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23. Wind Shear PIREPs
23.1 Because unexpected changes in wind speed and
direction can be hazardous to aircraft operations at
low altitudes on approach to and departing from
airports, pilots are urged to promptly volunteer
reports to controllers of wind shear conditions they
encounter. An advance warning of this information
will assist other pilots in avoiding or coping with a
wind shear on approach or departure.
23.2 When describing conditions, the use of the
terms “negative” or “positive” wind shear should be
avoided. PIREPs of negative wind shear on final,
intended to describe loss of airspeed and lift, have
been interpreted to mean that no wind shear was
encountered. The recommended method for wind
shear reporting is to state the loss/gain of airspeed and
the altitude(s) at which it was encountered.
EXAMPLE-
1. Denver Tower, Cessna 1234 encountered wind shear,
loss of 20 knots at 400.
2. Tulsa Tower, American 721 encountered wind shear on
final, gained 25 knots between 600 and 400 feet followed
by loss of 40 knots between 400 feet and surface.
Pilots using Inertial Navigation Systems should
report the wind and altitude both above and below the
shear layer.
EXAMPLEMiami
Tower, Gulfstream 403 Charlie encountered an
abrupt wind shear at 800 feet on final, max thrust required.
Pilots who are not able to report wind shear in these
specific terms are encouraged to make reports in
terms of the effect upon their aircraft.
24. Clear Air Turbulence (CAT) PIREPs
24.1 Clear air turbulence (CAT) has become a very
serious operational factor to flight operations at all
levels and especially to jet traffic flying in excess of
15,000 feet. The best available information on this
phenomenon must come from pilots via the PIREP
procedures. All pilots encountering CAT conditions
are urgently requested to report time, location, and
intensity (light, moderate, severe, or extreme) of the
element to the FAA facility with which they are
maintaining radio contact. If time and conditions
permit, elements should be reported according to the
standards for other PIREPs and position reports. See
TBL GEN 3.5-10, Turbulence Reporting Criteria
Table.
25. Microbursts
25.1 Relatively recent meteorological studies have
confirmed the existence of microburst phenomena.
Microbursts are small-scale intense downdrafts
which, on reaching the surface, spread outward in all
directions from the downdraft center. This causes the
presence of both vertical and horizontal wind shears
that can be extremely hazardous to all types and
categories of aircraft, especially at low altitudes. Due
to their small size, short life-span, and the fact that
they can occur over areas without surface precipitation,
microbursts are not easily detectable using
conventional weather radar or wind shear alert
systems.
25.2 Parent clouds producing microburst activity
can be any of the low or middle layer convective
cloud types. Note however, that microbursts
commonly occur within the heavy rain portion of
thunderstorms, and in much weaker, benign-appearing
convective cells that have little or no precipitation
reaching the ground.
25.3 The life cycle of a microburst as it descends in
a convective rain shaft is seen in FIG GEN 3.5-8,
Evolution of a Microburst. An important consideration
for pilots is the fact that the microburst intensifies for
about 5 minutes after it strikes the ground.
25.4 Characteristics of microbursts include:
25.4.1 Size. The microburst downdraft is typically
less than 1 mile in diameter as it descends from the
cloud base to about 1,000-3,000 feet above the
ground. In the transition zone near the ground, the
downdraft changes to a horizontal outflow that can
extend to approximately 2 1/2 miles in diameter.
25.4.2 Intensity. The downdrafts can be as strong
as 6,000 feet per minute. Horizontal winds near the
surface can be as strong as 45 knots resulting in a
90-knot 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.
25.4.3 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.
30 AUG 07
AIP
　
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