Wind shear is a sudden change in wind speed or direction over a short distance, and it is especially dangerous near the ground. During takeoff or landing, an aircraft has little altitude and time to recover from a rapid loss of airspeed or lift. A microburst is one of the most hazardous forms of wind shear because it can create strong downward and outward winds near a runway.
Understanding it helps explain why pilots avoid some storms even when the runway is visible.
Understanding Aviation: Wind Shear and Microbursts
Aircraft do not fly by their speed over the ground. They fly through the air around them. A cockpit airspeed indicator measures this motion through the air.
This distinction matters when the air itself changes speed. An airplane may appear to keep the same ground track while its useful flying speed falls quickly.
Near a runway, pilots normally use a planned extra margin above the minimum safe speed. That margin can disappear in seconds if the moving air changes unexpectedly.
A microburst begins inside a strong rain shower or thunderstorm. Falling rain, hail, and evaporating water can cool the surrounding air. Cooler air is denser, so it sinks rapidly.
When the descending air reaches the surface, it cannot continue downward. It spreads sideways in every direction, producing a changing pattern of winds. The strongest part is often short lived and confined to a small area, which makes it hard to see from the cockpit.
Rain may mark the danger, but some microbursts produce little rain at the ground. These are sometimes called dry microbursts.
The sequence during an approach is especially difficult because the early change can feel helpful. Stronger airflow from ahead raises indicated airspeed and can make the airplane briefly climb above its intended path. A pilot may reduce power or adjust the nose to regain the path.
Soon afterward, sinking air pushes the aircraft downward. Then the flow can reverse relative to the flight path, removing airspeed just when more lift is needed. Raising the nose alone cannot create enough lift if airflow over the wings is too slow.
A steep nose angle can increase drag and bring the wing closer to a stall. Fast power response, correct pitch control, and an early decision to abandon the landing are therefore important.
Pilots learn to treat storm proximity, sudden airspeed changes, unusual vertical speed, and reports from other crews as warning signs. Airport sensors can compare winds at several points around the field. Doppler radar can identify motion within precipitation.
Aircraft warning systems use flight data to recognize patterns that may be unsafe. These tools do not remove the need for judgment. A warning on approach commonly calls for an immediate escape maneuver using a practiced procedure and maximum available thrust.
Students should connect this subject to energy. Altitude, airspeed, engine power, and aircraft attitude are parts of an energy balance. Close to the ground there is little stored altitude to trade for speed, so avoiding the hazardous air is safer than trying to outfly it.
Key Facts
- Wind shear is a rapid change in wind velocity: wind velocity includes both speed and direction.
- A microburst is a small, intense downdraft that spreads outward when it hits the ground.
- Lift depends strongly on airspeed: L = 1/2 rho v^2 S CL.
- On approach, a microburst can first create a headwind, then a downdraft, then a tailwind.
- A sudden tailwind lowers airspeed over the wings, which can reduce lift at a dangerous moment.
- Doppler radar, LLWAS, and onboard wind shear warning systems help detect dangerous wind patterns.
Vocabulary
- Wind shear
- Wind shear is a sudden change in wind speed or direction over a short distance.
- Microburst
- A microburst is a powerful localized downdraft that hits the ground and spreads outward in all directions.
- Downdraft
- A downdraft is a column of sinking air, often produced by thunderstorms or heavy rain.
- Airspeed
- Airspeed is the speed of an aircraft relative to the surrounding air, not the ground.
- Doppler radar
- Doppler radar measures the motion of precipitation or air particles to detect wind speed and direction changes.
Common Mistakes to Avoid
- Confusing airspeed with ground speed, because lift depends on airspeed over the wings while ground speed only tells how fast the aircraft moves over the ground.
- Thinking a microburst is just heavy rain, because the main danger is the powerful sinking and spreading air that changes the aircraft's lift and flight path.
- Assuming a headwind is always helpful, because in a microburst the aircraft may quickly pass from a headwind into a downdraft and then a tailwind.
- Ignoring wind shear warnings during approach, because low altitude leaves very little time for pilots to regain speed, climb, or go around.
Practice Questions
- 1 An aircraft on approach has an airspeed of 70 m/s in a 10 m/s headwind. If the wind suddenly becomes a 20 m/s tailwind while the aircraft's ground speed has not yet changed, what is the new airspeed?
- 2 Lift is proportional to v^2. If an aircraft's airspeed drops from 80 m/s to 60 m/s, what fraction of its original lift is available, assuming all other factors stay the same?
- 3 Explain why the sequence of headwind, downdraft, and tailwind in a microburst is especially dangerous for an aircraft on final approach.