Low level windshear alert system
Encyclopedia
A Low level windshear alert system (LLWAS) measures wind speed and direction at remote sensor station sites situated around an airport.
Each equipped airport may have as few as 6 or as many as 32 remote anemometer
stations. The remote sensor data received is transmitted to a master station, which generates warnings when windshear
or microburst
conditions are detected. Current wind speed and direction data and warnings are displayed for approach controllers in the Terminal Radar Approach Control Facility (TRACON) and for ground controllers in the Air Traffic Control Tower (ATCT).
Air Traffic Controllers (ATCs) relay the LLWAS runway specific alerts to pilots via voice radio communication. LLWAS alerts assist pilots during critical times when they must determine whether to attempt to land or take off in hazardous weather conditions.
(FAA) in 1976 in response to the 1975 Eastern Air Lines Flight 66
windshear accident in New York. LLWAS I used a center field anemometer along with five pole mounted anemometers sited around the periphery of the airport. It was installed at 110 FAA towered airports between 1977 and 1987. Windshear was detected used a simple vector difference algorithm, triggering an alarm when the magnitude of the difference vector between the center field anemometer and any of the five remotes exceeded 15 knots. The LLWAS II deployment included software and hardware upgrades to the existing LLWAS I to improve the windshear detection. Between 1988 and 1991, all of the LLWAS I systems were also upgraded to be LLWAS II compliant.
The Low Level Windshear Alert System Relocation/Sustainment (LLWAS-RS) is intended to upgrade the current LLWAS at 40 LLWAS-2 operating sites and 4 support sites, to extend their service life another 20 years. The LLWAS-RS program is divided into two efforts: pole relocation and system sustainment. The program began in response to the National Transportation Safety Board
(NTSB) investigation of the USAir Flight 1016
accident at Charlotte, NC, in 1994. From that accident, a determination was made that LLWAS must regain and retain its original capability. Due to increased obstructions around remote station wind sensors and equipment obsolescence, the capability has been lost over the years.
In 1993 the Federal Aviation Administration (FAA) began deploying two new wind shear detection systems: the Terminal Doppler Weather Radar
(TDWR) and the third-generation Low Level Windshear Alert System (LLWAS 3).
Each equipped airport may have as few as 6 or as many as 32 remote anemometer
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...
stations. The remote sensor data received is transmitted to a master station, which generates warnings when windshear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
or microburst
Microburst
A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to, but distinguishable from, tornadoes, which generally have convergent damage. There are two types of microbursts: wet microbursts and dry microbursts...
conditions are detected. Current wind speed and direction data and warnings are displayed for approach controllers in the Terminal Radar Approach Control Facility (TRACON) and for ground controllers in the Air Traffic Control Tower (ATCT).
Air Traffic Controllers (ATCs) relay the LLWAS runway specific alerts to pilots via voice radio communication. LLWAS alerts assist pilots during critical times when they must determine whether to attempt to land or take off in hazardous weather conditions.
Related activities in the United States
The original LLWAS system (LLWAS I) was developed by the Federal Aviation AdministrationFederal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...
(FAA) in 1976 in response to the 1975 Eastern Air Lines Flight 66
Eastern Air Lines Flight 66
Eastern Air Lines Flight 66, a Boeing 727-225 with registration number N8845E, departed from New Orleans Moisant Field, bound for John F. Kennedy International Airport on the afternoon of June 24, 1975. The aircraft carried 124 persons, including 116 passengers and 8 crew.As the aircraft was on its...
windshear accident in New York. LLWAS I used a center field anemometer along with five pole mounted anemometers sited around the periphery of the airport. It was installed at 110 FAA towered airports between 1977 and 1987. Windshear was detected used a simple vector difference algorithm, triggering an alarm when the magnitude of the difference vector between the center field anemometer and any of the five remotes exceeded 15 knots. The LLWAS II deployment included software and hardware upgrades to the existing LLWAS I to improve the windshear detection. Between 1988 and 1991, all of the LLWAS I systems were also upgraded to be LLWAS II compliant.
The Low Level Windshear Alert System Relocation/Sustainment (LLWAS-RS) is intended to upgrade the current LLWAS at 40 LLWAS-2 operating sites and 4 support sites, to extend their service life another 20 years. The LLWAS-RS program is divided into two efforts: pole relocation and system sustainment. The program began in response to the National Transportation Safety Board
National Transportation Safety Board
The National Transportation Safety Board is an independent U.S. government investigative agency responsible for civil transportation accident investigation. In this role, the NTSB investigates and reports on aviation accidents and incidents, certain types of highway crashes, ship and marine...
(NTSB) investigation of the USAir Flight 1016
USAir Flight 1016
USAir Flight 1016 was a regularly scheduled flight between Columbia, South Carolina and Charlotte, North Carolina. On Saturday, July 2, 1994, the plane, a McDonnell Douglas DC-9 registered N954VJ, departed Columbia Metropolitan Airport at 18:15 EST...
accident at Charlotte, NC, in 1994. From that accident, a determination was made that LLWAS must regain and retain its original capability. Due to increased obstructions around remote station wind sensors and equipment obsolescence, the capability has been lost over the years.
In 1993 the Federal Aviation Administration (FAA) began deploying two new wind shear detection systems: the Terminal Doppler Weather Radar
Terminal Doppler Weather Radar
Terminal Doppler Weather Radar is a doppler weather radar system used primarily for the detection of hazardous wind shear conditions on and near major airports in the United States. As of 2011, there were 48 active radars, across the United States & Puerto Rico. Several more radars have also been...
(TDWR) and the third-generation Low Level Windshear Alert System (LLWAS 3).
See also
- MicroburstMicroburstA microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to, but distinguishable from, tornadoes, which generally have convergent damage. There are two types of microbursts: wet microbursts and dry microbursts...
- Weather radarWeather radarWeather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...