View Full Version : CMV conundrum

15th May 2018, 03:23
Hi All, quick question on converted met visibility (CMV). I'll use the LFPG (Paris DeGaulle) runway 08L VOR approach as an example....

CMV takes credit for lighting systems that make it easier to identify the runway environment. At night with high intensity approach and runway lighting the reported met vis can be multiplied by 2 and by day, by 1.5. If there is no approach lighting system (ALS) operating, (ALS out on the chart), the multiplier is 1. That is, there is no benefit to be gained by using CMV as the reported met vis is considered to be the same as an RVR.

So, considering the LFPG 08R VOR, if ATC report a met vis of 900m and you require an RVR of 1500m for the approach, and assuming all the high intensity lights are operating, if it were daytime you would have a CMV of 1350m and if it were night you would have a CMV of 1800m.

In this instance you could not pass your approach ban point if it is day, but you can if it is night as the CMV is greater than the required RVR.

My question is this. On the (non-customised) minima panel on the Jeppesen LFPG 08R VOR approach chart, under 'ALS out' it quotes a CMV of 2300m. If the ALS is 'out' the reported vis is equivalent to an RVR (multiplier is 1), so why quote a CMV minima of 2300m when the reported met vis would be required to also be 2300m? Why not just quote vis?

Thanks in advance.

15th May 2018, 05:57
Im not familiar with this airport, however the published minima could quote a CMV value when RVR is not reported for that runway (i.e. No RVR equipment) or the published visibility is above the maximum reported value for RVR. Where I work RVR is not reported when the value exceeds 1800m.

15th May 2018, 16:37
Does your Jepp chart say minima box say "STANDARD"?--
why is there no 08R VOR IAP chart in my database ...

20th May 2018, 06:15
Hi Rumbear, CMV is only used when RVR is not available. RVR is 'generally' not quoted above 2000m.

FlightDetent, yes the chart is JAR OPS (it is labelled STANDARD).

20th May 2018, 07:08
Yes I agree. However when RVR is above 2000 m and not report it is still not available. I commonly see CMV displayed on approach minima where RVR equipment is not installed ( depends on the authority you fly in )

Empty Cruise
20th May 2018, 11:58
Visibility is different to CMV - it's generally used to obtain an 'RVR equivalent' when outside RVR range. The Aerodrome Meteorological Observation and Forecast Study Group (AMOFSG) 10th meeting, Agenda Item 5: Aerodrome observations: Inconsistency Between Visibility and CMV, a Converted Meteorological Visibility:

"Straight-in approach minima are often expressed in terms of RVR. Following widely used procedures, many airline operators convert the reported meteorological visibility into an equivalent RVR value, called CMV. This conversion is applied by the pilot only for landing, when the required RVR minimum is equal or above 800 m (1/2 sm) and when the RVR is not available (a reported RVR above its 2000 m limit is considered as being not available)."

Visibility is only used for those approaches that are not straight-in, ie offset more than 4˚ for Cat C/D, classed as side-step or circling. The credit for light sources differs - basically, in visibility reporting, only limited account is taken for looking at a light-source - very different from looking at a battery of approach lights.

20th May 2018, 23:36
flyonthewall: post the chart, as an example. If it is a mis-print or a logical error, we'll help you hunt it down. :)

EU-OPS is dead.
You're from Australia, making a query about a non-existent approach in Paris. And then I checked your occupation in the personal profile - what do I make of all that? :O