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Mountain waves;
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Frontal surfaces;
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Thunderstorms and convective clouds; and,
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Microbursts.
Wind Shear Awareness
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Microbursts combine two distinct threats to aviation safety:
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The downburst part, resulting in strong downdrafts (reaching up to 40 kt of vertical velocity); and,
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The outburst part, resulting in large horizontal wind shear and wind component shift from head wind to tail wind (horizontal winds may reach up to 100 kt).
Wind Shear Avoidance
The following information should be used to avoid areas of potential or observed wind shear:
. Weather reports and forecast:
. The low level wind shear alert system (LLWAS) allows controllers to warn pilots of existing or impending wind shear conditions.
LLWAS consists of a central wind sensor (sensing wind velocity and direction) and peripheral wind sensors located at approximately 2 nm from the center.
Center wind sensor data are averaged over a rolling 2-minute period and compared every 10 seconds to the data of the peripheral wind sensors.
An alert is generated whenever a difference in excess of 15 kt is detected.
LLWAS may not detect downbursts with a diameter of 2 nm or less.
. A Terminal Doppler Weather Radar (TDWR) allows to detect approaching wind shear areas and, thus, to provide pilots with more advance warning of wind shear hazard.
. Pilot’s reports:
. PIREPS of wind shear in excess of 20 kt or downdraft / updraft of 500 ft/mn below 1000 ft above airfield elevation should be cause for caution.
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Visual observation:
. Blowing dust, rings of dust, dust devils (i.e., whirlwinds containing dust and stand), or any other evidence of strong local air outflow near the surface often are indication of potential or existing wind shear.
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On-board wind component and ground speed monitoring:
. On approach, a comparison of the head wind or tail wind component aloft (as available) and the surface head or tail wind component indicates the potential and likely degree of vertical wind shear.
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On-board weather radar; and,
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On-board predictive wind shear system.
Wind Shear Recognition
Timely recognition of a wind shear condition is vital for the successful implementation of the wind shear recovery/escape procedure.
The aircraft flight envelope protection provides an automatic detection of a wind shear condition during takeoff, approach or go-around, based on the assessment of the aircraft performance (flight parameters and accelerations).
Wind Shear Awareness
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The following deviations should be considered as indications of a possible wind shear condition:
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Indicated airspeed variations in excess of 15 kt;
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Ground speed variations (decreasing head wind or increasing tail wind or shift from head wind to tail wind);
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Vertical speed excursions of 500 ft/mn;
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Pitch attitude excursions of 5 degrees;
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Glide slope deviation of 1 dot;
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Heading variations of 10 degrees;
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Unusual autothrottle activity or throttle levers position.
Wind Shear Survival Strategy
In case of wind shear encounter, a survival strategy needs to be adopted to minimize the altitude loss and the associated risk of ground contact.
The following describes the wind shear survival strategy implemented in the flight director (FD) for conventional aircraft models.
The FD wind shear recovery guidance attempts to:
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Maintain the speed target (speed selected on FCU + 10 kt) as long as a positive vertical speed is possible;
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