Wind Shear & Microbursts

Wind shear is a sudden, drastic change in wind speed and/or direction over a small area. Microbursts are the most dangerous form — concentrated downbursts that produce headwind/tailwind reversals up to 90 kt at low altitude, exactly where you have no altitude to trade. Primary references: FAA-H-8083-28 (Ch. 19) and AC 00-54 — Pilot Windshear Guide.

Vertical cross-section diagram of a microburst. A cumulonimbus cloud at the top produces a concentrated downdraft of up to 6,000 fpm that strikes the ground and spreads outward as surface outflow approximately 2.5 NM wide. An aircraft on approach is shown transitioning through three positions: headwind on entry, descending in the central downdraft, then losing airspeed in the trailing tailwind, with about a 90 kt headwind-to-tailwind differential across the cell. — Microburst cross-section: concentrated downdraft up to 6,000 fpm, ~2.5 NM diameter at the surface, ~90 kt headwind-to-tailwind differential. The aircraft on approach experiences airspeed gain (headwind) → descent (downdraft) → airspeed loss (tailwind).

Wind shear — definitions

From AC 00-54:

Severe wind shear: a change of more than 15 kt in horizontal wind speed, any change exceeding aircraft performance capability, or more than 500 fpm of vertical wind change.
Low-Level Wind Shear (LLWS): wind shear of 10 kt or more per 100 ft, in a layer at least 200 ft thick, occurring within 2,000 ft of the surface. Reported in TAFs as WS.
Wind shear can be vertical, horizontal, or both, and exists wherever air masses with different velocities meet.

Conditions producing wind shear

Passing frontal systems — especially fast-moving cold fronts (frontal slope and temperature contrast both steep)
Temperature inversions — common at night and at sunrise; the inversion top is a shear surface
Strong upper-level winds > 25 kt coupled with weak surface winds
Thunderstorm gust fronts — outflow can extend 15+ NM ahead of the parent cell, producing shear in clear air
Mountain wave activity — see turbulence and Mountain Flying
Sea breezes at coastal airports during onshore-flow afternoons

Microbursts — geometry & lethality

A microburst is a small, intense downburst that creates strong, often damaging surface winds. Per AC 00-54 and the AIM 7-1-26:

Diameter: typically < 2.5 NM at the surface
Lifespan: < 15 minutes total; intense phase often only 5
Vertical wind: downdrafts up to 6,000 fpm
Headwind/tailwind differential: up to 90 kt across the cell
Wet vs dry: wet microbursts visible with rain shaft; dry microbursts develop from high-based cumulus with virga only — virtually invisible until you're in it

The classic accident sequence: aircraft on approach hits the leading-edge headwind (airspeed spike, climbs above glide path), pilot reduces power to correct, aircraft transits the downdraft and exits into the trailing tailwind (airspeed collapses), aircraft now low + slow + reduced power. This is why the canonical recovery rule is "do not chase airspeed."

If suspected — recovery technique

Per AC 00-54:

Maximum power — collective up to torque/EGT limit, fixed-wing equivalent is firewall thrust.
Pitch for best climb attitude, not best climb airspeed. Hold the attitude even as airspeed wanders.
Do not chase airspeed — pilots who pitch down to recover lost airspeed feed altitude into the microburst.
Communicate — declare the encounter, request priority, deviate.

If you see a low-level wet microburst on approach, the correct action is to not be there — go around early and reposition.

Helicopter exposure — why we are extra-vulnerable

Helicopter operations include hovering and slow flight at exactly the altitudes microbursts dominate (surface to ~1,000 ft).
Light helicopters have lower mass and less momentum to ride through a downdraft — you don't punch through, you sink with it.
Downdrafts over forested areas can be strong enough to force a helicopter into the trees even at best rate-of-climb airspeed and full power.
External-load operations and confined-area approaches put you in slow flight with limited maneuver options — exactly the wrong place when shear arrives.
Helicopters cannot use airline-style escape vectors at altitude; the shear envelopes the entire approach.

Detection & warning systems

LLWAS (Low-Level Wind Shear Alert System) — surface anemometer network at large airports; tower issues alerts to pilots
TDWR (Terminal Doppler Weather Radar) — automated microburst detection at TDWR-equipped airports; alerts include "Microburst Alert, expect 40-knot loss on 3-mile final"
PIREPs — by far the best low-altitude warning. Listen on tower freq during convective weather.
Visible cues — virga (rain falling but not reaching ground), dust ring on the surface (microburst foot), localized rain shaft