M11-2 LLWS and Microbursts PDF

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Summary

Thomas Guinn...

Description

Wind Shear

WX 301

Wind shear is a change in wind direction and/or speed over some distance. Vertical Shear (change in horizontal wind with height)

~30 kt vertical shear

Arrows indicate 15 kt wind speed

~30 kt horizontal shear

Horizontal Shear (change in the horizontal wind over a horizontal distance)

For aviation purposes, low level wind shear (LLWS) is wind shear that occurs below 2,000 ft AGL along either the final approach path or takeoff and initial climb-out path.

Most critical when wind change occurs rapidly over short horizontal distances (i.e., < 1 SM)

Causes of Low Level Wind Shear

A downburst is a strong precipitation-induced downdraft from a thunderstorm that generates horizontal and vertical wind shear when it hits the ground.

Note: Technically a “microburst” is a downburst with a radius ≤ 4km; however, the terms are often used interchangeably

A wet downburst includes rain extending to the surface, which adds an additional hazard of reduced visibility.

If the precipitation associated with a downburst evaporates before reaching the ground, it is known as a dry downburst.

Dry downbursts can still produce hazardous wind shear  Important not to fly under a thunderstorm even if you can clearly see through to the other side!

Downbursts/microbursts are particularly dangerous if encountered by an aircraft on final approach.

Points 1-2: Increased headwind causes aircraft to rise above the normal glideslope. Pilot likely to reduce thrust to get back on glide slope. Point 3: Downdraft causes aircraft to sink rapidly. Point 4: Most lethal zone, sudden tailwind causes the aircraft to lose airspeed and lift. Difficult to regain thrust in time to avoid collision. Delta Flight 191: https://www.youtube.com/watch?v=HDfodeURad0...