On a hot afternoon, a thunderstorm drifts close to the airport. An aircraft on final approach suddenly gains speed, then moments later loses it and starts sinking toward the ground. That sequence is the signature of a microburst, one of the most dangerous weather hazards in aviation.
What is a microburst?
A microburst is a powerful, concentrated downdraft of air, usually from a thunderstorm or a large shower cloud. When it hits the ground it spreads outward in all directions, like water poured onto a floor. It is small, typically less than 4 km across, and short-lived, usually lasting only a few minutes, but the downdraft and the outflow winds can be violent.
Why it is so dangerous on approach and take-off
Imagine an aircraft flying through a microburst on final approach:
- Headwind: entering the outflow, the aircraft meets a strong headwind. Airspeed increases and the aircraft rises above the glide path. The natural reaction is to reduce power.
- Downdraft: in the centre, a strong downdraft pushes the aircraft toward the ground.
- Tailwind: on the far side, the wind turns into a tailwind. Airspeed drops sharply, just when the aircraft is low, slow and with reduced power.
Close to the ground there is very little height to recover. Several fatal accidents, including Delta Air Lines Flight 191 at Dallas in 1985, led to the windshear training and detection systems used today.
Watch: a microburst in 60 seconds
Wet and dry microbursts
- Wet microbursts come with heavy rain and are easier to see.
- Dry microbursts happen when rain evaporates before reaching the ground. Look for virga (streaks of rain hanging below a cloud), dust rings on the ground and gusty, shifting winds.
How microbursts are detected
- On the aircraft: predictive windshear systems in the weather radar look ahead for wind patterns, and reactive systems warn ("WINDSHEAR, WINDSHEAR") when the aircraft is already in it.
- At the airport: low-level windshear alert systems and Doppler weather radar can detect microbursts and let ATC warn crews.
- Pilot reports: crews report windshear to ATC so following aircraft are warned.
What pilots do
The best defence is avoidance: delay the take-off or approach until the storm has moved away. If windshear is encountered, crews follow the windshear recovery procedure: apply maximum thrust, follow the flight director or pitch guidance up to the stall-warning limit, and do not change the flap or landing gear configuration until clear.
Quick revision
- Microburst: intense, small, short-lived downdraft spreading out near the ground.
- Sequence on approach: headwind (speed gain), downdraft, tailwind (speed loss).
- Signs: thunderstorms, virga, dust rings, gusty winds.
- Defence: avoid; if caught, windshear recovery with maximum thrust.
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