Some refer to RVR but this is measured by 3 transmissometers alongside the runway (touchdown, midpoint and stop end). RVR's are not reported when the vis is above 1,500m so if the tower are reporting all RVR's below 1500m then the IMCR is subject to an approach ban and can't descend below 1,000' above the airfield/past the FAF etc.
FrightLevel
For a prosecution to be successful there has to be an accurate way of determining the visibility. ie with a IMCR you are illegal at 1799 metres but legal at 1800 metres?
An estimated visibility without the pilot admitting the infringement would be like a policeman estimating your speed as 65 mph in a 50 mph zone. Unless you admitted to driving at 65mph the prosecution would be hard pushed to make a successful prosecution against you.
But if you went through a speed trap at 65 mph they have evidence of your speed to take to court.
RVR is simular to the speed trap as the CAA would have evidence to take to court.
Cloudbase is not evidence as even with a solid overcast no one could prove that a hole didnt open giving you the lights and runway numbers.
If an IMCR pilot took an approach and landed and then had his licence checked with a runway RVR of 1000 metres then the prosecution would have a good case against the IMCR pilot.
You are correct on RVR variations on each end of the runway. It is the RVR on the touchdown point which is relevant.
I have made approaches where the RVR was below minima on one end but above on the other end. ATC will normally offer you the above minima end approach even if there is a slight tailwind on that end.
Landing RVR maybe 550 metres but takeoff RVR minima could be 200 metres allowing takeoffs but not landings.
As Fuji stated the pilot who landed on the 100 metres sticking out of the fog could have argued the fact that he landed in VFR and then used the takeoff /taxi minima for the roll out. He could have questioned the RVR equiptment as the readings were obviosly not correct on the touchdown.
I believe he was advised to admit guilt and took the £2000 fine
Pace