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.
For the following reasons inaccuracies in the reports are unavoidable:
a. Fog is seldom uniform. Transmissometer(s) may be located in areas of good/bad visibility compared with general visibility.
b. Locations of observation is, in nearly all cases not on the runway (see also subpara 4.-c.).
c. 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.
d. 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).
e. Changes in visual range with a pilot’s eye-level can be caused by variations in atmospheric transparency with height.
f. Processing of RVRs involves averaging. Transmissometer readings are averaged over a period not less than 30 seconds and not more than one minute.
g. Reporting of RVRs is in increments (see also subpara 6.-a.).
h. 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.
Rapid changes in visual range also raise difficulties for ATS units when passing information to aircraft. Some smoothing of observations, by averaging over a certain period or otherwise, is therefore desirable (see also subpara 7.-f.).
i. In all cases where the RVR observing/reporting procedures are not automated a delay in reporting to aircraft concerned is unavoidable. (See also subpara 5).
1. Horizontal visibility aspects
2. Visual range
3. Observing techniques of RVR
4. Observing of RVR
5. Availability of RVR observations at ATS units
6. Reporting procedures of RVR reports
7. Accuracy of RVR reports
8. Slant visual range
9. The visual segment
10. Variation in visual segment
11. Flight visibility and vision at high altitudes