Originally Posted by
FlightDetent
Although it is a full circle from the OP's question, here's a familiar sample case. EASA rules.
Your destination WS is ok, however company preferred
alternate is not looking great. TAF LIMC (MXP) V2kt 2000 BR NSC 3/1 Q1003 TEMPO 14-18 0600 FG BECMG 20-24 220/5 6000
A well equipped major hub with dual runways, CATIIIB and given the forecast it still is a respectable choice. BTW destination is CAVOK, and a non-preferred alternate is 8000 m VIS half way between DEST and LIMC.
Alternate planning minima is one grade more restrictive than what's available, i.e. for Milano CAT I ILS R550 / VIS 800.
As much as we agreed the paragraphs we're looking at do not allow conversion of TAF 600 VIS to 900 CMV, because the target minimum is R550 (less than 800) ...
... when you ran the above scenario through the flight planning system, the answer is ACCEPTABLE.
Where's the catch?
Hi FD,
in your example, the forecasted visibility is greater than the required RVR for CAT I, hence LIDO (example of what we use in my company) will accept it and will not mark it as "U".
Regarding the conversion, you can convert 600 m of visibility. What the rule says is that you can't convert in order to find RVRs lower than 800 m. If I have a general visibility of 400 m and a conversion factor of 2.0 I will find a CMV of 800m, I'm ok. On the other hand, if I have a conversion factor of 1.5 with the same visibility (daytime, HIRLS) I will find 600 m, less than 800 m , so not allowed.
Let me know your thoughts.