Low-Level Wind Shear Alert System (LLWAS)

Low-Level Wind Shear Alert System (LLWAS)

The LLWAS system was originally developed by the FAA in the 1970s to detect large scale wind shifts (sea breeze fronts, gust fronts and cold and warm fronts). It was developed by the FAA in response to an accident at JFK airport in New York. The aircraft (Eastern 66) landed during a wind shift caused by interacting sea breeze and thunderstorm outflows.

Artist rendition of a microburst and its effect on a landing aircraft.
Artist rendition of a microburst and its effect on a landing aircraft.

This Phase–1 LLWAS was very simple. It compared a center field wind to 5 other sensors around the airport. When there was a 15–knot vector difference, it would flash the wind data to the air traffic controller and the controller would read the raw winds, e.g., 120/35 (120 degrees at 35 knots), 110/20, 350/15, etc. from each sensor to the pilot landing or about to take off and the pilot had to do the vector addition in his head to determine the headwind/tailwind components.

This simple system worked for large scale weather features, but it also had a serious false alarm problem and the sensors were too far apart to capture small, but intense windshear events important to aircraft. Also, the center field wind could be variable and this would trigger windshear alarms at all the outer sensors since all the other sensor winds were compared to center field. Research conducted at the National Center for Atmospheric Research (NCAR) in the 1980s indicated that microburst windshear was very dangerous to aircraft below 1000 ft. Several major accidents during the 1980s also implicated windshear as a factor.

LLWAS History

In 1983, the FAA asked NCAR to develop a version of LLWAS that could detect microbursts. Between 1983 and 1988, NCAR developed and tested a new LLWAS system, called enhanced LLWAS or LLWAS–Network Expansion that detected microbursts, determined the strength in terms of headwind/tailwind gains or losses (in knots) and located the event (on the runway, at 1, 2, or 3 nm on departure or arrival). The system was designed to provide alerts specific to each runway operation. It was designed to have a probability of detection of 90 percent or greater and a false alarm rate of 10 percent or less.

This system was later improved and is now called the Phase–3 LLWAS. A typical Phase–3 LLWAS will have enough sensors to be spaced 2–km apart (∼1 nm apart) and cover out to 2 nm from the end of each major runway. The largest LLWAS is at Denver International Airport. It has 32 wind sensors. Most Phase–3 systems have between 12 and 16 wind sensors. A siting evaluation is done for each airport to determine the network geometry since it depends on terrain, # of runways, obstructions, etc.

The Phase–3 LLWAS alert information is described here. If a pilot is landing on runway 08, and there is a microburst on his path, the controller would have a display that reads: 08A MBA 30K–3MF 350/25. This is read to a pilot arriving on runway 08 (08A) by a final controller as "microburst alert (MBA), expect a thirty knot loss (30K–) at three miles final (3MF), threshold wind three–five–zero at 25 (knots)".

If there was a wind shear with a wind speed gain at 1 mile departure (headwind gain), for a pilot departing runway 25 left, the final controller's LLWAS display would show: 25LD WSA 15k+ 1MD. This would be read as "winds hear alert, expect a fifteen knot gain at one mile departure". There are Phase–3 LLWAS systems at 9 US airports and Phase 2 LLWAS at more than 100 airports. Taiwan, Korea, Singapore, Saudi Arabia, and Kuwait are now implementing LLWAS Phase–3 systems. Note, the FAA also has Terminal Doppler Weather Radars for wind shear detection at 45 airports and has ASR–9 based wind shear detection systems at another 37 airports. The FAA originally had 110 Phase–1 LLWAS systems, which were upgraded to Phase–2 systems.

A Phase–2 LLWAS has the same number of sensors (5–6) as a Phase–1 system (described above), but the wind shear algorithm was upgraded to significantly decrease the number of false alarms. As mentioned above, a Phase–1 or Phase–2 LLWAS was not designed to detect microbursts per se, but if the flow is large and strong, it may alert.

When NCAR developed the Phase–3 LLWAS, it gave the specifications to the FAA. The University Corporation for Atmospheric Research Foundation (UCARF),  owned the intellectual property for the wind shear algorithm during the lifetime of the patent. A license agreement was required for companies to implement LLWAS technology until early 2013 when the patent exclusion expired. A license from UCARF is no longer required to utilize the LLWAS algorithm. The UCARF does however, provide technical materials such as test datasets, test airport configuration files, test alert outputs, etc. to aid companies in the implementation and testing of the LLWAS Phase-3 algorithm.


Wind shear experts in RAL provide consultancy services to public and private organizations and governments around the world to help them understand wind shear and various wind shear detection system solutions. The consultancy services include identifying the exposure to wind shear, providing technical information on wind shear detection system solutions, siting systems, training aviation personnel on the impacts of wind shear on aviation, preparing technical specifications for wind shear systems, supporting the tendering process, and assisting with the implementation of wind shear detection solutions. For more information go to our page on wind shear system consultancy services.

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