Low-Level Wind Shear

Low-level wind shear (LLWS) is a sudden, large change in wind speed and/or direction over a short distance at low altitude — typically below 2,000 ft AGL. It is one of the most dangerous weather hazards for aircraft on approach and departure, capable of causing sudden loss of airspeed and loss of control.

Sources of LLWS

• Microburst: A localized, intense downdraft from a thunderstorm — the most dangerous form of LLWS; can cause airspeed changes exceeding 50 knots in seconds
• Frontal wind shear: Wind direction and speed changes at frontal boundaries; significant on approach when passing through a frontal surface
• Low-level jet: Sharp transition at the top of the nocturnal LLJ creates severe wind shear
• Sea breeze / land breeze interface: Sharp wind shift at the coastal boundary
• Wake turbulence: The descending vortices from a preceding heavy aircraft

LLWS on Approach

An aircraft on approach encountering a headwind shear (headwind increasing then decreasing, or changing to tailwind) will first pitch up and gain airspeed (the aircraft is going into stronger headwind), then pitch down and lose airspeed as the headwind decreases. If the airspeed loss is severe enough and the aircraft is near the stall, the result can be catastrophic.

Detection and Avoidance

• PIREPs are the most current information on LLWS
• PIREP reports of “moderate wind shear on approach” should be taken seriously
• LLWS alert systems (LLWAS) at some airports provide real-time data
• Fly recommended approach speeds — do not slow below V_REF in known shear conditions
• If shear is suspected, add an airspeed buffer (typically 5–10 knots)

On the TC Written Exam

LLWS is tested in both meteorology and airmanship sections. Know: microburst as the most severe form; the headwind-to-tailwind shear sequence on approach; PIREPs as the key information source; speed adjustment technique.

Related Terms

See also: Microburst, Low-Level Jet, Thunderstorm.