What kind of weather prevents flights from taking off?
Meteorological problems that have a greater impact on aviation include: clouds, fog, precipitation, smoke, haze, sand and floating dust, etc., which can reduce visibility. When the horizontal and tilt visibility of the airport is reduced to critical When the value is lower than the visual range, it will be difficult for the aircraft to take off and land. When the horizontal visibility is less than 1,500 meters, at airports with instrument landing facilities, the runway visual distance must be observed. Although aircraft can land in low visibility at airports with instrument landing systems, at current larger airports in the world, when the runway visual distance is less than 400 meters and the judged altitude is less than 30 meters, it is difficult for the aircraft to land.
There is still a lack of effective instruments for observing strabismus visibility, so it can only be inferred based on horizontal visibility. Atmospheric turbulence can cause instantaneous or long-term turbulence when an aircraft is flying. When the scale of the turbulence is comparable to the scale of the aircraft, the turbulence is severe. The aircraft's response to turbulence is related to flight speed, flight attitude and wing load. Strong turbulence can cause the aircraft to lose control and even cause the fuselage structure to deform or break due to overload. Clear-air turbulence, low-altitude wind shear and terrain waves have a greater impact on flight.
Clear-air turbulence is a small-scale atmospheric turbulence phenomenon that mostly occurs at altitudes above 5,000 meters. It often occurs in the jet stream area where the wind speed shear is greatest near the center of the maximum wind speed, and its vertical thickness is only a few hundred meters to more than a thousand meters. Clear-air turbulence can cause persistent aircraft turbulence. Because it is not accompanied by visible weather phenomena, it is difficult for pilots to detect it in advance. Great impact on flight. The physical mechanisms of clear-air turbulence are not well understood, and there are no practical forecasting methods. There have been studies on airborne instruments that use infrared or laser to detect clear-air turbulence ahead 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 of the aircraft, changes the lift, and causes sudden changes in flight altitude, it often causes serious flight accidents in large aircraft that have lowered their altitude and are decelerating for landing. Thunderstorms, low-level jets and frontal activities are the main weather conditions that form low-level wind shear. Strong downdrafts from thunderstorms or convective cells are accompanied by strong wind shear. The temporal and spatial scales of this phenomenon are very small, and its detection and prediction are difficult.
Topographic waves are wave-like vertical movements caused by the influence of terrain when airflow passes through mountainous areas. When the air flow is strong, the vertical motion is also strong. According to the vertical distribution of airflow and wind, Furchtgott divided topographic waves into four types: laminar flow, steady eddy flow, wavy flow and tumble flow. The vertical airflow in the terrain wave can cause the flight height of the aircraft to drop suddenly, which may cause a serious mountain collision accident; the strong turbulence in the terrain wave can cause the aircraft to turbulence; in places with large vertical acceleration in the terrain wave, it can cause the aircraft to crash. The aircraft's barometric altimeter indicates an error. In daily forecasting operations, it is not possible to make quantitative forecasts of terrain waves.
When an aircraft flies through clouds, freezing rain and wet snow areas containing supercooled water droplets, ice may form on the protruding parts of the aircraft's surface. Ice accumulation will change the aerodynamic shape of the aircraft, increase flight resistance, consume fuel, and cause the Pitot static pressure system instruments and communication equipment to malfunction. Icing on aircraft is related to the water content and temperature in the clouds. For propeller aircraft, the most likely temperature for icing is around -10°C, and sometimes icing is also likely to occur at around -30°C to -40°C. For jet aircraft, the power of high-speed flight increases the temperature, making the surface temperature of the fuselage higher than the atmospheric temperature. Therefore, the temperature at which icing occurs is related to the flight speed. Ice accretion was once one of the major threats to flight safety. After the 1950s, the cruising altitude of aircraft has generally been higher than the altitude where icing is prone to occur, and the aircraft is equipped with anti-icing and de-icing devices. However, when taking off, climbing, hovering and descending, you may still encounter relatively high altitudes. Severe ice accumulation.
Thunderstorm is a kind of strong convective weather that develops vigorously. Strong vertical movement of airflow in the cloud can cause the aircraft to lose control; supercooled water droplets in the cloud can cause severe icing on the aircraft; hail can damage the aircraft; lightning can cause interference and damage to radio compasses and communication equipment; lightning strikes Can damage aircraft skin. Therefore, thunderstorm areas have always been regarded as "air forbidden zones" and aircraft are prohibited from crossing them. Since the advent of weather radar, people have been able to detect thunderstorms promptly and accurately, monitor and avoid them. Modern aircraft use a large number of electronic devices, especially electronic computers that control flight status. Lightning can cause serious damage to these devices and directly affect the normal navigation of the aircraft. Thunderstorms are small- to medium-scale weather systems and are difficult to predict accurately.
The temporal and spatial distributions of high-altitude wind and air temperature vary greatly, and the actual atmospheric temperature is also very different from the standard atmospheric temperature based on which aircraft are designed. In the case of high-speed flight, changes in temperature cause changes in air compressibility, affecting the aerodynamic characteristics of the aircraft. When making a long-distance flight plan, in order to shorten flight time and save fuel, the best route, best flight altitude and flight speed must be selected based on observation data and forecasts of high-altitude wind and actual atmospheric temperature.
In addition, surface wind direction and speed, especially strong winds and gusty changes in wind, have a serious impact on the takeoff and landing of aircraft. This is also a subject of aviation meteorology research.
When a spacecraft is launched, it must understand the distribution of wind, temperature and thunderstorms in the area. When returning to the atmosphere, it must select the reentry angle and altitude based on the temperature and density of the atmosphere. The spacecraft also needs accurate aeronautical meteorological information when landing.
With the further improvement of aircraft performance and the gradual practicalization of automatic flight technology, the problem of "all-weather" flight has emerged. The relationship between flight activities and meteorological conditions is changing from whether meteorological conditions determine whether to fly to how to fly under complex meteorological conditions. The all-weather flight system still needs to adjust the system's working state according to actual atmospheric conditions, and has higher requirements for meteorological data during takeoff and landing.
In future aviation activities, in addition to the detection and forecasting of low visibility, squinting visibility, atmospheric end currents, thunderstorms, and high-altitude meteorological conditions that still need to be gradually solved, there will also be severe disturbances and hazards to flight. , forecasting methods for small-scale weather systems, high-function automated aviation meteorological service systems that process, transmit and display large amounts of meteorological information at high speed, and theories and methods for artificially influencing or changing weather processes that hinder flight, are all aviation meteorology that need further exploration and solution. problem.