Specific example.
We returned to BHX last year, at night. Now the location of their IRVR detectors is awful - just to the right of 15, in a dip, first to catch the fog.
We saw the airfield from 20 miles out, they were giving RVR's below Cat 1, rwy 15 in use, centre line lights out due work in progress.
We are Cat 3 but now can't conduct Cat 3 landing because of no centre line lights.
Decide to continue until the 4 mile point (our approach limit for RVR) then enter the hold if below Cat 1.
It was, we held. Other operators were making autolands, their ops manuals may have been different.
ATC asked us to go up a thousand to let someone take off from 33, our cunning plan was hatched ! 33 was localiser only, 900m RVR from memory was needed, we asked for immediate vectors for 33 as we knew the RVR would improve. It did but was dropping fast as we approached 4 miles, we were above Cat 1 RVR at 4 miles ( never lost sight of the runway throughout), then it dropped below, he asked for our intentions, we said continue.
The controller then came out with a whole long statement about being below state minima and there were no known obstructions to prevent our landing - we landed.
We queried the statement as we taxied in and he said it was for the tape recorder and was to protect the controller in the event of an incident.
To conclude - we landed when the RVR was below the minima for the runway, however we had crossed our 4 mile mark and were entitled to continue to our MDA which we did. We never lost sight of the runway so we were legal - complicated isn't it
