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 18th Apr 2007, 21:26 #1 (permalink) Join Date: Jan 2000 Posts: 1,030 RVR & Visibility JAR OPS has a table giving a conversion factor for converting RVR to visibility. Generally speaking, the factors indicate that RVR will be equal to or greater than the associated visibility - I would have thought it was the other way round, but I'm hardly an expert. Any one got an explanation or view on this?
18th Apr 2007, 23:40   #2 (permalink)

Join Date: Jan 2004
Location: UK
Posts: 101
Quote:
 Any one got an explanation or view on this?
I'll try.............

Met Vis is the LOWEST vis observered in all directions. RVR is the vis observed only in the direction of the runway (at the average pilot's eye ht.....blah)

Some examples:

(1)I am on the threshold of R/W 27 looking west. Down the R/W I can see for 2000m, to the north I can see for 4000m , to the south I can see for 4000m and to the east I can see for 4000m. Now, here the met vis would be 2000m as would the RVR be.

(2) I am, again, stood on the threshold of R/W 27 looking west. Down the R/W I can see for 4000m, to the north I can see for 2000m, to the south I can see for 2000m and to the east I can see for 2000m. In this case the met vis would be 2000m; however, the RVR would be 4000m.

Hopefully you can see from these examples that if the direction of worst vis is in the same direction as the R/W RVR will equal met vis; however, the RVR can never be less than the met vis.

No doubt somebody will get all technical about the exact definitions of met vis and RVR, but hopefully that helps?

 19th Apr 2007, 15:18 #3 (permalink) Join Date: Jan 2007 Location: Czech Republic Age: 31 Posts: 3 RVR - Runway Visual Range - the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity Visibility - The greatest distance under given weather conditions to which it is possible to see without instrumental assistance. The important is: "RVR is calculated from....runway light intensity" So typically High Intensity Runway or Approach Lights are more visible than unlit objects and thats why RVR is greater than visibility. David
 19th Apr 2007, 16:27 #4 (permalink) Spitoon Guest   Posts: n/a Is this the bit in JAR-OPS about calculating an equivalent RVR from the met vis if RVR is not available? If so, I would suggest that there is little, if any, real relationship between the two values. It is simply a way of controlling whether an approach can be made or continued in poor visibility. As such, the conversion is is just an approximation rather than an attempt to relate the two (different) measuring systems in a real-world situation. BTW, met vis is now reported in terms of prevailing visibility, at least that is the ICAO standard. The exact rules escape me for the moment but it means that the vis in a met report may not be the lowest in any direction. PS - Just my understanding as a simple controller. Last edited by Spitoon; 19th Apr 2007 at 16:38.
 19th Apr 2007, 17:44 #5 (permalink) Join Date: Aug 2000 Location: UK Posts: 3,495 During the day visibility is judged using an unlit object. At night, the benchmark (according to wikipedia at least -- can't find a reference) is a 1,000 candela light. RVR uses the runways lights as they really exist, so high intensity lights may be 15,000 candela, and can be seen from much further away. Hence the higher factors for higher intensity lights.
 19th Apr 2007, 20:34 #6 (permalink) Join Date: Jan 2000 Posts: 1,030 Thank you, gentlemen - it sort of makes sense now .....
 12th Dec 2007, 10:46 #7 (permalink) Join Date: Nov 2004 Location: Europe Posts: 39 Another question regarding RVR and Visibility: May I convert the reported visibility (e.g in a METAR or TAF) to a RVR for alternate preplanning or only in-flight, when there is no RVR reported? Thanks in advance!
 12th Dec 2007, 13:04 #8 (permalink) Join Date: Apr 2001 Location: Brussels, Belgium Age: 64 Posts: 66 ICAO references... Not so simple : Visibility. Visibility for aeronautical purposes is the greater of: a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background; b) the greatest distance at which lights in the vicinity of 1 000 candelas can be seen and identified against an unlit background. Note.— The two distances have different values in air of a given extinction coefficient, and the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range (MOR). Runway visual range (RVR). 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 or identifying its centre line. 4.3.1.1 Recommendation.— Runway visual range should be assessed at a height of approximately 2.5 m (7.5 ft) above the runway. 4.3.1.2 Recommendation.— Runway visual range should be assessed at a lateral distance from the runway centre line of not more than 120 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 stop-end of the runway should be located at a distance of 1 000 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, if necessary, additional sites should be decided after considering aeronautical, meteorological and climatological factors such as long runways, swamps and other fog-prone areas. 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 runway visual range displays in air traffic services units; and b) 10 minutes for METAR and SPECI, except that when the 10-minute period immediately preceding the observation includes a marked discontinuity in runway visual range values, only those values occurring after the discontinuity shall be used for obtaining mean values. 4.3.6 Reporting 4.3.6.1 In local routine and special reports and in METAR and SPECI, the runway visual range shall be reported in steps of 25 m when the runway visual range is less than 400 m; in steps of 50 m when it is between 400 m and 800 m; and in steps of 100 m when the runway visual range is more than 800 m. Any observed value which does not fit the reporting scale in use shall be rounded down to the nearest lower step in the scale. 4.3.6.2 Recommendation.— Fifty metres should be considered the lower limit and 2 000 metres the upper limit for runway visual range. Outside of these limits, local routine and special reports and METAR and SPECI should merely indicate that the runway visual range is less than 50 m or more than 2 000 m. -ICAO Annex 3 to the Convention on International Civil Aviation Meteorological Service for International Air Navigation -Manual of Runway Visual Range Observing and Reporting Practices (Doc 9328)
 12th Dec 2007, 13:09 #9 (permalink) ECON cruise, LR cruise...   Join Date: Apr 2002 Location: MIRSI hold - give or take... Age: 41 Posts: 516 No - a close look in your OM part A ch. 8 should produce words to the effect that "...the observed visibility may be converted...". The only time you can use converted visibility is: a) No RVR measurement available and either b) CAT I ILS, or c) Straight-in NPA That means that you cannot convert during the planning stage, to detemine if RVR required for TO exists, during CAT II/III ops and (for obvious reasons) for circling approaches. Brgds, Empty

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