Low-level wind shear and microbursts

Low-Level Wind Shear and Microbursts

Wind shear is a change in wind speed and/or direction over a short distance (vertical or horizontal). Microbursts are an extreme form of wind shear under thunderstorms. Both are life-threatening in take-off and approach.

Source: FAA AC 00-54 Pilot Windshear Guide; ICAO Doc 9817 Manual of Low-Level Wind Shear; WMO.

Definition wind shear

Wind shear is a significant change of wind direction and/or speed over a small height difference:

• Classification: vertical vs horizontal.

Horizontal wind shear

A significant change of wind direction and/or speed over a small horizontal distance is a "horizontal wind shear" — e.g. at a front, at an outflow boundary.

Vertical wind shear

• Change with altitude — e.g. at inversion, at jet-stream edge.
• Critical on final approach (between ground and 500 ft AGL).

Conditions for wind shear

• Inversions: between cool surface and warm upper air.
• Thunderstorms: outflow boundaries, downbursts.
• Fronts: especially fast cold fronts.
• Topography: mountain waves, lee.
• Jet streams: at the edges.

Microburst — extreme phenomenon

• Strongly descending air column beneath a Cb or TCu.
• Diameter: 1-4 km.
• Lifetime: 5-15 min.
• Vertical speed: up to 6000 fpm downward.
• Horizontal outflow at surface: up to 80-90 kt → strong wind shear.

Wind-shear detection

• METAR/TAF with WS annotation.
• LLWAS at major airports.
• TDWR (Terminal Doppler Weather Radar).
• PIREP.
• Visual cues: virga, dust ring beneath Cb.

Pilot response

Detail: Subject 070 lesson "Low-Level Wind Shear and Microbursts — operational" and Subject 050 met-products.

Short:

1. Avoid take-off when LLWAS warns.
2. Monitor wind-shear indicators on final.
3. On microburst suspicion: immediate go-around with max power, Vy or stick-shaker.
4. Approach wind shear: immediate go-around at IAS loss > 10 kt or altitude loss > 100 ft.

Four wind-shear cases on approach (no correction)

(Detail in Subject 070 LLWS lesson.)