9-2 Weather Hazard (Low-Level Wind Shear)
Wind shear is dangerous to an aircraft. It can rapidly
change the performance of the aircraft and disrupt the normal flight attitude. Wind
shear is a sudden, drastic change in wind speed and/or direction over a very
small area. Wind shear can subject an aircraft to violent updrafts and downdrafts,
as well as abrupt changes to the horizontal movement of the aircraft. While
wind shear can occur at any altitude, low-level wind shear is especially
hazardous due to the proximity of an aircraft to the ground.
Low-level wind shear is commonly associated with passing
frontal systems, thunderstorms, temperature inversions, and strong upper-level
winds greater than 25 knots. The most severe type of low-level wind shear, a
microburst, is associated with convective precipitation into dry air at cloud
base. Microburst activity may be indicated by an intense rain shaft at the
surface but virga at cloud base and a ring of blowing dust is often the only
visible clue.
The FAA has made a substantial investment in microburst
accident prevention. The totally redesigned LLWAS-NE, the TDWR, and the ASR-9
WSP are skillful microburst alerting systems installed at major airports. These
three systems were extensively evaluated over a 3-year period. Each was seen to
issue very few false alerts and to detect microbursts well above the 90 percent
detection requirement established by Congress. Many flights involve airports
that lack microburst alert equipment, so the FAA has also prepared wind shear
training material: Advisory Circular (AC) 00-54, FAA Pilot Wind Shear Guide.
Included is information on how to recognize the risk of a microburst encounter,
how to avoid an encounter, and the best flight strategy for successful escape
should an encounter occur.
Word Count: 279 words
References
- Weather Theory Ch 12
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