Revision for “7. Accuracy of RVR reports” created on November 19, 2013 @ 13:18:54
7. Accuracy of RVR reports
<strong>Accuracy of RVR reports</strong>
As already indicated previously, an RVR report is not more than an ’assessment’ what a pilot would see form the position on a particular runway for which the RVR is issued.
<strong>a.</strong> Fog is seldom uniform. Transmissometer(s) may be located in areas of good/bad visibility compared with general visibility.
<strong>b.</strong> Locations of observation is, in nearly all cases not on the runway (see also <a href="http://flightcrewguide.com/wiki/meteorology/visibility/4-observing-rvr/" title="4. Observing of RVR">subpara 4.-c.</a>).
<strong>c.</strong> Observations are seldom made at the eye-level of the pilot who uses the RVR report. Since most runway edge lights are inclined a few degrees above the horizontal the pilot’s eyes are, depending on circumstances, close to or away from the axis of the beam of the lights and therefore visual range respectively increases or decreases.
<strong>d.</strong> Components used in the theoretical conversion of transmissivity of the air into RVR may change in actual practice (e.g. runway edge lights used by the pilot may need cleaning).
<strong>e.</strong> Changes in visual range with a pilot’s eye-level can be caused by variations in atmospheric transparency with height.
<strong>f.</strong> Processing of RVRs involves averaging. Transmissometer readings are averaged over a period not less than 30 seconds and not more than one minute.
<strong>g.</strong> Reporting of RVRs is in increments (see also <a href="http://flightcrewguide.com/wiki/meteorology/visibility/6-reporting-procedures-rvr-reports/" title="6. Reporting procedures of RVR reports">subpara 6.-a.</a>).
<strong>h.</strong> RVR sometimes fluctuates rapidly, occasionally changing by several hundred metres in less than one minute. This causes difficulties to pilots landing, although some of the fluctuations may be of little significance, for example, those experienced when passing through small patches of fog.
<strong>i.</strong> In all cases where the RVR observing/reporting procedures are not automated a delay in reporting to aircraft concerned is unavoidable. (See also <a href="http://flightcrewguide.com/wiki/meteorology/visibility/5-availability-rvr-observations-ats-units/" title="5. Availability of RVR observations at ATS units">subpara 5</a>).