Low-Level Wind Sheer & Microburst Detection Systems


Weather Data Sources
Figure 1: Weather Data Sources

FAA's Integrated Wind Shear Detection Plan:

    NAS Wind Shear Product Systems
    Figure 2: NAS Wind Shear Product Systems
  • The wind shear/microburst information and warnings are displayed on the ribbon display terminals (RBDT) located in the tower cabs. They are identical (and standardized) in the LLWAS, TDWR and WSP systems, and so designed that the controller does not need to interpret the data, but simply read the displayed information to the pilot. The RBDTs are constantly monitored by the controller to ensure the rapid and timely dissemination of any hazardous event(s) to the pilot
  • The early detection of a wind shear/micro-burst event, and the subsequent warning(s) issued to an aircraft on approach or departure, will alert the pilot/crew to the potential of, and to be prepared for, a situation that could become very dangerous! Without these warnings, the aircraft may NOT be able to climb out of, or safely transition, the event, resulting in a catastrophe. The air carriers, working with the FAA, have developed specialized training programs using their simulators to train and prepare their pilots on the demanding aircraft procedures required to escape these very dangerous wind shear and/or microburst encounters

Low-Level Wind Shear Alert System (LLWAS):

  • The LLWAS provides wind data and software processes to detect the presence of hazardous wind shear and microbursts in the vicinity of an airport
    • Controllers will provide this information to pilots by giving the pilot the airport wind followed by the boundary wind
    • ATC: "Wind shear alert, airport wind 230 at 8, south boundary wind 170 at 20"
  • The system senses wind shear occurrences through comparison of reading from the various wind sensors
  • LLWAS was fielded in 1988 at 110 airports across the United States
    • Many of these systems have been replaced by new TDWR and WSP technology
    • Eventually all LLWAS systems will be phased out; however, 39 airports will be upgraded to the LLWAS−NE (Network Expansion) system, which employs the very latest software and sensor technology
    • The new LLWAS-NE systems will not only provide the controller with wind shear warnings and alerts, including wind shear/microburst detection at the airport wind sensor location, but will also provide the location of the hazards relative to the airport runway(s)
    • It will also have the flexibility and capability to grow with the airport as new runways are built
    • As many as 32 sensors, strategically located around the airport and in relationship to its runway configuration, can be accommodated by the LLWAS-NE network
  • Unfortunately, some microbursts are so small that they can fit between the sensors
  • Doppler radar has proven effective in detecting microbursts and is being installed at major airports, integrated with the LLWAS
  • Wind sensors, mounted on poles sometimes as high as 150 feet, are (ideally) located 2,000 - 3,500', but not more than 5,000', from the centerline of the runway [Figure 3:]
LLWAS Siting Criteria
Figure 3: LLWAS Siting Criteria

Terminal Doppler Weather Radar (TDWR):

  • TDWRs are being deployed at 45 locations across the U.S. Optimum locations for TDWRs are 8 to 12 miles off of the airport proper, and designed to look at the airspace around and over the airport to detect microbursts, gust fronts, wind shifts and precipitation intensities
    • TDWR products advise the controller of wind shear and microburst events impacting all runways and the areas 1/2 mile on either side of the extended centerline of the runways out to 3 miles on final approach and 2 miles out on departure
  • Software uses warning boxes to determine the location(s) of wind shear or microbursts [Figure 4]
  • These warnings are displayed on the RBDT
  • It is very important to understand what TDWR does NOT DO:
    • It DOES NOT warn of wind shear outside of the alert boxes (on the arrival and departure ends of the runways);
    • It DOES NOT detect wind shear that is NOT a microburst or a gust front;
    • It DOES NOT detect gusty or cross wind conditions; and
    • It DOES NOT detect turbulence
  • Future improvements may include such areas as storm motion (movement), improved gust front detection, storm growth and decay, microburst prediction, and turbulence detection
  • TDWR also provides a geographical situation display (GSD) for supervisors and traffic management specialists for planning purposes
    • The GSD displays (in color) 6 levels of weather (precipitation), gust fronts and predicted storm movement(s)
    • This data is used by the tower supervisor(s), traffic management specialists and controllers to plan for runway changes and arrival/departure route changes in order to both reduce aircraft delays and increase airport capacity
  • LLWAS "network expansion," (LLWAS NE) and LLWAS Relocation/Sustainment (LLWAS-RS) are systems integrated with TDWR
    • These systems provide the capability of detecting microburst alerts and wind shear alerts
    • Controllers will issue the appropriate wind shear alerts or microburst alerts
    • In some of these systems controllers also have the ability to issue wind information oriented to the threshold or departure end of the runway
      • ATC: "Runway 17 arrival microburst alert, 40 knot loss 3 mile final"
Warning Boxes
Figure 4: Warning Boxes

Integrated Terminal Weather System (ITWS):

  • Provides alerts for microbursts, wind shear, and significant thunderstorm activity
  • Displays wind information oriented to the threshold or departure end of the runway

Weather System Processor:

  • The WSP provides weather processor enhancements to selected Airport Surveillance Radar (ASR)-9 facilities, thus eliminating the requirements for a separate radar location, land acquisition, support facilities and the associated communication landlines and expenses
  • The WSP provides the controller, supervisor, traffic management specialist, and ultimately the pilot, with the same products as the terminal doppler weather radar (TDWR) at a fraction of the cost of a TDWR which improves detection and alerting of hazardous weather such as wind shear, microbursts, and significant thunderstorm activity
    • This is accomplished by utilizing new technologies to access the weather channel capabilities of the existing ASR−9 radar located on or near the airport, thus eliminating the requirements for a separate radar location, land acquisition, support facilities and the associated communication landlines and expenses
  • Displays terminal area 6 level weather, storm cell locations and movement, as well as the location and predicted future position and intensity of wind shifts that may affect airport operations
  • Controllers will receive and issue alerts based on Areas Noted for Attention (ARENA)
  • An ARENA extends on the runway centerline from a 3 mile final to the runway to a 2 mile departure
  • The WSP utilizes the same RBDT display as the TDWR and LLWAS, and, just like TDWR, also has a GSD for planning purposes by supervisors, traffic management specialists and controllers
  • The WSP GSD emulates the TDWR display, i.e., it also depicts 6 levels of precipitation, gust fronts and predicted storm movement, and like the TDWR GSD, is used to plan for runway changes and arrival/departure route changes in order to reduce aircraft delays and to increase airport capacity

Operational aspects of LLWAS, TDWR and WSP:

  • To demonstrate how this data is used by both the controller and the pilot, 3 ribbon display examples and their explanations are presented:
  • Microburst Alerts:

    • The controller sees a ribbon display in the tower cab: 27A MBA 35K- 2MF 250 20 [Figure 5]
    • Warning Boxes
      Figure 5: Microburst Alert
    • ATC: "Runway 27 arrival, microburst alert, 35 knot loss 2 mile final, threshold wind 250 @ 20"
    • In plain language, the controller is telling the pilot that on approach to runway 27, there is a microburst alert on the approach lane to the runway, and to anticipate or expect a 35 knot loss of airspeed at approximately 2 miles out on final approach (where it will first encounter the phenomena). With that information, the aircrew is forewarned, and should be prepared to apply wind shear/microburst escape procedures should they decide to continue the approach. Additionally, the surface winds at the airport for landing runway 27 are reported as 250 degrees at 20 knots
    • Threshold wind is at pilot's request or as deemed appropriate by the controller
  • Wind Shear Alerts:

    • The controller sees a ribbon display in the tower cab: 27A WSA 20K- 3MF 200 15 [Figure 6]
    • Warning Boxes
      Figure 6: Microburst Alert
    • ATC: "Runway 27 arrival, wind shear alert, 20 knot loss, 3 mile final, threshold wind 200 @ 15"
    • In plain language, the controller is advising the aircraft arriving on runway 27 that at about 3 miles out they can expect to encounter a wind shear condition that will decrease their airspeed by 20 knots and possibly encounter turbulence. Additionally, the airport surface winds for landing runway 27 are reported as 200 degrees at 15 knots
    • Threshold wind is at pilot's request or as deemed appropriate by the controller
  • Multiple Wind Shear (gust front) Alerts:

    • The controller sees a ribbon display in the tower cab: 27A WSA 20K+ RWY 250 20 & 27D WSA 20K+ RWY 250 20 [Figure 7]
    • Warning Boxes
      Figure 7: Gust Front Alert
    • ATC: "Multiple wind shear alerts, runway 27 arrival, wind shear alert, 20 knot gain on runway; 27 departure, wind shear alert, 20 knot gain on runway, wind 250 @ 20"
    • In this example, the controller is advising arriving and departing aircraft that they could encounter a wind shear condition right on the runway due to a gust front (significant change of wind direction) with the possibility of a 20 knot gain in airspeed associated with the gust front. Additionally, the airport surface winds (for the runway in use) are reported as 250 degrees at 20 knots

Terminal Weather Information for Pilots System (TWIP):

  • With the increase in the quantity and quality of terminal weather information available through TDWR, the next step is to provide this information directly to pilots rather than relying on voice communications from ATC. The National Airspace System has long been in need of a means of delivering terminal weather information to the cockpit more efficiently in terms of both speed and accuracy to enhance pilot awareness of weather hazards and reduce air traffic controller workload. With the TWIP capability, terminal weather information, both alphanumerically and graphically, is now available directly to the cockpit on a test basis at 9 locations
  • TWIP products are generated using weather data from the TDWR or the Integrated Terminal Weather System (ITWS) testbed. TWIP products are generated and stored in the form of text and character graphic messages. Software has been developed to allow TDWR or ITWS to format the data and send the TWIP products to a database resident at Aeronautical Radio, Inc. (ARINC). These products can then be accessed by pilots using the ARINC Aircraft Communications Addressing and Reporting System (ACARS) data link services. Airline dispatchers can also access this database and send messages to specific aircraft whenever wind shear activity begins or ends at an airport
  • TWIP products include descriptions and character graphics of microburst alerts, wind shear alerts, significant precipitation, convective activity within 30 NM surrounding the terminal area, and expected weather that will impact airport operations. During inclement weather, i.e., whenever a predetermined level of precipitation or wind shear is detected within 15 miles of the terminal area, TWIP products are updated once each minute for text messages and once every five minutes for character graphic messages. During good weather (below the predetermined precipitation or wind shear parameters) each message is updated every 10 minutes. These products are intended to improve the situational awareness of the pilot/flight crew, and to aid in flight planning prior to arriving or departing the terminal area. It is important to understand that, in the context of TWIP, the predetermined levels for inclement versus good weather has nothing to do with the criteria for VFR/MVFR/IFR/LIFR; it only deals with precipitation, wind shears and microbursts


  • The Weather System Processor system is currently under development and is operating in a developmental test status at the Albuquerque, New Mexico, airport. When fielded, the WSP is expected to be installed at 34 airports across the nation, substantially increasing the safety of the American flying public
  • An airport equipped with the LLWAS, ITWS, or WSP is so indicated in the Airport/Facility Directory under Weather Data Sources for that particular airport