This chapter presents procedures and practices for measuring and recording runway visual range (RVR). RVR is an estimate of how far a pilot can see down a runway. It is used to define operational limits on the use of precision instrument runways.

10-2.   DEFINITIONS

a.  Runway Visual Range. The RVR is an estimate of the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specific point on its center line. This value is normally determined by visibility sensors or transmissometers located alongside and higher than the center line of the runway. RVR is used operationally to assess whether visibility conditions are good enough to allow a particular operation, such as an instrument landing.

b.  Designated RVR Runway. The designated RVR runway is the runway officially designated by the airport authority for reporting RVR values. The designated RVR runway is typically the runway with the lowest approach minimums.

c.  Runway Light Intensity. Runway light intensity is a numerical scale of the brightness of runway lights.

d.  Long-Line RVR. The RVR reported in surface observations and disseminated long line is the highest RVR achievable for the measured visibility at the touchdown zone of a specified runway. Typically, this is the RVR calculated for the highest and lowest values of visibility over the previous 10 minutes at runway light intensity step five. With New Generation RVR (NGRVR), this is an automated report. When the automated interface fails, RVR will not be reported long-line.

An automated RVR system uses three sensors to estimate the RVR value: Extinction coefficient sensor (or visibility), background luminance (or ambient light sensor) and runway light intensity monitor. The transmissometer or forward scatter meter can be used as an RVR visibility sensor.

a.  Transmissometer. A transmissometer measures the fraction of light (transmittance) that has not been absorbed or scattered out of a light beam after it has traveled a certain distance through the atmosphere. The extinction coefficient is computed from transmittance. The transmissometer measures the extinction coefficient using a projector and a receiver. A projector transmits a light beam toward a receiver located some specified distance away. The receiver measures the intensity of the beam after it has passed through the atmosphere. RVR is then derived from algorithms that also account for ambient light (background luminance) and runway light intensity. RVR tables are contained in Figure 10-1, RVR Transmittance Conversion Table for Tasker 400 and Equivalent Systems with 250-Foot Baseline - Contrast Threshold 5.5 Percent, and Figure 10-2, Tasker 500 RVR Transmittance Conversion Table for 250-Foot Baseline - Contrast Threshold 5.0 Percent.

b.  Forward Scatter Meter. A forward scatter meter measures a small portion of light scattered out of a light beam into a narrow band of scattering angles. The scatter meter is used to estimate the extinction coefficient. The forward scatter meter measures the extinction coefficient using visibility sensors (VS) (transmitter and receiver). The VS receiver measures the amount of infrared light scattered into the receiving window by airborne particles. RVR is then derived from algorithms.

The RVR visibility sensor should be located within 500 feet of the runway center line and, relative to the center of the glide slope antenna, within distances of 1000 feet towards the runway threshold and 1500 feet away from the runway threshold. The midpoint RVR is placed within 1000 feet of half the distance of the runway length.

The day scale should be used in the evening until low-intensity lights on or near the airport complex are clearly visible; the night scale should be used in the morning until these lights begin to fade. Alternatively, a day-night switch may be used to determine which scale should be used.

For automated long-line RVR observing, the RVR transmits the designated runway RVR to the ASOS for long-line dissemination. The New Generation RVR (forward scatter meter) will provide automated long-line service to the ASOS. The Tasker (transmissometer) provides RVR long-line information manually.

At certain automated stations, RVR values for as many as four designated runways may be reported for long-line dissemination. At manual stations, only RVR for the designated runway shall be reported.

RVR is measured in feet whenever the prevailing visibility is 1 statute mile or less and/or the RVR for the designated instrument runway is 6000 feet or less. RVR up to 1000 feet is reported in increments of 100 feet. RVR between 1000 and 3000 feet is reported in increments of 200 feet. RVR between 3000 and 6000 feet is reported in increments of 500 feet. For RVR based on the forward scatter meter, RVR up to 800 feet is reported in increments of 100 feet; RVR between 800 and 3000 feet is reported in increments of 200 feet; RVR between 3000 and 6500 feet is reported in increments of 500 feet.

At manual stations, 10-minute extreme values (highest and lowest) of transmittance shall be read from the transmissometer strip chart. RVR shall be reported based on light setting 5 for either day or night time conditions, regardless of the light setting actually in use. One RVR value shall be reported if the 10-minute high and low value are the same.

RVR is automatically provided to the automated surface observing system (ASOS). ASOS shall calculate and report extreme RVR values.

When the observed RVR is above the maximum value that can be determined by the system in use, it should be reported as "P6000FT", where the figure 6000 is the maximum value that can be determined by the system. Similarly, when the RVR is below the minimum value that can be determined by the system in use, it should be reported as "M0600FT", where the figure 600 is the minimum value that can be determined by the system. Automated RVR exceeding its upper reporting limit shall be reported as 6500+.

When RVR varies by more than a reportable increment during the 10-minute period preceding the observation time, report the lowest reportable value and the highest reportable value in feet. The RVR format is given in paragraph 15-13, Runway Visual Range Group.

Observers at stations with the capability of measuring RVR should report RVR in the body of the METAR/SPECI whenever the prevailing visibility is 1 mile or less and/or the RVR is 6000 feet or less (6500 feet or less for automated RVR). The format is given in Chapter 15, Coding and Dissemination. The 10-minute runway visual range values for the designated RVR runway shall be included in METAR and SPECI observations. The values shall be based on runway light setting 5 and reported in the increments identified in paragraph 10-8, Units of Measure. Transmissometer-determined values shall be applicable only to the specified runway near which the instrument is located. After transmissivity values are obtained, background correction shall be applied and the appropriate figure used to determine runway visual range. Figures 10-1, RVR Transmittance Conversion Table, and 10-2, Tasker 500 RVR Transmittance Conversion Table, present RVR transmittance conversion data for two different operating conditions. In determining runway visual range, the observer shall select the appropriate time for changing from day to night values or vice versa. In general, the day scale should be used in the evening until low-intensity lights on or near the airport complex are clearly visible, and the night scale should be used in the morning until these lights begin to fade. Alternatively, a day-night switch may be used to determine which scale should be used. When reliable reports are unavailable, or the observer determines that the instrument values are not representative for the associated runway, the data shall not be used. Automated RVR values are complete as forwarded to ASOS and require no external compensation for day/night conditions.

In order to correctly determine the RVR to be reported, the observer shall have the following information:

a.  Which recorder indicates RVR values at the approach end of the designated RVR runway.

b.  The relation of RVR sensors and readouts to the runway approaches.

c.  The lowest RVR instrument minimums for the designated RVR runway.

d.  Whether the day or night tables are to be used.

Ten-minute extreme values (highest and lowest) of RVR shall be determined by selecting the highest and lowest values on the recorder chart and converting them to hundreds of feet by using the appropriate RVR table. Values based on light setting 5 shall always be used, regardless of the light setting actually in use. The 10-minute values are considered more representative for longer periods after observation and shall be used for long-line transmission.

When necessary to determine RVR values manually (digital readout inoperative or not available), the observer shall obtain readings from the transmissivity meter or the recorder trace. Because of the lag in the transmissometer recording system, these values may be considered nominal one-minute measures of atmospheric transmission. These indications shall be converted to RVR equivalents whenever the appropriate light settings and day or night condition are known. Background correction shall be applied to the observed value and the appropriate RVR table used.

When notified that RVR readouts in the traffic control facility are inoperative, but a readout (digital, recorder, or meter) in the weather station/contract observer location is operating, the weather station observer shall provide to the control facility for the runways(s) of concern:

a.  The RVR at the time of notification.

b.  Notification when the RVR is observed to decrease to equal or become less than, or to increase to equal or become more than 2400 feet RVR, or the lowest landing minimum.

The RVR provided in accordance with this paragraph shall be the one-minute mean value, based on light setting 5, unless another light setting is specially requested. If digital readouts are unavailable, the observer shall manually determine one-minute RVR using the instructions in paragraph 10-16, Manually Determined One-Minute RVR Values. All values furnished to the control facility in accordance with this paragraph shall be recorded and retained for 30 days. Telewriter copy or voice tapes may serve this purpose.

At the beginning and end of each chart roll, the observer shall enter station name, runway number, length of transmissometer baseline, time check, and date-time group. If the chart, or any part of the chart, is provided for special studies, an aircraft accident investigation, etc., the observer shall enter other identification as necessary; e.g., station name, runway, length of transmissometer baseline. The observer shall indicate maintenance shut-downs or other inoperative periods by entering time checks and date-time groups at the end of one period of operation and the beginning of the next. The observer shall enter a time check and date-time group near the trace whenever notified of an aircraft mishap. The observer shall adjust the chart to the correct time whenever the time error is 5 minutes or more. The observer shall note the time of adjustment, and enter a new time check on the chart.

When the chart on a recorder roll has been exhausted, the observer shall insert the used roll into an empty chart carton and enter on the carton the station name, dates for beginning and ending of the roll, and runway identification. The used roll shall be held on station for 30 days. If no request is received within this time for review or a copy of any portion of the roll, it may be discarded. It shall not be sent to the National Climatic Data Center (NCDC).

Practices and procedures for the operation of visibility measuring instruments and related equipment are presented in Chapter 17, Operation of Equipment.

10-21. - 10-23. RESERVED