It looks like tomorrow morning there will also be some fog around Kassel-Calden EDVK. For that reason I’m thinking about what my options are going to be.
I operate IFR under FAA part 91. Takeoff is at a German airport with full ATC.
The METAR of today’s morning has partial fog with an RVR between 350m and 500m. From where I am located as I write this I can see the fog in the lower parts of the terrain around here. Other than that we have blue sky :-)
As this is a mixed case of FAA/EASA rules, I think it makes for an interesting question.
Otherwise there is the old Finnair method:
ATC: “Confirm you have 4 runway lights” (implying 200m visbility)
Finnair: “Affirm”
after take off
Finnair: “two on each side”…..
(Story from an airtrafic controller somewhere in scandinavia) 
If it’s a cloud layer and blues sky above, I would depart in the Cirrus if they let you. With the Cirrus the risk is almost the same with or without fog … While I have a pretty high minimum for approaches, a low viz t.o. in the Cirrus (!) is easy and pretty safe if you know what you’re doing (IMHO)
A year or two ago I read somewhere a very very long analysis of the rules which concluded that 150m is the lowest for departures if you have the full IR.
1800m for the UK IMC Rating (the “IR(R)”). Bizzarely, after April 2012, you can depart “VFR” down to 1500m 
Until a few years ago, an N-reg could depart in zero-zero (it can in the USA, Part 91).
For arrivals, it is something like 550m if the airport has HIAL, otherwise about 800m. This is all official RVR; if there is no RVR (where the first “R” means “runway” ) reported then it is less clear.
References are almost impossible to find nowadays because the regs are in multiple places and there are amendments all over the place.
Isn’t there the CMV involved, calculated from the met visibility ? With some coefficient to apply, depending on type of lighting and night or day ?
I’d say the biggest risk is a runway incursion, like with wild animals, but at a proper airport (with a fence) that risk is minimal.
Not too long ago i did such a t.o. to get the plane home from a shop. It was obvious that the cloud layer was 500 ft thick and that my home field, 30 miles away, was in the sun. But since the departure was over the Danube river, which is almost 400 meters wide there I made sure to do a turn away from the river after take-off. Landing in the middle of the river in dense fog after an EFATO, even with the parachute would have been just too stupid ;-)
For arrivals, it is something like 550m if the airport has HIAL, otherwise about 800m. This is all official RVR
I believe that for SINGLE PILOT with an EASA IR in addition to the appropriate lighting you also need an approved coupled autopilot to go
down to 550m RVR, otherwise it’s 800m. Multi Crew have different rules which I don’t know.
550m is tricky, I did a few in the simulator recently, and on one I went around because I didn’t see anything; the instructor said the lights were there but I looked in the wrong place, i.e. too far ahead. They were almost under the nose. We also did 150m RVR takeoffs in the sim, The big question is what to do if there is a problem, return is likely to be impossible.
Flyer59 wrote:
With the Cirrus the risk is almost the same with or without fog …
Doesn’t make sense to me up to 500ft AGL, where you can make use of CAPS.
Yes, of course, 400-500 ft you have to reach, of course. But really, if I wasn’t sure the plane can’t make it to 500 ft … I wouldn’t fly at all.
How can you be sure??
