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To my humble aviation knowledge, RVR is derived from RVR measuring equipment, and the CMV can be converted from a reported visibility.
Converted Meteorological Visibility (CMV). A value (equivalent to an RVR) which is derived from the reported meteoro- logical visibility, as converted in accordance with the requirements in this subpart.
But what I am also trying to find out for a while is, why is CMV only used in conjunction with CAT C & D aircraft?
Yea you're right, I was also confused reading my post but your reply addressed the issue I was trying to nail.
The thing is, you say "if you are not given a RVR, use CMV for planning purposes.", but no where in JAR OPS does it say this can be done for planning. If I'm wrong please point me in the right direction.
JAR OPS 1.430 sub paragraph L and table 11 is as close as I can get. Notice CMV is based on REPORTED met visibility and not forecasted. It seems to me met Vis would have to be directly translated to RVR to make a flight legal. 600vis = 600rvr.
(l) Conversion of reported meteorological visibility to RVR/CMV.
1. An operator must ensure that a meteorological visibility to RVR/CMV conversion is not used for takeoff, for cal-culating any other required RVR minimum less than 800 m, or when reported RVR is available.
Note: If the RVR is reported as being above the maximum value assessed by the aerodrome operator, e.g. “RVR more than 1 500 metres”, it is not considered to be a reported value for the purpose of this paragraph.
2. When converting meteorological visibility to RVR in all other circumstances than those in subparagraph (l)1. above, an operator must ensure that the following Table is used:
Conversion of met visibility to RVR/CMV
Lighting elements in operation RVR/CMV = Reported met. Visibility ×
HI approach and runway lighting 1,5 2,0
Any type of lighting installation other than above 1,0 1,5
I think your point 2 confirms this with regarding planning purposes, to use CMV when no RVR is presented or available.
When no RVR is reported, you will use CMV to know what this number would be in RVR.
The table tells you what conditions apply for RVR vs CMV, and must be used for planning purpose.
All other circumstances besides take off, this means your approach etc.
Normally you will need to concentrate about Visual, Non Precision and Cat 1, as your normal approaches.
If you during flight get Visibility given, you will need to convert it to RVR to know if you are above your minima.
Example single pilot Cat 1 approach requires 800m RVR, if you recieve a met report in your TAF where your visibility is 600m, then you need to use the table to see if you will be legal. Normally RVR will be given in METAR and ATIS, however you might be interested to know how it is going to be, as the TAF is a forecast, and the METAR is actual conditions, which might not require and RVR report at current time.
If your TV says that during your approach time, visibility will be 600m, you need to use this for your planning, and use the CMV to get your RVR calculations correct. If your approach has HI Int. light in night, you will multiplie the 600 with 2 and have an RVR of 1200m, however if you doing approach at daylight, and no Hi Int. approach lights available you can only use the factor 1, so your RVR will be 600 m, so you will not be legal to start the approach after FAP.
Now if in Daylight with Hi Int. app light, factor will be 1.5, this means 900m RVR, so you could legally plan for that approach.
Note however that a METAR with a TREND in the end, will take priority ahead of any forecast in the TAF, in that first 2 hour period which the TREND is valid for.
You have to think a little of the situations when you would need to use the conversion, the table is given in JAR OPS, so that you know how to convert any Visibility distances given to useful RVR measures. If you was not going to use it, it would be useless for them to put the table in the Ops. Also it specifies when to NOT use it, so besides this you can use it all the time, unless you have RVR, but you will most likely never have the RVR during planning part of flight (as only in METAR and ATIS)
An Operator should ensure that a meteorological visibility to RVR conversion is not used for takeoff, for calculating any other required RVR minimum less than 800 m, for visual/circling approaches, or when reported RVR is available.
When converting meteorological visibility to RVR in all other circumstances than those in sub-paragraph above, an operator should ensure that Table below is used:
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
Assume for the moment that you want to land somewhere and there is no RVR available. You only have the visibility of 275M at night with HI app lights.
Your minimums are 550m 300M mid-end and 300m stop-end.
Are you allowed to convert the visibility to 550RVR ? 275x2
So for take off you can't use CMV if the visibility is less than 800m but what about if you are already in the air?