Originally Posted by
Don Coyote
You may be considering when you have a legal TAF but a METAR below minimums as you are very unlikely to get RVR in a TAF. For planning purposes, you would use the TAF to determine dispatch requirements.
For EASA land CAT operations, the following would be applicable:
The table in GM1 CAT.OP.MPA.185 has useful guidance for applying TAF criteria to determine suitability of weather from a TAF. Once you have determined that the forecast is suitable (or not) then CAT.OP.MPA.185 would also require alternate weather of better than NPA RVR/VIS with ceiling at or above MDH for the most applicable runway as only CAT I is available at destination. Provided those conditions are met then you could dispatch with one alternate.
If the destination forecast is below limits, as per yunje’s example of RKSI/ICN TAF of 600m visibility (800m visibility required), then you would require two alternates with suitable weather forecasts as per CAT.OP.MPA.185.
Once you arrive at the destination then CAT.OP.MPA.305 becomes applicable. This requires that you cannot proceed below 1000 ft above the aerodrome if the reported RVR/VIS is less than the applicable minimum.
The conversion of reported met vis to RVR, detailed in AMC10 CAT.OP.MPA.110, would not be applicable at the planning stage.
It’s not unlikely, it’s impossible to have RVR in a TAF as it is an actual measurement.
AMC10 CAT.OP.MPA.110 does not state that the CMV is not applicable at planning stage.