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near the runway measure visibility for the RVR report.
If multiple transmissometers are installed, they provide
reports for multiple locations, including touchdown
RVR, mid-RVR, and rollout RVR. RVR visibility may
be reported as RVR 5-5-5. This directly relates to the multiple locations from which RVR is reported and indicates
500 feet visibility at touchdown RVR, 500 feet at mid-
RVR, and 500 feet at the rollout RVR stations.
RVR is the primary visibility measurement used by Part
121 and 135 operators, with specific visibility reports
and controlling values outlined in their respective
OpsSpecs. Under their OpsSpecs agreements, the operator must have specific, current RVR reports, if available,
to proceed with an instrument departure. OpsSpecs also
outline which visibility report is controlling in various
departure scenarios.
RUNWAY VISIBILITY VALUE
Runway visibility value (RVV) is the distance down
the runway that a pilot can see unlighted objects. It is
reported in statute miles for individual runways. RVV,
like RVR, is derived from a transmissometer for a particular runway. RVV is used in lieu of prevailing visibility in determining specific runway minimums.
PREVAILING VISIBILITY
Prevailing visibility is the horizontal distance over
which objects or bright lights can be seen and identified
over at least half of the horizon circle. If the prevailing
visibility varies from area to area, the visibility of the
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
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Instrument Procedures Handbook (IPH)仪表程序手册下(41)
