision accident; Strong turbulence in topographic waves will cause aircraft bumps; Where the vertical acceleration in topographic waves is large, the indication of aircraft barometric altimeter may be wrong. In the daily forecasting business, it is impossible to quantitatively forecast the topographic waves. When the plane flies over clouds, freezing rain and wet snow areas containing supercooled water droplets, the protruding part of the plane surface freezes. Ice accumulation will change the aerodynamic shape of aircraft, increase flight resistance, consume fuel, and make the instruments and communication equipment of pitot static pressure system fail. Aircraft icing is related to the water content and temperature in the cloud. For propeller aircraft, the most prone to freezing temperature is around-10℃, and sometimes it is easy to freeze at -30℃ ~-40℃. For jet aircraft, the dynamic temperature rise in high-speed flight makes the surface temperature of the fuselage higher than the atmospheric temperature, so the icing temperature is related to the flight speed. Ice accumulation was once one of the main problems that threatened flight safety. Since 1950s, the cruising altitude of aircraft is generally higher than that which is easy to freeze, and there are anti-icing devices and deicing devices on board. However, when taking off, climbing, hovering and descending in the air, serious icing may still be encountered. Thunderstorm is a strong convective weather. The strong vertical movement of airflow in the cloud makes the plane out of control; The supercooled water droplets in the cloud will cause serious aircraft icing; Hail will damage the plane; Lightning causes interference and damage to radio compasses and communication equipment; Lightning can damage the outer shell of an airplane. Therefore, the thunderstorm area has always been regarded as an "air restricted area", and aircraft are prohibited from crossing. Since the appearance of weather radar, people can find thunderstorms in time and accurately, and monitor and avoid them. Modern aircraft use a large number of electronic devices, especially computers, to control the flight state. Lightning will cause serious damage to these equipment and directly affect the normal navigation of aircraft. Thunderstorm is a small and medium-scale weather system, so it is difficult to predict it accurately. The temporal and spatial distribution of wind and temperature in the upper air varies greatly, and the actual atmospheric temperature is also very different from the standard atmospheric temperature on which the aircraft design is based. In the case of high-speed flight, the change of temperature causes the change of air compressibility and affects the aerodynamic characteristics of the aircraft. When making a long-distance flight plan, in order to shorten the flight time and save fuel, we must choose the best flight route, the best flight altitude and the best flight speed according to the observation data and forecast of high-altitude wind and actual atmospheric temperature. In addition, the wind direction and speed on the ground, especially the sudden change of strong wind and wind, have a serious impact on the take-off and landing of aircraft. This is also the subject of aviation meteorology research. When the space shuttle is launched, it is necessary to know the distribution of wind, temperature and thunderstorm in the field. When it returns to the atmosphere, the reentry angle and altitude should be selected according to the temperature and density of the atmosphere. The space shuttle also needs accurate aviation meteorological information when landing. With the further improvement of aircraft performance and the gradual practical application of automatic flight technology, the problem of "all-weather" flight has appeared. The relationship between flight activities and meteorological conditions is changing from meteorological conditions to how to fly under complex meteorological conditions. All-weather flight system still needs to adjust the working state of the system according to the actual atmospheric conditions, and the requirements for meteorological data are higher when taking off and landing. In the future aviation activities, in addition to the detection and prediction of low visibility, squint visibility, atmospheric turbulence, thunderstorms and high-altitude meteorological conditions, the prediction methods of small and medium-scale weather systems that have strong interference and harm to flight, the high-function automatic aviation meteorological service system that processes, transmits and displays a large amount of meteorological information at high speed, and the theories and methods that artificially affect or change the weather process that hinders flight need to be further explored and solved.