Wind shear. Wind tunnel

Wind shear is a sudden change of wind speed or direction over a short distance. Wind shear has caused numerous crashes or near-crashes of large airplanes. Milder wind shear causes "bumps' felt by passengers during take-off or landing.

Weather conditions that lead to wind shear include showers and thunderstorms. Wind shear can also develop when air flows over mountains or through cities.

Wind shear produced by a microburst, a downward flow of cool air, causes more airplane accidents than any other weather hazard. Microbursts often occur during showers and thunderstorms. The evaporation of rain or the melting and evaporation of snow or other ice particles cool the air in the microburst, making it heavier than the surrounding air. Raindrops or ice particles add weight to the air in the microburst. This air gains speed as it plunges to earth. When it hits the ground, it spreads in all directions like water from a faucet hitting the sink. Microbursts measure up to 2 ½ miles (4 kilometers) across and last from 2 to 10 minutes.

When an airplane enters a microburst, it encounters a headwind. A head wind blows from the front to the back of the aircraft. Airflow increases over the wings, and the plane moves upward. The pilot may try to restore the aircraft to its previous path by leveling it and decreasing its speed. However, after the plane passes the downward air current at the center of the microburst, it enters a tail wind. A tail wind blows from the back to the front of the plane. Airflow decreases over the wings, and the airplane suddenly loses altitude. If the airplane is too close to the ground or moving too slowly when it enters the tail wind, it may crash.

Pilot education helps reduce airplane accidents from wind shear. Pilots learn to recognize and avoid microbursts. Machines called flight simulators that imitate an airplane in flight teach pilots how to react to a microburst.

Many major airports have wind shear detection systems that measure changes in wind speed and direction. If conditions seem dangerous, the control tower warns the pilots so they can postpone take-off or landing. A type of radar called Doppler radar is most effective because it can detect wind shear early. Doppler radar sends out radar waves that bounce off raindrops, ice particles, insects, or air currents. A change in wind speed or direction alters the frequency of the radar waves as they return to the radar antenna.

Wind tunnel is a ground-based testing facility used to study the effects of wind, or airflow, on aircraft and other vehicles and structures. Wind tunnels are built in many shapes and for different purposes. Some of them are very large and can test full-sized experimental aircraft But most wind tunnels test scaled-down models.

Most wind tunnels have a segment called the test section through which a stream of air is blown at an object at a uniform speed. Air pressure and temperature can be controlled as well. The air is generally blown by electric fans, but other devices, such as pressurized tanks, may be used. A large nozzle in front of the test section accelerates the air to the desired speed. After the wind passes through the test section, a duct called a diffuser slows down the airflow.

The vehicle or structure being tested is secured by supports that extend from the ground or from behind the object. The supports are fastened to measuring devices outside the test section that record the force of the airflow on the vehicle or structure. Instruments also can measure surface pressure at many places on the object.

Wind tunnels in which the air speed is close to the speed of sound—that is, about 760 miles (1,225 kilometers) per hour—are called transonic tunnels. In subsonic tunnels, the air travels slower than the speed of sound. Wind tunnels in which air travels faster than the speed of sound are supersonic tunnel. In hypersonic tunnels, air speeds are at least five times as fast as sound.

Highly compressed air or other gases may be blown through wind tunnels to simulate various flight conditions. In some wind tunnels, very high or low temperatures can be achieved, enabling experts to study such subjects as aircraft icing and automobile performance in polar or tropical climates.