Of all the hazards that thunderstorms produce, the microburst stands apart as the single most deadly mechanism responsible for numerous catastrophic accidents near airports. A pilot who encounters a microburst during takeoff or approach has, at best, five to fifteen seconds to recognize what is happening and respond correctly. In many cases, no response is sufficient. Understanding what microbursts and downbursts are, how they form, what they look like from the cockpit, and how to survive an inadvertent encounter is therefore not merely an exam topic — it is a survival skill.
The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 22, treats microbursts as the most severe type of wind shear, and the accident record bears that out. This article explains the phenomenon in depth, covering the physics, the visual cues, the specific phases of flight where the hazard peaks, and the counter-intuitive recovery technique that the FAA specifically mandates.
What Downbursts and Microbursts Are
A downburst is an intense column of descending air produced by a thunderstorm cell, a convective shower cell, or even an organized convective cloud. While every thunderstorm contains a downdraft as part of its normal circulation, a downburst is a discrete, localized intensification of that descending air — strong enough to produce damaging surface winds and severe wind shear. Downbursts can be large-scale events covering several miles, persisting for tens of minutes.
A microburst is the smaller, shorter-lived, but often more intense variant. By definition it has a horizontal diameter of less than 2.5 miles and a nominal vertical depth of about 1,000 feet. Its lifespan is only 5 to 15 minutes, yet during that window it can generate downdrafts exceeding 6,000 feet per minute and impose airspeed changes — first a headwind gain, then a catastrophic headwind loss — of 30 to 90 knots. Because of its small size and brief duration, it may not appear on conventional weather radar in time to warn pilots already on approach or just after liftoff.
When the descending core of a microburst strikes the surface, it spreads outward in all directions from the impact point. This outflow can be symmetric — spreading equally in every direction like a radial burst — or asymmetric, where environmental winds distort the outflow so that one side has a much stronger wind than the other. The asymmetric case is particularly insidious because the initial headwind increase upon entering the outflow may be mild or even absent, while the subsequent airspeed loss as the aircraft exits through the far side can be dramatic and sudden.
How a Microburst Encounter Unfolds
The classic microburst encounter on approach follows a three-act sequence that has been reconstructed from accident data. First, as the aircraft enters the leading edge of the outflow, it experiences a sudden increase in headwind. Indicated airspeed rises, the aircraft climbs above the glidepath, and the pilot — correctly interpreting the climb deviation — reduces power and lowers the nose to return to the glidepath. This is exactly what normal instrument scan and technique demand in calm air.
Second, the aircraft passes through the core of the downburst itself. The vertical component of the downdraft pushes the aircraft toward the ground. The pilot adds power and raises the nose, but the downdraft may be so strong that climb is impossible.
Third — and most lethal — the aircraft exits the far side of the microburst into a strong tailwind. Indicated airspeed plunges by tens of knots. Lift collapses. The aircraft is already in a nose-low, power-reduced configuration from the pilot's earlier correction in phase one. Recovery from this configuration at low altitude is often impossible. The FAA specifically warns that during landing, if the pilot has reduced power and lowered the nose in response to the initial headwind shear, the aircraft is left in exactly the wrong configuration when the tailwind shear hits — dramatically increasing the risk of landing short of the runway or stalling.
Takeoff Encounters: On the Runway and After Liftoff
Microbursts are equally dangerous during the departure phase. In documented accidents involving wind shear on the takeoff ground roll, the encounter with an increasing tailwind caused the aircraft to accelerate more slowly than normal. The airplane did not reach rotation speed until it was near the end of the runway. Even after liftoff, the continuing tailwind prevented further airspeed buildup, and the aircraft could not climb away from obstacles off the departure end.
In after-liftoff encounters, the initial seconds appear completely normal — airspeed, pitch attitude, vertical speed, and altitude all look correct. Then the wind shear strikes before a stabilized climb is established. As airspeed drops, the aircraft develops a pitch-down tendency. Past training instilled a reflex to lower the nose to regain airspeed, but this reflex is fatal in a microburst. The FAA is explicit: reducing pitch attitude to chase lost airspeed prevents recovery. The correct technique is to maintain or increase pitch attitude and accept the lower-than-normal airspeed. Only by holding pitch can the flightpath be preserved long enough for the aircraft to fly through the shear and regain performance. Some aircraft may require unusually high stick forces — up to 30 pounds of pull — to hold the correct pitch against the airplane's natural tendency to pitch down as lift decreases.
The time available to recognize and respond to a wind shear encounter after liftoff is typically only 5 to 15 seconds. This is not time for analysis; it is time for immediate, pre-briefed action.
Visual and Operational Clues
Detecting a microburst before entering it is the safest strategy, but visual cues can be subtle or misleading. The FAA identifies the following indicators:
- Intense rain shaft at the surface — a heavy, concentrated curtain of precipitation reaching the ground beneath a convective cloud is a classic sign of a downburst core.
- Virga — streaks of precipitation that evaporate before reaching the ground. Virga is particularly dangerous in dry climates because the evaporative cooling it produces intensifies the downdraft even with no surface rain to warn of the hazard.
- Ring of blowing dust — at the surface, the outflow from a microburst can loft dust and debris in a visible ring expanding outward from the impact point. This may be the only visible clue in a dry microburst environment.
- PIREP and ATIS wind shear advisories — reports from preceding aircraft and tower observations are critical. Any report of wind shear or microburst activity should be treated as an immediate reason to delay or divert.
- Low-Level Wind Shear Alert System (LLWAS) — many airports are equipped with networks of anemometers that detect diverging surface winds indicating outflow. LLWAS alerts transmitted via ATIS or ATC are authoritative warnings.
Crucially, more than one microburst can be embedded in the same weather system. If one microburst has been encountered or reported, pilots must remain alert for additional events. Multiple adjacent microbursts can generate a series of horizontal vortices near the ground, capable of producing violent roll forces and powerful updrafts in addition to downdrafts — a scenario that can overwhelm even high-performance aircraft.
Why This Is the Most Dangerous Wind Shear
The FAA designates microbursts as the most severe type of wind shear for three compounding reasons. First, the combination of a powerful vertical downdraft and a massive horizontal airspeed swing — up to 90 knots — can exceed the climb performance of virtually any transport aircraft at low altitude. Second, the phenomenon is brief enough to miss on weather radar and localized enough to affect one aircraft while the one ahead has a normal approach. Third, the natural pilot response to the initial cues (power reduction and nose-down pitch) puts the aircraft in the worst possible configuration for the subsequent and more dangerous tailwind phase. The FAA states plainly that some microbursts cannot be successfully escaped with any known technique, and that some wind shears within the theoretical performance capability of the aircraft have still caused fatal accidents.
Key Numbers and Rules
- Microburst horizontal diameter: less than 2.5 miles
- Nominal depth: approximately 1,000 feet
- Lifespan: 5 to 15 minutes
- Downdraft intensity: up to 6,000 feet per minute
- Airspeed variation: 30 to 90 knots headwind gain followed by loss
- Time to recognize and respond after liftoff: 5 to 15 seconds
- Recovery technique: maintain or increase pitch, accept reduced airspeed
- Stick force required in some aircraft: up to 30 pounds of pull
- Multiple microbursts are possible in one system — one encounter does not mean the hazard is over
Common Test Traps
- Confusing virga with a safe condition. Virga means precipitation is evaporating aloft, which can intensify the downdraft. The absence of rain at the surface does not mean the absence of a microburst.
- Thinking a headwind increase means the worst is over. The initial headwind gain is actually the entry into the microburst. The most dangerous phase — the tailwind loss — comes after the aircraft has passed through the core.
- Applying the wrong recovery technique. Every instinct tells a pilot to lower the nose when airspeed drops. In a microburst, this is the wrong response. The FAA is specific: maintain or increase pitch attitude.
- Assuming one microburst means the area is clear afterward. Multiple microbursts embedded in a single weather system are specifically warned against in FAA-H-8083-28B.
- Underestimating an asymmetric microburst. The airspeed increase entering an asymmetric microburst may be small or absent, removing the normal warning cue before the catastrophic exit-side airspeed loss occurs.