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can pit the windscreens, thus reducing or eliminating forward
visibility. The pitot-static system may become clogged,
causing instrument failure. Severe engine damage is probable
in both piston and jet-powered aircraft.
Every effort must be made to avoid volcanic ash. Since
volcanic ash clouds are carried by the wind, pilots should plan
their flights to remain upwind of the ash-producing volcano.
Visual detection and airborne radar are not considered
a reliable means of avoiding volcanic ash clouds. Pilots
witnessing volcanic eruptions or encountering volcanic
ash should immediately pass this information along in
the form of a pilot report. The National Weather Service
10-25
Figure 10-16. A thunderstorm packs just about every weather hazard
known to aviation into one vicious bundle.
monitors volcanic eruptions and estimates ash trajectories.
This information is passed along to pilots in the form of
SIGMETs.
As for many other hazards to flight, the best source of
volcanic information comes from PIREPs. Pilots who witness
a volcanic eruption or encounter volcanic ash in flight
should immediately inform the nearest agency. Volcanic
Ash Forecast Transport and Dispersion (VAFTAD) charts
are also available; these depict volcanic ash cloud locations
in the atmosphere following an eruption, and also forecast
dispersion of the ash concentrations over 6- and 12-hour time
intervals. See AC 00-45, Aviation Weather Services.
Thunderstorms
A thunderstorm packs just about every weather hazard known
to aviation into one vicious bundle. Turbulence, hail, rain,
snow, lightning, sustained updrafts and downdrafts, and icing
conditions are all present in thunderstorms. Do not take off in
the face of an approaching thunderstorm or fly an aircraft that is
not equipped with thunderstorm detection in clouds or at night
in areas of suspected thunderstorm activity. [Figure 10-16]
There is no useful correlation between the external visual
appearance of thunderstorms and the severity or amount of
turbulence or hail within them. All thunderstorms should be
considered hazardous, and thunderstorms with tops above
35,000 feet should be considered extremely hazardous.
Weather radar, airborne or ground based, will normally
reflect the areas of moderate to heavy precipitation (radar
does not detect turbulence). The frequency and severity of
turbulence generally increases with the radar reflectivity
closely associated with the areas of highest liquid water
content of the storm. A flight path through an area of strong
or very strong radar echoes separated by 20 to 30 miles or
less may not be considered free of severe turbulence.
The probability of lightning strikes occurring to aircraft is
greatest when operating at altitudes where temperatures are
between -5 ° C and +5 ° C. In addition, an aircraft flying in the
clear air near a thunderstorm is also susceptible to lightning
strikes. Thunderstorm avoidance is always the best policy.
Wind Shear
Wind shear can be defined as a change in wind speed and/or
wind direction in a short distance. It can exist in a horizontal
or vertical direction and occasionally in both. Wind shear can
occur at all levels of the atmosphere but is of greatest concern
during takeoffs and landings. It is typically associated
with thunderstorms and low-level temperature inversions;
however, the jet stream and weather fronts are also sources
of wind shear.
As Figure 10-17 illustrates, while an aircraft is on an
instrument approach, a shear from a tailwind to a headwind
causes the airspeed to increase and the nose to pitch up with
a corresponding balloon above the glide path. A shear from
a headwind to a tailwind has the opposite effect, and the
aircraft will sink below the glide path.
A headwind shear followed by a tailwind/downdraft shear is
particularly dangerous because the pilot has reduced power
and lowered the nose in response to the headwind shear. This
leaves the aircraft in a nose-low, power-low configuration
when the tailwind shear occurs, which makes recovery more
difficult, particularly near the ground. This type of wind
shear scenario is likely while making an approach in the
face of an oncoming thunderstorm. Pilots should be alert for
indications of wind shear early in the approach phase and be
ready to initiate a missed approach at the first indication. It
may be impossible to recover from a wind shear encounter
at low altitude.
To inform pilots of hazardous wind shear activity, some
airports have installed a Low-Level Wind Shear Alert
System (LLWAS) consisting of a centerfield wind indicator
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