INSTRUMENT PROCEDURES HANDBOOK
 

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

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 operatorís 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.

 
 
HOME  |   LATEST NEWS  |     |   -  |   BOOKS YOU CAN READ ONLINE  |   EDITORIALS  |   AVIATION HISTORY