Wind Shear in Aviation: Definition, Causes and Hazards

Wind shear is defined as a sudden change in wind speed and/or direction over a short distance. It can occur in both the horizontal and vertical planes.

Types and Causes

• Vertical Wind Shear: Change in wind with height (knots per 100 feet).
• Horizontal Wind Shear: Change in wind in the horizontal plane (knots per 1000 feet).

The main causes include:

1. Thunderstorms and Microbursts: The most common and dangerous cause at low altitude.
2. Fronts: In the transition zone between air masses.
3. Temperature Inversions: Especially strong low-level inversions with strong winds above the stagnant cold air layer.
4. Jet Streams: High-altitude wind shear (CAT).
5. Topography: Wind flowing around obstacles or terrain.

Microbursts

A microburst is a localized column of sinking air (downdraft) within a thunderstorm, which upon hitting the ground spreads out violently in all directions.

• Dimensions: Typically 1 to 4 km in diameter.
• Duration: Short-lived, generally less than 5 minutes.
• Intensity: Downdrafts of up to 6000 fpm and horizontal wind changes of 50 to 90 knots.

Types of Microbursts

• Wet: Associated with heavy rain.
• Dry: Associated with Virga (rain that evaporates before touching the ground). Evaporation cools the air, making it denser and accelerating its fall. A common visual sign is a ring of dust rising from the ground.

Effect on Aircraft (Encounter Sequence)

When traversing a microburst (e.g., during landing):

1. Entry: The aircraft encounters a strong headwind.
   • Effect: Sudden increase in Indicated Airspeed (IAS) and lift.
   • Aircraft Reaction: Tends to rise ("balloon") above the glide path.
2. Center: The aircraft enters the strong downdraft.
   • Effect: The aircraft is pushed down, losing altitude rapidly.
3. Exit: The aircraft encounters a strong tailwind.
   • Effect: Sudden decrease in IAS and dramatic loss of lift.
   • Danger: If the aircraft is close to the ground and has low energy, it may impact the terrain before it can recover.

Effects on Approach (General Rules)

• Sudden increase in headwind (or decrease in tailwind):
  • Increases IAS and lift.
  • The aircraft rises above the glide path (Glideslope).
  • Risk: Long landing (overrun).
• Sudden decrease in headwind (or increase in tailwind):
  • Decreases IAS and lift.
  • The aircraft drops below the glide path (Glideslope).
  • Risk: Short landing (underrun) or impact with terrain.

Detection and Avoidance

• LLWAS (Low-Level Wind Shear Alert System): A system of sensors at airports that compares wind speed and direction at different points to detect wind shear.
• Pilot Reports: Vital information on actual conditions encountered.
• Action: If severe wind shear (especially microbursts) is reported or suspected, the recommended action is to delay takeoff or go-around and wait for conditions to improve.