Peter "Just Loves Flying"

KEY  FACTOR : 

Flight crew awareness and alertness are key factors in the successful application of windshear avoidance and escape / recovery techniques. This Flight Briefing Note provides an overview of operational recommendations and training guidelines for aircraft operation in forecast or suspected windshear or downburst conditions.

REAL DATA : 

Adverse wind conditions (i.e., strong cross winds, tailwind and windshear) are involved in more than 30 percent of approach-and-landing accidents and in 15 percent of events involving CFIT. Windshear is the primary causal factor in 4 percent of approach-and-landing accidents and is the ninth cause of fatalities.

Defining Windshear : 

Windshear is defined as a sudden change of wind velocity and/or direction.
Windshear occurs in all directions, but for convenience, it is measured along vertical and horizontal axis, thus becoming vertical and horizontal windshear:

Vertical windshear: − Variations of the horizontal wind component along the vertical axis, resulting in turbulence that may affect the aircraft airspeed when climbing or descending through the windshear layer − Variations of the wind component of 20 kt per 1000 ft to 30 kt per 1000 ft are typical values, but a vertical windshear may reach up to 10 kt per 100 ft.
 Horizontal windshear: − Variations of the wind component along the horizontal axis (e.g., decreasing headwind or increasing tailwind, or a shift from a headwind to a tailwind) − Variations of wind component may reach up to 100 kt per nautical mile.
Windshear conditions usually are associated with the following weather situations:

• Jet streams • Mountain waves • Frontal surfaces • Thunderstorms and convective clouds • Microbursts.

MICROBURSTS:

 Microbursts combine two distinct threats to aviation safety :

• The downburst part, resulting in strong downdrafts (reaching up to 6000 ft/mn of vertical velocity) •

The outburst part, resulting in large horizontal windshear and wind component shift from headwind to tailwind (horizontal winds may reach up to 45 kt).

Windshear and Aircraft Performance :

• Headwind gust instantaneously increases the aircraft speed and thus tends to make the aircraft fly above intended path and/or accelerate ( item 1).

. A downdraft affects both the aircraft Angle-Of-Attack (AOA), that increases, and the aircraft path since it makes the aircraft sink ( item 2).

• Tailwind gust instantaneously decreases the aircraft speed and thus tends to make the aircraft fly below intended path and/or decelerate ( item 3).

NOTE: 
Windshears associated to jet streams, mountain waves and frontal surfaces usually occur at altitudes that do not present the same risk than microbursts, which occur closer to the ground.