Originally Posted by
Smokey Lomcevak
I can’t add any more to the regulatory side of things, but is it not the case that if we assume a uniform distribution of cloud/mist/fog giving a met vis of 600m, we could reasonably expect an RVR measurement to give more than than 600m?
Of course it doesn’t always work like that in the real world - especially in the ranges of visibility we’re talking about here. It’s not unforeseeable that a rogue fog bank could envelope the TDZ RVR on such a day. But the chances of that happening on a day when one didn’t have the ability to carry extra fuel for another div and or holding without - say - offloading freight are pretty low, and one would still have their original alternate which must have met the more stringent requirements - from an EASA OPS pint of view. In that case - if you gotta go (to the alternate) then you gotta go...
As
'safetypee' and
'ShyTorque' said,
Low VIS is a highly unpredictable situation.
In this case,
We have some regulation for Weather forecast destination airport
* When Below the weather minimum or weather information is not available.
1. More than 2 hours of flight time :
NO DISPATCH
2. When flight time is less than 2 hours, If Current weather at the destination airport is above weather minimum :
DISPATCH
So If METAR or AMOS's RVR is above WX minimum, We can dispatch
less than 2 hours flight regardless of TAF
But More than 2 hours flight, I don't know which one to apply
VIS 800m or RVR 550m.
Because No RVR is forecast for TAF.