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. 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. A 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 six 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 have been observed.
