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majority of the sky is reported. When critical differences
exist in various sectors of the sky and the prevailing visibility is less than three miles, these differences will be
reported at manned stations. Typically, this is referred to
as sector visibility in the remarks section of a METAR
report. Prevailing visibility is reported in statute miles or
fractions of miles.
TOWER VISIBILITY
Tower visibility is the prevailing visibility as determined from the air traffic control tower (ATCT). If
visibility is determined from only one point on the
airport and it is the tower, then it is considered the
usual point of observation. Otherwise, when the visibility is measured from multiple points, the control
tower observation is referred to as the tower visibility.
It too is measured in statute miles or fractions of
miles.
ADEQUATE VISUAL REFERENCE
Another set of lower-than-standard takeoff minimums
is available to Part 121 and 135 operations as outlined
in their respective OpsSpecs document. When certain
types of visibility reports are unavailable or specific
equipment is out of service, the flight can still depart
the airport if the pilot can maintain adequate visual
reference. An appropriate visual aid must be available
to ensure the takeoff surface can be continuously identified and directional control can be maintained
throughout the takeoff run. Appropriate visual aids
include high intensity runway lights, runway centerline
lights, runway centerline markings, or other runway
lighting and markings. A visibility of 1600 feet RVR or
1/4 SM is below standard and may be considered adequate for specific commercial operators if contained in
an OpsSpecs approval.
AUTOMATED WEATHER SYSTEM
An automated weather system consists of any of the
automated weather sensor platforms that collect weather
data at airports and disseminate the weather information
via radio and/or landline. The systems consist of the
Automated Surface Observing System
(ASOS)/Automated Weather Sensor System (AWSS),
and the Automated Weather Observation System (AWOS).
These systems are installed and maintained at airports
across the United States (U.S.) by both government (FAA
and NWS) and private entities. They are relatively inexpensive to operate because they require no outside observer,
and they provide invaluable weather information for airports without operating control towers. [Figure 2-12 on
page 2-10]
AWOS and ASOS/AWSS offer a wide variety of capabilities and progressively broader weather reports. Automated
systems typically transmit weather every one to two minutes
RVR Visibility
(FT) (SM)
1,600 . . . . . . . . . . . . 1/4
2,400 . . . . . . . . . . . . 1/2
3,200 . . . . . . . . . . . . 5/8
4,000 . . . . . . . . . . . . 3/4
4,500 . . . . . . . . . . . . 7/8
5,000 . . . . . . . . . . . . 1
6,000 . . . . . . . . . . . . 1 1/4
Conversion
Figure 2-11. RVR Conversion Table.
2-10
so the most up-to-date weather information is constantly
broadcast. Basic AWOS includes only altimeter setting, wind
speed, wind direction, temperature, and dew point information. More advanced systems such as the ASOS/AWSS and
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
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Instrument Procedures Handbook (IPH)仪表程序手册上(46)
