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4.3 Runway visual range
4.2.4 Reporting
4.2.4.1 In local routine and special reports and METAR
and SPECI, the visibility shall be reported in steps of 50 m
when the visibility is less than 800 m; in steps of 100 m, when
it is 800 m or more but less than 5 km, in kilometre steps,
when the visibility is 5 lan or more but less than 10 lan, and
it shall be given as 10 lan when the visibility is 10 lan or more,
except when the conditions for the use of CAVOK apply. Any
observed value which does not fit the reporting scale in use
shall be rounded down to the nearest lower step in the scale.
Note.- Specifications concerning the use of CAVOK are
given in 2.2.
4.2.4.2 In local routine and special reports, visibility
along the runway(s) shall be reported together with the units of
measurement.
4.2.4.3 Recommendation.- In local routine and special
reports, when instrumented systems are used for the measurement
of visibility:
4.3.1 Siting
4.3.1.1 Recommendation.- Runway visual range
should be assessed at a height of approximately 2.5 m (7.5ft)
above the runway.
4.3.1.2 Recommendation.- Runway visual range
should be assessed at a lateral distance fhm the runway
centre line of not more than I20 m. The site for observations
to be representative of the touchdown zone should be located
about 300 m along the runway from the threshold. The sites for
observations to be representative of the mid-point and stopend
of the runway should be located at a distance of I 000 to
1 500 m along the runway from the threshold and at a distance
of about 300 m from the other end of the runway. The exact
position of these sites and, ifnecessary, additional sites should
be decided after considering aeronautical, meteorological and
climatological factors such as long runways, swamps and
other fog-prone areas.
4.3.2 Instrumented systems
a) ifthe visibility is observed from more than one location
along the runway as specified in Chapter 4, 4.6.2.2, the Note.- Since accuracy can vary fhm one instrument
values representative of the touchdown zone should be design to another; pet$ormance characteristics are to be
reported first, followed, as necessary, by the values checked before selecting an instrument for assessing RVR. The
representative of the mid-point and stop-end of the run- calibration of a forward-scatter meter has to be traceable and
way, and the locations for which these values are verifiable to a transmissometer standard, the accuracy of
representative should be indicated; and which has been verified over the intended operational range.
APP 3-5 25/11/04
Annex 3 - Meteorological Service for International Air Navigation Appendix 3
Guidance on the use of transmissometers and forward-scatter
meters in instrumented RVR systems is given in the Manual of
Runway Visual Range Observing and Reporting Practices
(Doc 9328).
4.3.2.1 Instrumented systems based on transmissometers
or forward-scatter meters shall be used to assess runway visual
range on runways intended for Category I1 and I11 instrument
approach and landing operations.
4.3.2.2 Recommendation.- Instrumented systems based
on transmissometers or forward-scatter meters should be used
to assess runway visual range on runways intended for
Category I instrument approach and landing operations.
4.3.3 Display
4.3.3.1 Where runway visual range is determined by
instrumented systems, one display or more if required, shall be
located in the meteorological station with corresponding displays,
in the appropriate air traffic services units. The displays
in the meteorological station and in the air traffic services
units shall be related to the same sensors, and where separate
sensors are required as specified in 4.3.1.2, the displays shall
be clearly marked to identify the runway and section of runway
monitored by each sensor.
4.3.3.2 Recommendation.- Where runway visual range
is determined by human observers, runway visual range
should be reported to the appropriate local air trafic services
units, whenever there is a change in the value to be reported
in accordance with the reporting scale (except where the provisions
of 3.2.2 a) or b) apply). n e transmission of such
reports should normally be completed within I5 seconak after
the termination of the observation.
4.3.4 Averaging
Where instrumented systems are used for the assessment of
runway visual range, their output shall be updated at least
every 60 seconds to permit the provision of current, representative
values. The' averaging period for runway visual range
values shall be:
.a) 1 minute for local routine and special reports and for
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