scope:

GENERAL

RVR, as defined in 2.1, is the range over which a pilot can see runway lights or runway surface markings. Assessment of RVR is by calculation, based on Koschmieder's law (in the case of objects or markings) or Allard's law (in the case of lights), taking into account the prevailing atmospheric conditions. Assessment of RVR by calculation should be made separately for each available runway in accordance with Annex 3, Appendix 3, 4.3.5.

The theoretical aspects of the visual range of objects and runway markings are discussed in Appendix B and summarized in Section 6.3. The theoretical background of the visual range of lights together with the basic relationships between the variables on which RVR depends are considered in Appendix A and summarized in Section 6.4. The following sections present the practical calculation processes involved in the assessment of RVR based on objects and lights.

In assessing RVR no account is taken of the effect on the pilot's vision of such factors as:

a) the transmittance of the windscreen of the aircraft (this aspect is discussed in Appendix C);

b) rain on the windscreen;

c) the level of cockpit lighting;

d) the illumination to which the pilot has been exposed prior to take-off or landing such as apron floodlighting, very bright fog and flying over bright approach lights;

e) physical and psychological conditions, e.g. tiredness or stress;

f) directionality of background luminance (may be reduced by the use of multiple background luminance sensors); and

g) increase in background luminance from backscatter of aircraft landing lights (especially significant in snow).

Ideally, the reported RVR value should accurately represent what the pilot will experience on landing or take-off. This requirement is implied in the statement of desirable and attainable RVR accuracies specified in Annex 3, Attachment A, which indicates that both negative and positive RVR errors are equal. However, due to a desire to prevent non-conservative RVR values (i.e. those higher than actual), RVR systems are intentionally biased in a conservative direction. This results in an inherent under-reporting of RVR. Ways in which States bias their respective systems are listed below:

a) most round down the estimated value to the nearest lower step in the reporting scale, as recommended by Annex 3, Appendix 3, 4.3.6.1;

b) all derate the runway light intensity to account for possible aging and contamination of lamps (see Section 6.4); and

c) at least one State applies a lag in the reported RVR value, dropping the reported value as soon as a lower value is indicated, but requiring an increase of 1.5 increments before increasing the reported value.

Care must be taken in applying multiple biases. If the RVR values are biased too far below the actual values, runway use may be unnecessarily curtailed under conditions where normal operations can be carried out without problem.