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ARINC Aircraft Communications Addressing and
Reporting System (ACARS) data link services.
Airline dispatchers can also access this database and
send messages to specific aircraft whenever wind
shear activity begins or ends at an airport.
(c) TWIP products include descriptions and
character graphics of microburst alerts, wind shear
alerts, significant precipitation, convective activity
within 30 NM surrounding the terminal area, and
expected weather that will impact airport operations.
During inclement weather, i.e., whenever a predetermined
level of precipitation or wind shear is detected
within 15 miles of the terminal area, TWIP products
are updated once each minute for text messages and
once every five minutes for character graphic
messages. During good weather (below the predetermined
precipitation or wind shear parameters) each
message is updated every 10 minutes. These products
are intended to improve the situational awareness of
the pilot/flight crew, and to aid in flight planning prior
to arriving or departing the terminal area. It is
important to understand that, in the context of TWIP,
the predetermined levels for inclement versus good
weather has nothing to do with the criteria for
VFR/MVFR/IFR/LIFR; it only deals with precipitation,
wind shears and microbursts.
7-1-27. PIREPs Relating to Volcanic Ash
Activity
a. Volcanic eruptions which send ash into the
upper atmosphere occur somewhere around the world
several times each year. Flying into a volcanic ash
cloud can be extremely dangerous. At least two
B747s have lost all power in all four engines after
such an encounter. Regardless of the type aircraft,
some damage is almost certain to ensue after an
encounter with a volcanic ash cloud.
b. While some volcanoes in the U.S. are
monitored, many in remote areas are not. These
unmonitored volcanoes may erupt without prior
warning to the aviation community. A pilot observing
a volcanic eruption who has not had previous
notification of it may be the only witness to the
eruption. Pilots are strongly encouraged to transmit a
PIREP regarding volcanic eruptions and any
observed volcanic ash clouds.
c. Pilots should submit PIREPs regarding volcanic
activity using the Volcanic Activity Reporting (VAR)
form as illustrated in Appendix 2. If a VAR form is
not immediately available, relay enough information
to identify the position and type of volcanic activity.
d. Pilots should verbally transmit the data required
in items 1 through 8 of the VAR as soon as possible.
The data required in items 9 through 16 of the VAR
should be relayed after landing if possible.
7-1-28. Thunderstorms
a. Turbulence, hail, rain, snow, lightning, sustained
updrafts and downdrafts, icing conditions-all
are present in thunderstorms. While there is some
evidence that maximum turbulence exists at the
middle level of a thunderstorm, recent studies show
little variation of turbulence intensity with altitude.
b. There is no useful correlation between the
external visual appearance of thunderstorms and the
severity or amount of turbulence or hail within them.
The visible thunderstorm cloud is only a portion of a
turbulent system whose updrafts and downdrafts
often extend far beyond the visible storm cloud.
Severe turbulence can be expected up to 20 miles
from severe thunderstorms. This distance decreases
to about 10 miles in less severe storms.
AIM 2/14/08
7-1-56 Meteorology
c. 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 which is closely
associated with the areas of highest liquid water
content of the storm. NO FLIGHT PATH THROUGH
AN AREA OF STRONG OR VERY STRONG
RADAR ECHOES SEPARATED BY 20-30 MILES
OR LESS MAY BE CONSIDERED FREE OF
SEVERE TURBULENCE.
d. Turbulence beneath a thunderstorm should not
be minimized. This is especially true when the
relative humidity is low in any layer between the
surface and 15,000 feet. Then the lower altitudes may
be characterized by strong out flowing winds and
severe turbulence.
e. The probability of lightning strikes occurring to
aircraft is greatest when operating at altitudes where
temperatures are between minus 5 degrees Celsius
and plus 5 degrees Celsius. Lightning can strike
aircraft flying in the clear in the vicinity of a
thunderstorm.
f. METAR reports do not include a descriptor for
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