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surface can be as strong as 45 knots resulting in a
90knot shear (headwind to tailwind change for a
traversing aircraft) across the microburst. These
strong horizontal winds occur within a few hundred
feet of the ground.
3. Visual Signs. Microbursts can be found
almost anywhere that there is convective activity.
They may be embedded in heavy rain associated with
a thunderstorm or in light rain in benign appearing
virga. When there is little or no precipitation at the
surface accompanying the microburst, a ring of
blowing dust may be the only visual clue of its
existence.
4. Duration. An individual microburst will
seldom last longer than 15 minutes from the time it
strikes the ground until dissipation. The horizontal
winds continue to increase during the first 5 minutes
with the maximum intensity winds lasting approximately
2-4 minutes. Sometimes microbursts are
concentrated into a line structure, and under these
conditions, activity may continue for as long as an
hour. Once microburst activity starts, multiple
microbursts in the same general area are not
uncommon and should be expected.
2/14/08 AIM
Meteorology 7-1-47
FIG 7-1-14
Microburst Encounter During Takeoff
A microburst encounter during takeoff. The airplane first encounters a headwind and experiences increasing
performance (1), this is followed in short succession by a decreasing headwind component (2), a downdraft
(3), and finally a strong tailwind (4), where 2 through 5 all result in decreasing performance of the airplane.
Position (5) represents an extreme situation just prior to impact. Figure courtesy of Walter Frost, FWG
Associates, Inc., Tullahoma, Tennessee.
e. Microburst wind shear may create a severe
hazard for aircraft within 1,000 feet of the ground,
particularly during the approach to landing and
landing and take‐off phases. The impact of a
microburst on aircraft which have the unfortunate
experience of penetrating one is characterized in
FIG 7-1-14. The aircraft may encounter a headwind
(performance increasing) followed by a downdraft
and tailwind (both performance decreasing), possibly
resulting in terrain impact.
AIM 2/14/08
7-1-48 Meteorology
FIG 7-1-15
NAS Wind Shear Product Systems
f. Detection of Microbursts, Wind Shear and
Gust Fronts.
1. FAA's Integrated Wind Shear Detection
Plan.
(a) The FAA currently employs an integrated
plan for wind shear detection that will significantly
improve both the safety and capacity of the majority
of the airports currently served by the air carriers.
This plan integrates several programs, such as the
Integrated Terminal Weather System (ITWS),
Terminal Doppler Weather Radar (TDWR), Weather
System Processor (WSP), and Low Level Wind Shear
Alert Systems (LLWAS) into a single strategic
concept that significantly improves the aviation
weather information in the terminal area. (See
FIG 7-1-15.)
(b) The wind shear/microburst information
and warnings are displayed on the ribbon display
terminals (RBDT) located in the tower cabs. They are
identical (and standardized) in the LLWAS, TDWR
and WSP systems, and so designed that the controller
does not need to interpret the data, but simply read the
displayed information to the pilot. The RBDTs are
constantly monitored by the controller to ensure the
rapid and timely dissemination of any hazardous
event(s) to the pilot.
2/14/08 AIM
Meteorology 7-1-49
FIG 7-1-16
LLWAS Siting Criteria
(c) The early detection of a wind shear/
micro-burst event, and the subsequent warning(s)
issued to an aircraft on approach or departure, will
alert the pilot/crew to the potential of, and to be
prepared for, a situation that could become very
dangerous! Without these warnings, the aircraft may
NOT be able to climb out of, or safely transition, the
event, resulting in a catastrophe. The air carriers,
working with the FAA, have developed specialized
training programs using their simulators to train and
prepare their pilots on the demanding aircraft
procedures required to escape these very dangerous
wind shear and/or microburst encounters.
2. Low Level Wind Shear Alert System
(LLWAS).
(a) The LLWAS provides wind data and
software processes to detect the presence of
hazardous wind shear and microbursts in the vicinity
of an airport. Wind sensors, mounted on poles
sometimes as high as 150 feet, are (ideally) located
2,000 - 3,500 feet, but not more than 5,000 feet, from
the centerline of the runway. (See FIG 7-1-16.)
AIM 2/14/08
　
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