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.
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.