MLG MADE VISUAL APCH IN WHAT HE PERCEIVED AS BELOW MINIMA CONDITIONS DUE TO RVR READING.

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ON APCH TO BOS APCH CTL WAS USING RWY 22L FOR LNDGS. THEY WERE GIVING RVR 1000 VAR 6000+. WE TOLD THEM WE NEEDED AT LEAST 6000 BEFORE OM TO CONTINUE APCH. THE LAST RVR GIVEN US OUTSIDE THE FAF WAS 1400 RVR; BUT WE WERE ABLE TO SEE THE RWY. A FOG BANK TO THE RIGHT OF THE RWY WAS CAUSING THE LOW RVR. ALL PARTS OF THE ARPT WERE VISIBLE AND THERE WERE NUMEROUS ACFT BEING GIVEN THE APCH TO RWY 22L WITH THE CLRNC; 'BE CAREFUL OF THE FOG BANK.' WE STARTED THE APCH AND WELL OUTSIDE OUR FAF CALLED THE TWR AND TOLD THEM WE HAD BOTH RWY 22R AND L IN SIGHT AND WANTED A VISUAL TO RWY 22R. THEY TOLD US; 'UNABLE DUE TO NOISE PROC CLRED TO LAND 22L.' I THEN SAID; 'I UNDERSTAND CLRED VISUAL FOR 22L.' THE TWR AT NO POINT GAVE US ANY UPDATED RVR INFO. THE LAST WE HAD WAS FROM APCH. AFTER LNDG AND FURTHER DISCUSSION WITH OUR COMPANY CHKPLT I WAS INFORMED EVEN THOUGH WE WERE CLRED FOR A VISUAL APCH TO RWY 22L THE RVR MADE IT BELOW LIMITS. THE APCH AND TWR WERE WELL AWARE OF LIMITS TO THE RWY AT LNDG BUT EVEN WHEN ASKED TO CHANGE THEY WOULD NOT. IT IS A SUCKER HOLE WHEN THE APCH IS GIVEN TO A RWY THAT IS CLR TO THE EYE 4 1/2 MI OFF; BUT THE RVR IS BELOW LIMITS DUE TO A LCL FOG CONDITION ONLY AFFECTING THE RVR READING.