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Low level windshear alert system
A Low level windshear alert system (LLWAS) measures average surface wind speed and direction using a network of remote sensor stations, situated near runways and along approach or departure corridors at an airport. Wind shear is the generic term for wind differences over an operationally short distance (in relation to flight) which encompass meteorological phenomena including gust fronts, microbursts, vertical shear, and derechos.
==Background==
LLWAS compares results over its operating area to determine whether calm, steady winds, wind shifts (in relation to runways), wind gusts, divergent winds, sustained divergent winds (indicative of shear), or strong and sustained divergent winds (indicative of microbursts) are observed. An LLWAS master station polls each remote station every system cycle (nominally every ten seconds) and provides prevailing airport wind averages, runway specific winds, gusts, may set new wind shear alerts or microburst alerts and reset countdown timers of elapsed time since the last alert. By airline rules, pilots must avoid microbursts if warnings are issued by an automated wind shear detection system, and must wait until a safe time interval passes, to assure departure or landing conditions are safe for the performance of the airframe. Pilots may decide whether to land (or conduct a missed approach) after wind shear alerts are issued. LLWAS wind shear alerts are defined as wind speed gain or loss of between 20 and 30 knots aligned with the active runway direction. "Low level" refers to altitudes of or less above ground level (AGL). Arriving aircraft on descent, generally within three nautical miles of touchdown will fly within this low level, maintaining a glide slope and may lack recovery altitude sufficient to avoid a stall or flight-into-terrain if caught unaware by a microburst. LLWAS microburst alerts are issued for greater than 30 knot loss of airspeed at the runway or within three nautical miles of approach or two nautical miles of departure. Microbursts in excess of 110 knots are known.
Each LLWAS equipped airport may have as few as six or as many as thirty-two remote stations. Each remote station uses a tall pole with anemometer and radio-telecommunication equipment mounted on a lowerable ring. Remote station wind measurements are transmitted to a master station at the Air Traffic Control Tower (ATCT), which polls the remote stations, runs wind shear and gust front algorithms, and generates warnings when windshear or microburst conditions are detected. Current observations and warnings are displayed for approach controllers in the Terminal Radar Approach Control Facility (TRACON) and for local and ground controllers in the Air Traffic Control Tower.
Air Traffic Controller (ATC) users at local, ground and departure positions in the ATCT relay the LLWAS runway specific alerts to pilots via voice radio communication. Recent wind shear alerts may also feature in radio broadcasts by the Automated Terminal Information System (ATIS). LLWAS wind shear and microburst alerts assist pilots during busy times on final approach and on departure, often when heavy traffic, low ceilings, obstructions to vision, and moderate to heavy precipitation add to the difficulty in determining in just a few seconds whether mounting wind and weather hazards should be risked or avoided.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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