Originally Posted by
Stanley Eevil
The conversion factors for CMV, I believe, are 1.5 for daytime operations and 2.0 for night time operations assuming full lighting/facilities etc.
Let`s assume a CAT1 ILS at dest`n with a minimum published RVR requirement of 550m.
The way I interpret the 800 m limitation is that notwithstanding our 550 m RVR requirement we can`t `play` with an RVR below 800 m when conducting the conversion back to an equivalent met visibility.
Therefore at night: 800/2 = 400 and so the min forecast visibility in the TAF at ETA +/-1 hr is 400 m in order to dispatch with just 1 dest`n alternate.
For a daytime approach 800/1.5 = 533.3, so effectively the min met vis in the TAF at ETA +/-1 hr is 550 m (although in a TAF they forecast in 100 m increments so I guess you`d be looking for a minimum forecast vis of 600 m).
Anyway, it`s purely an opinion (and probably wrong!)
Any thoughts?
I'm not really sure what you're trying to say, Stanley.
EASA:
First of all it is possible to convert visibility to CMV/RVR, I don't think you can reverse it and convert a required RVR to visibility that should be present.
The conversion factor -in case of HI approach lights/runway lights- 1,5 day and 2.0 night.
The conversion cannot be used for:
- Take off
- For calculating a required RVR less than 800 m (i.e. 550 meters for a standard ILS)
- When RVR is available.
Secondly, when the VIS drops below 800 m YOU SHOULD PROVIDED with RVR information.