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 2,000 ft (610 m) 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.
Each LLWAS equipped airport may have as few as six or as many as thirty-two remote stations. Each remote station uses a 150 ft (46 m) 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.
The original LLWAS system (LLWAS I) was developed by the Federal Aviation Administration (FAA) in 1976 in response to the 1975 Eastern Air Lines Flight 66 windshear accident in New York and the findings of Project NIMROD by Ted Fujita. LLWAS I used a center field anemometer along with five pole mounted anemometers sited around the periphery of a single runway. It was installed at 110 FAA towered airports between 1977 and 1987. Windshear was detected using 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 and reduce false alarms. Between 1988 and 1991, all of the LLWAS I systems were upgraded to be LLWAS II compliant. Windshear deployment studies conducted from 1989 through 1994 determined at which LLWAS-II sites weather exposure justified upgrade to a weather radar (Terminal Doppler Weather Radar (TDWR) or Weather Systems Processor (WSP)) an LLWAS Network Expansion (LLWAS-NE) or LLWAS-Relocate/Sustain (LLWAS-RS) upgrade, singly or in combination. By 2005 all LLWAS-II had been decommissioned for one of these replacement wind shear detection systems or for two in combination.
The LLWAS-NE added the ability to cover more than a single runway, using up to 32 remote stations to provide runway specific alerts for parallel and crossing runways at ten large airports in combination with TDWR. The LLWAS-RS further upgrades service at 40 remaining LLWAS-2 operating sites (not justified for a radar solution) to employ LLWAS-NE algorithms and extend service life by 20 years, in part by adding ultrasonic anemometers with no moving parts. The LLWAS-RS program began in response to the National Transportation Safety Board (NTSB) investigation of the USAir Flight 1016 accident at Charlotte, North Carolina, in 1994. From that accident, a determination was made that LLWAS-II must regain and retain its original capability, often degraded by tree growth and airport construction such as hangars that obstruct or deflect wind near LLWAS remote station sensors.
Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC is to prevent collisions, organize and expedite the flow of traffic in the air, and provide information and other support for pilots.
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind shear, sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a given altitude.
Delta Air Lines Flight 191 was a regularly scheduled Delta Air Lines domestic service from Fort Lauderdale, Florida, to Los Angeles, California, with an intermediate stop at Dallas/Fort Worth International Airport (DFW). On August 2, 1985, the Lockheed L-1011 TriStar operating Flight 191 encountered a microburst while on approach to land at DFW. The aircraft impacted ground just over one mile (1.6 km) short of the runway, struck a car near the airport, collided with two water tanks and disintegrated. Out of the 163 occupants on board, 137 people died and 25 others were injured in the accident.
In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. It originates under deep, moist convective conditions like cumulus congestus or cumulonimbus. Capable of producing damaging winds, it may sometimes be confused with a tornado, where high-velocity winds circle a central area, and air moves inward and upward. These usually last for seconds to minutes. Downbursts are particularly strong downdrafts within thunderstorms . Downbursts are most often created by an area of significantly precipitation-cooled air that, after reaching the surface (subsiding), spreads out in all directions producing strong winds.
Pan Am Flight 759 was a regularly scheduled domestic passenger flight from Miami to San Diego, with en route stops in New Orleans and Las Vegas. On July 9, 1982, the Boeing 727 flying this route crashed in the New Orleans suburb of Kenner after being forced down by a microburst shortly after takeoff. All 145 on board, as well as eight people on the ground, were killed.
Eastern Air Lines Flight 66 was a regularly scheduled flight from New Orleans to New York City that crashed on June 24, 1975, while on approach to New York's John F. Kennedy International Airport, killing 113 of the 124 people on board. The crash was determined to be caused by wind shear caused by a microburst, but the failure of the airport and the flight crew to recognize the severe weather hazard was also a contributing factor.
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.
USAir Flight 1016 was a regularly scheduled flight in the southeastern United States, between Columbia, South Carolina, and Charlotte, North Carolina. On July 2, 1994, the flight encountered heavy thunderstorms and microburst-induced windshear while attempting to land, and crashed into heavy trees and a private residence near the airport. The crash and ensuing fire caused 37 fatalities and seriously injured 20 others.
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
American Airlines Flight 1572 was a flight from Chicago O'Hare International Airport to Bradley International Airport on November 12, 1995. The McDonnell Douglas MD-83 struck trees and an instrument landing system (ILS) antenna during landing, causing $9 million in damage to the aircraft.
ADC Airlines Flight 053 (ADK053) was a scheduled passenger flight operated by ADC Airlines from Nigeria's capital of Abuja to Sokoto. On 29 October 2006, the Boeing 737-2B7 crashed onto a corn field shortly after take-off from Nnamdi Azikiwe International Airport in Abuja, killing 96 out of 105 people on board.
The airborne wind shear detection and alert system, fitted in an aircraft, detects and alerts the pilot both visually and aurally of a wind shear condition. A reactive wind shear detection system is activated by the aircraft flying into an area with a wind shear condition of sufficient force to pose a hazard to the aircraft. A predictive wind shear detection system is activated by the presence of a wind shear condition ahead of the aircraft. In 1988, the U.S. Federal Aviation Administration (FAA) mandated that all turbine-powered commercial aircraft must have on-board wind shear detection systems by 1993. Airlines successfully lobbied to have commercial turbo-prop aircraft exempted from this requirement.
Terminal Doppler Weather Radar (TDWR) is a Doppler weather radar system with a three-dimensional "pencil beam" used primarily for the detection of hazardous wind shear conditions, precipitation, and winds aloft on and near major airports situated in climates with great exposure to thunderstorms in the United States. As of 2011, all were in-service with 45 operational radars, some covering multiple airports in major metropolitan locations, across the United States & Puerto Rico. Several similar weather radars have also been sold to other countries such as China (Hong Kong). Funded by the United States Federal Aviation Administration (FAA), TDWR technology was developed in the early 1990s at Lincoln Laboratory, part of the Massachusetts Institute of Technology, to assist air traffic controllers by providing real-time wind shear detection and high-resolution precipitation data.
On 4 April 2011, Georgian Airways Flight 834, a Bombardier CRJ100 passenger jet of Georgian Airways operating a domestic flight from Kisangani to Kinshasa in the Democratic Republic of Congo (DRC) crashed while attempting to land at Kinshasa Airport. The aircraft, which was chartered by the United Nations, was trying to land during a thunderstorm. Of the 33 people on board, only one person survived. The incident remains as the United Nations' deadliest aviation disaster. It is also the third-deadliest air disaster involving the CRJ100/200, behind Comair Flight 5191 and China Eastern Airlines Flight 5210.
Bhoja Air Flight 213 was a domestic scheduled passenger flight operated by Pakistani private airline Bhoja Air from Karachi to Islamabad. On 20 April 2012, the Boeing 737-236A aircraft serving the route crashed in bad weather during the final approach killing all 121 passengers and 6 crew on board, it remains as the second deadliest air disaster in Pakistan.
Leslie R. Lemon was an American meteorologist bridging research and forecasting with expertise in weather radar, particularly regarding severe convective storms. Lemon was, along with Charles A. Doswell III, a seminal contributor to the modern conception of the supercell convective storm which was first identified by Keith Browning, and he developed the Lemon technique to estimate updraft strength and thunderstorm organization also as a continuation of Browning's work.
Aeroméxico Connect Flight 2431 (SLI2431/5D2431) was a Mexican domestic scheduled passenger flight bound for Mexico City that crashed on takeoff from Durango International Airport on July 31, 2018. Shortly after becoming airborne, the plane encountered sudden wind shear caused by a microburst. The plane rapidly lost speed and altitude and impacted the runway, detaching the engines and skidding to a halt about 1,000 feet (300 m) beyond the runway. The plane caught fire and was destroyed. All 103 people on board survived, but 39 passengers and crew members were injured.
Project NIMROD was a meteorological field study of severe thunderstorms and their damaging winds conducted by the National Center for Atmospheric Research (NCAR). It took place in the Greater Chicago area from May 15 to June 30, 1978. Data collected was from single cell thunderstorms as well as mesoscale convective systems, such as bow echoes. Using Doppler weather radars and damage clues on the ground, the team studied mesocyclones, downbursts and gust fronts. NIMROD was the first time that microbursts, very localized strong downdrafts under thunderstorms, were detected; this helped improve airport and public safety by the development of systems like the Terminal Doppler Weather Radar and the Low-level windshear alert system.
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