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2. Visual Range

Wikis > Meteorology > Visibility > 2. Visual Range
Visual Range

The purpose of providing runway visual range (RVR) information is to permit pilots to appraise aerodrome visibility conditions and in particular to determine whether these conditions are above or below established aerodrome operating minima.

RVR is defined as the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway of identifying its centre line.

For this purpose, a height of approximately 5 metres is regarded as corresponding to the average eye-level of a pilot in an aircraft on the centre line of a runway.

In practice, RVR cannot be measured directly from the position specified in the definition but is an assessment of what a pilot would see from that position.

One should realize, however, that in assessing RVR no account is taken of the effect on vision of such variables and imponderables as:

    – rain on the windshield of the aircraft;
    – the level of cockpit lighting, which is adjustable;
    – the illumination to which a pilot has been exposed during the preceding few minutes (for example,
    when passing over approach lights);
    – any effect connected with the motion of a pilot with respect to the runway lights, e.g. the time taken for
    a pilot to react to a light coming into view;
    – the pilot’s vision and any physical or psychological factors affecting it.

Thus, RVR is merely a method of assessing ’seeing conditions’ for take-off and landing and is not a statement of what a pilot would actually see.

There is no doubt that, for landing, an assessment of the visual guidance available to the pilot during final approach, i.e. ’slant visual range’ (SVR) would be even more useful than RVR. No practical system for assessing SVR has yet been developed, although several experimental SVR systems have been developed and are being tested. It is felt that progress might be made with the development of an observing system, if a specific reference point in space could be defined to which the SVR could be related. Such a value should relate to a particular height, on a particular approach slope and with a visual segment intercept on the ground of a certain length.

The longer term objective is to meet in full the complete aeronautical requirement, which includes the two basic concepts:

    – the height at which ground cues become evident, i.e. probable contact height; and
    – the degree to which this reference is maintained as the descent is continued.

ICAO and WMO are encouraging States to pursue efforts both to satisfy the long term ends, and to provide a more immediate limited solution.

Therefore, RVR is often used, not only for its primary purpose of indicating visual guidance along the runway but also for the secondary purpose of giving some indication of ’seeing conditions’ on final approach.

RVR information is issued for one or more observation sites along a runway but although ideally desirable, it is impracticable to make the observation from the pilot’s position over the runway. The various techniques generally approach as closely the ideal as is possible without risk either to aircraft or to human observers. The observations are used to assess values that are satisfactorily representative of what a pilot would see at the appropriate part of the runway.

Source: FCRG

See also:
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

 
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