AIM ¶ 4-3-7 — Wind Shear Detection Systems
AIM 4-3-7 explains LLWAS, TDWR, WSP, and ITWS wind shear and microburst alert systems. Learn alert phraseology and ARENA for your checkride.
In Plain English
AIM 4-3-7 describes the ground-based systems that detect dangerous low-level winds near airports and how controllers relay that information to you.
Key systems:
- LLWAS (Low Level Wind Shear Alert System): Detects wind shear and gust fronts around the airport perimeter only — no microburst alerts. Controllers report airport wind followed by boundary wind (e.g., "airport wind 230 at 8, south boundary wind 170 at 20").
- LLWAS-NE / LLWAS-RS: Integrated with TDWR; can issue microburst and wind shear alerts keyed to runway threshold or departure end (e.g., "Runway 17 arrival microburst alert, 40 knot loss 3 mile final").
- TDWR (Terminal Doppler Weather Radar): Provides microburst and wind shear detection.
- WSP (Weather Systems Processor): Enhancement to ASR-9 radar; detects wind shear, microbursts, and thunderstorms. Alerts are based on the ARENA — extending from a 3-mile final to a 2-mile departure along the runway centerline.
- ITWS (Integrated Terminal Weather System): Most advanced; alerts for microbursts, wind shear, and significant thunderstorms with runway-oriented winds.
Why it matters: knowing which alerts mean what — and reacting promptly to a microburst alert — can prevent loss of control on approach or departure. Equipped airports are noted in the Chart Supplement under Weather Data Sources.
AIM Source Text
FAA AIM ¶ 4-3-74-3-7. 4-3-7. Low Level Wind Shear/Microburst Detection Systems
Low Level Wind Shear Alert System (LLWAS), Terminal Doppler Weather Radar (TDWR), Weather Systems Processor (WSP), and Integrated Terminal Weather System (ITWS) display information on hazardous wind shear and microburst activity in the vicinity of an airport to air traffic controllers who relay this information to pilots. LLWAS provides wind shear alert and gust front information but does not provide microburst alerts. The LLWAS is designed to detect low level wind shear conditions around the periphery of an airport. It does not detect wind shear beyond that limitation. Controllers will provide this information to pilots by giving the pilot the airport wind followed by the boundary wind. EXAMPLE- Wind shear alert, airport wind 230 at 8, south boundary wind 170 at 20. 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. EXAMPLE- Runway 17 arrival microburst alert, 40 knot loss 3 mile final. REFERENCE- AIM, Para 7-1-24 , Microbursts. More advanced systems are in the field or being developed such as ITWS. ITWS provides alerts for microbursts, wind shear, and significant thunderstorm activity. ITWS displays wind information oriented to the threshold or departure end of the runway. The WSP provides weather processor enhancements to selected Airport Surveillance Radar (ASR)-9 facilities. The WSP provides Air Traffic with detection and alerting of hazardous weather such as wind shear, microbursts, and significant thunderstorm activity. The WSP 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 center line from a 3 mile final to the runway to a 2 mile departure. An airport equipped with the LLWAS, ITWS, or WSP is so indicated in the Chart Supplement under Weather Data Sources for that particular airport.
Oral Exam Questions a DPE Might Ask
Q1What is the key limitation of LLWAS compared to TDWR, WSP, or ITWS?
Per AIM 4-3-7, LLWAS detects only low-level wind shear and gust fronts around the airport periphery and does NOT provide microburst alerts. TDWR, WSP, and ITWS (and LLWAS-NE/RS integrated with TDWR) all provide microburst alerts in addition to wind shear detection.
Q2What is an ARENA and which system uses it?
Per AIM 4-3-7, an ARENA (Area Noted for Attention) is used by the WSP and extends along the runway centerline from a 3-mile final to a 2-mile departure. Controllers issue alerts based on hazardous weather detected within this area.
Q3How would a controller phrase a microburst alert, and how can you tell if an airport has these systems?
Per AIM 4-3-7, an example phraseology is: "Runway 17 arrival microburst alert, 40 knot loss 3 mile final." Airports equipped with LLWAS, ITWS, or WSP are indicated in the Chart Supplement under Weather Data Sources.
Related Paragraphs in AIM Chapter 4