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 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).
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.