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AWOS-3 are able to provide additional information such as
cloud and ceiling data and precipitation type. ASOS/AWSS
stations providing service levels A or B also report RVR. The
specific type of equipment found at a given facility is listed
in the A/FD. [Figure 2-13]
Automated weather information is available both over a
radio frequency specific to each site and via telephone.
When an automated system is brought online, it first goes
through a period of testing. Although you can listen to the
reports on the radio and over the phone during the test
phase, they are not legal for use until they are fully operational, and the test message is removed.
The use of the aforementioned visibility reports and
weather services are not limited for Part 91 operators. Part
121 and 135 operators are bound by their individual
OpsSpecs documents and are required to use weather
reports that come from the National Weather Service or
other approved sources. While every operator’s specifications are individually tailored, most operators are required
to use ATIS information, RVR reports, and selected
reports from automated weather stations. All reports coming from an AWOS-3 station are usable for Part 121 and
135 operators. Each type of automated station has different levels of approval as outlined in FAA Order 8400.10
and individual OpsSpecs. Ceiling and visibility reports
given by the tower with the departure information are
always considered official weather, and RVR reports are
typically the controlling visibility reference.
AUTOMATIC TERMINAL
INFORMATION SERVICE AND DIGITAL ATIS
The automatic terminal information service (ATIS) is
another valuable tool for gaining weather information. ATIS
is available at most airports that have an operating control
tower, which means the reports on the ATIS frequency are
only available during the regular hours of tower operation.
At some airports that operate part-time towers, ASOS/AWSS
information is broadcast over the ATIS frequency when the
tower is closed. This service is available only at those airports that have both an ASOS/AWSS on the field and an
ATIS-ASOS/AWSS interface switch installed in the tower.
Each ATIS report includes crucial information about runways and instrument approaches in use, specific outages,
and current weather conditions including visibility.
Visibility is reported in statute miles and may be omitted
if the visibility is greater than five miles. ATIS weather
information comes from a variety of sources depending
on the particular airport and the equipment installed there.
The reported weather may come from a manual weather
observer, weather instruments located in the tower, or
from automated weather stations. This information, no
matter the origin, must be from National Weather Service
approved weather sources for it to be used in the ATIS
report.
The digital ATIS (D-ATIS) is an alternative method of
receiving ATIS reports. The service provides text messages to aircraft, airlines, and other users outside the
standard reception range of conventional ATIS via
landline and data link communications to the cockpit.
Aircraft equipped with data link services are capable of
receiving ATIS information over their Aircraft
Communications Addressing and Reporting System
(ACARS) unit. This allows the pilots to read and print out
the ATIS report inside the aircraft, thereby increasing
report accuracy and decreasing pilot workload.
Also, the service provides a
computer-synthesized voice
message that can be transmitted to all aircraft within range
of existing transmitters. The
Terminal Data Link System
(TDLS) D-ATIS application
uses weather inputs from
local automated weather
sources or manually entered
meteorological data together
with preprogrammed menus
to provide standard information to users. Airports with D-
ATIS capability are listed in
the A/FD.
Figure 2-13. A/FD Entry for an AWOS Station.
Figure 2-12. ASOS Station Installation.
ings lower than 2,000 feet and/or visibility less than 3 SM. A
simple way to remember the rules for determining the necessity of filing an alternate for airplanes is the “1, 2, 3 Rule.”
For helicopter Part 91, similar alternate filing requirements
apply. An alternate must be listed on an IFR flight plan if the
forecast weather at the destination airport or heliport, from
the ETA and for one hour after the ETA, includes ceilings
lower than 1,000 feet, or less than 400 feet above the lowest
applicable approach minima, whichever is higher, and the
visibility less than 2 SM.
Not all airports can be used as alternate airports. An airport
　
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