Wind shear at low altitudes is one of the most insidious and lethal hazards in aviation. Unlike turbulence at cruise altitude, low-level wind shear strikes during takeoff and landing — the two phases of flight where airspeed margins are smallest, terrain clearance is minimal, and the time available to react is measured in seconds. The history of commercial aviation is punctuated by catastrophic accidents directly attributed to low-level wind shear and the microburst phenomenon, tragedies that ultimately drove the development of the sophisticated alerting systems in use today.
The FAA and the aviation community have responded with a layered detection architecture: ground-based sensor networks, dedicated Doppler radar systems, and onboard predictive alerting equipment. Understanding how each system works, what it can and cannot detect, and how its alerts are communicated is essential knowledge for any pilot operating at Part 121, Part 135, or complex Part 91 levels — and it is core material tested on the Airline Transport Pilot knowledge exam.
The Threat: Microbursts and Low-Level Wind Shear
Wind shear is a rapid change in wind speed and/or direction over a short distance. At low altitudes, the most dangerous form is the microburst — a concentrated downdraft that strikes the surface and spreads outward in all directions. An aircraft penetrating a microburst first encounters a headwind that momentarily increases lift, tempting a pilot to reduce power. As the aircraft passes through the core, it encounters a powerful downdraft and then a tailwind, causing a sudden, severe loss of airspeed and lift. This sequence can produce a net performance loss exceeding the climb capability of many jet aircraft. Microbursts can produce wind speed differentials of more than 100 knots and vertical speeds as high as 6,000 feet per minute, and their horizontal extent is typically less than 2.5 nautical miles. They are brief — most last only 5 to 15 minutes — making timely detection critical.
How the Alerting Systems Work
Low-Level Wind Shear Alert System (LLWAS)
The Low-Level Wind Shear Alert System (LLWAS) is a network of anemometers (wind sensors) placed around an airport — on the field itself and at distances extending outward from the runway ends and sides. A central processor continuously compares the wind readings at each remote sensor against a centerfield reference sensor. When the difference between any remote sensor and the centerfield sensor exceeds a defined threshold — typically 15 knots — the system generates an alert. The alert is passed to air traffic control, who then issues a wind shear advisory to pilots on approach or departure.
LLWAS is a surface-based system, so it only detects wind shear that has already reached the ground and spread across the sensor network. By the time the sensor spread is detected, the shear event may already be affecting the approach corridor. Modern LLWAS-NE (Network Expansion) and LLWAS-RS (Relocation/Sustainment) upgrades have added more sensors and improved algorithms to detect convergence zones and provide better short-term prediction of where a microburst is likely to be encountered, but LLWAS remains inherently a reactive, surface-level detection tool.
Terminal Doppler Weather Radar (TDWR)
The Terminal Doppler Weather Radar (TDWR) is a high-resolution Doppler radar specifically designed and sited to monitor the airspace around major airports for wind shear and microburst activity. TDWR operates at C-band (5 cm wavelength) and is positioned approximately 8–12 miles from the airport to optimize its scan angle through the low-level airspace critical to approach and departure corridors. Unlike older weather radars, TDWR uses Doppler velocity processing to measure the actual motion of precipitation targets — detecting the divergence signature of a microburst (air moving away from a central point) even before the rain reaches the surface in some cases.
TDWR generates automated alerts that are transmitted directly to the ATCT (Air Traffic Control Tower). These alerts include microburst alerts, wind shear alerts, and gust front advisories. A microburst alert is issued when TDWR detects a loss or gain of airspeed of 30 knots or more within the runway corridor. A wind shear alert is issued for losses or gains of 15 to 29 knots. Gust front information warns of the advancing boundary of outflow winds that may produce shear conditions prior to precipitation arrival. Because TDWR samples the entire approach and departure envelope volumetrically, it can detect shear events that occur above the surface — something LLWAS sensors cannot do. TDWR data is also fed into the Integrated Terminal Weather System (ITWS) to improve overall situational awareness for terminal controllers.
Predictive Windshear Systems (PWS)
While LLWAS and TDWR are ground-based systems that rely on air traffic control to relay alerts to flight crews, Predictive Windshear Systems (PWS) are onboard avionics that put the detection capability directly in the aircraft. PWS uses the aircraft's own forward-looking weather radar — operating in a special windshear detection mode — to interrogate the atmosphere ahead of the aircraft along the flight path. By analyzing Doppler velocity returns from precipitation, the system can detect the divergence pattern of a microburst and alert the crew while the aircraft is still up to approximately 3 nautical miles from the hazard, providing as much as 40 seconds of warning during a typical approach.
PWS alerts are displayed on the weather radar display and/or the EFIS Navigation Display, accompanied by an aural warning such as "WINDSHEAR AHEAD, WINDSHEAR AHEAD" or a caution-level advisory such as "MONITOR RADAR DISPLAY." Because PWS is forward-looking, it is most effective during the final approach and takeoff roll when the aircraft is pointed directly at the threat corridor. The limitation of PWS is that it requires precipitation to be present — it detects Doppler returns from rain droplets and cannot detect dry microbursts (those that evaporate before reaching the surface). Dry microbursts are more common in the high desert environments of the western United States.
How Alerts Are Communicated to Pilots
Per AIM 7-1-26, controllers are required to issue wind shear alerts to pilots whenever LLWAS or TDWR generates an advisory for the runway in use. The phraseology is specific: a controller might say, "Caution, microburst alert, 40-knot loss 3-mile final, threshold wind 300 at 20." Pilots should acknowledge receipt of the alert. Importantly, not all airports have TDWR or LLWAS — pilots operating into smaller airports must rely on PIREPs, onboard radar, and their own monitoring of weather conditions. Even at equipped airports, the absence of an alert does not guarantee the absence of wind shear; sensor coverage gaps and dry microbursts can go undetected by ground systems.
Why It Matters Operationally
Wind shear alerting systems directly inform go/no-go and continuation decisions. Upon receiving a microburst alert, the appropriate response is generally to delay the approach, execute a missed approach if already established, or hold until the hazard clears. 14 CFR Part 121 operators are required to comply with company wind shear policies spelled out in their Ops Specs and Operations Manuals. Many carriers use performance-based wind shear escape maneuver procedures — typically a full thrust, attitude hold, no flap retraction, immediate climb to a safe altitude — trained extensively in the simulator.
The integration of LLWAS, TDWR, and PWS creates overlapping layers of protection. Ground systems give controllers early situational awareness to issue advisories before aircraft are established on approach; PWS gives the flight crew a direct, independent look at what lies ahead. Neither system replaces the other, and a thorough ATP-level understanding recognizes both the capabilities and limitations of each layer.
Key Numbers and Rules
- LLWAS alert threshold: wind difference of 15 knots or more between any remote sensor and the centerfield sensor.
- TDWR microburst alert: detected airspeed loss or gain of 30 knots or more in the runway corridor.
- TDWR wind shear alert: detected airspeed loss or gain of 15–29 knots in the runway corridor.
- PWS look-ahead range: approximately 3 nautical miles ahead of the aircraft (~40 seconds warning on approach).
- Microburst wind differential: can exceed 100 knots; downdraft can reach 6,000 fpm.
- Microburst horizontal extent: typically less than 2.5 nautical miles.
- Microburst duration: typically 5–15 minutes at peak intensity.
- PWS requires precipitation returns — cannot detect dry microbursts.
- LLWAS detects only surface-level shear — cannot detect shear above the ground sensor network.
Common Test Traps
- LLWAS vs. TDWR capability: LLWAS is a surface network and reacts to shear already at the ground. TDWR samples the full low-level volume and can detect airborne shear before it reaches the surface. Students often mix up which system has the volumetric advantage.
- PWS limitations: Predictive windshear systems are often described as if they work in all conditions. Remember they require precipitation — a classic exam question describes a clear-air or dry-air microburst environment where PWS provides no warning.
- Alert thresholds: The 15-knot LLWAS threshold and the 30-knot TDWR microburst threshold are commonly confused. Know which number belongs to which system and what it represents.
- No alert does not mean no shear: Examiners test whether candidates understand that an absence of an advisory is not a guarantee of safe conditions, especially at airports without TDWR or LLWAS.
- Who issues the alert: PWS alerts come directly to the flight crew from the aircraft's own avionics. LLWAS and TDWR alerts are relayed through ATC — pilots do not directly access those raw system outputs.