What is LLWAS?
A typical Phase-3 LLWAS system includes a network of anemometers (wind sensors) atop tall poles located around the airport (a.k.a. remote stations) out to no more than 3 nm from the end of the runways, a master station that processes system data and communicates with the remote stations, an archiving system, operator console, alphanumeric alarm displays, and in some instances graphical displays.
How does it work?
The wind data at each remote station is processed every 10 seconds to determine if there is divergence or convergence within the network, or station-to-station wind differences between stations aligned with the runways. The divergence/convergence information is processed and if the intensity of the event is large enough, the system will calculate the strength of the along-runway wind losses or gains and generate windshear or microburst alerts (depending on strength), and identify the location of the event.
What is a microburst?
A microburst is an intense windshear. By definition: Microburst n: A small, very intense downdraft that descends to the ground resulting in a strong wind divergence. The size of the event is typically less than 4 kilometers across. Microbursts are capable of producing winds of more than 100 mph causing significant damage. The life span of a microburst is around 5-15 minutes.
What causes microbursts?
Microbursts are strong windshears (greater than 30-knot winds speed losses over distances of 1-4 km) that are primarily generated by evaporative cooling and rain loading. When precipitation (rain or snow) descends below cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst. In humid climates, microbursts can also generate from heavy precipitation. The weight and drag associated with the falling precipitation can result in a downdraft that will descend to the ground and spread out.
Why is it a problem for airplanes?
Windshear is a rapid change of wind speed or direction over a short distance. In general, windshear becomes a hazard for aircraft if the wind changes more than 20 knots over a distance of 1-4 km (0.5 to 2.5 nm). On either takeoff or landing, aircraft are near stall speeds. When going through a windshear, the headwind decreases resulting in a loss of lift. If the aircraft is near stall, then a little loss of lift can make all the difference to whether the aircraft can continue the flight.
How and what kind of decisions do airports or airlines make using LLWAS?
LLWAS provides information on windshear type, location, and intensity. Windshear alerts are issued via radio to arriving and departing aircraft by final air traffic controllers. In the U.S., most airlines require that pilots not continue their arrival or departure if there is a microburst alert valid for their operation. Although most aviation authorities do not close the runways when microbursts are occurring, the air traffic controllers will work with the pilots to reroute aircraft away from the event to a runway that is not impacted by the windshear.
How and what kind of decisions do airports make using LLWAS?
LLWAS provides information on windshear type, location, and intensity. The pilots get the alert from the controller and the pilot is supposed to determine if they feel comfortable continuing the operation. In the U.S., airlines require that pilots do not continue if there is a microburst alert.
Where have LLWAS systems been implemented?
Phase-3 LLWAS systems have been implemented in the USA, Taiwan, Korea, Saudi Arabia, and Hong Kong. Systems are being implemented (as of 2001) in Singapore, Kuwait, and Italy.
What are some of the windshear problems associated with airports?
Any airport that has thunderstorms, will be exposed to convective windshear. Any airport near mountains will experience terrain-induced windshear from time to time. Any airport near a coast, will experience windshear due to sea breezes. In the drier climates, even a light shower (or virga) can produce severe windshears, so in some places, the convection does not have to be as strong as a thunderstorm (e.g., Denver, Phoenix, Reno, Albuquerque, etc.).