MLG FLT LANDED ORD BELOW RPTED LNDG MINS.

Narrative

AFTER CONTACTING APCH CTL; WE WERE INSTRUCTED TO PLAN ON VECTORS TO RWY 27L. NOTICING A LOW FOG BANK MOVING OFF THE LAKE; WE BRIEFED FOR AN ILS CAT I APCH TO RWY 27L. THIS APCH RESULTED IN A MISSED APCH AT DECISION HEIGHT WITHOUT RWY IN SIGHT. VISUAL APCHS TO THE PARALLEL RWY; 27R; WERE STILL IN PROGRESS! WE WERE VECTORED E AND TOLD TO 'PLAN' RWY 27R. WE WERE LATER TOLD TO 'PLAN' RWY 14R; AND LATER AGAIN TOLD TO 'PLAN' RWY 14L. THE UNEXPECTED FOG WAS FAST MOVING; VERY LOW; AND VISIBILITY WAS DECREASING RAPIDLY. ON OUR L DOWNWIND FOR RWY 14L; RVR WAS RPTED AS 1800; AND WE BRIEFED AND SET UP FOR A CAT III APCH. AFTER HAVING STARTED THE FINAL APCH SEGMENT; ESTABLISHED AND PAST THE GSIA; RVR WAS RPTED BELOW MINS. WE CONTINUED THE APCH PER FAR 121.651. AT 250 FT AGL WE HAD THE APCH LIGHTS IN SIGHT; AND AT DECISION HEIGHT (50 FT AGL); I HAD THE THRESHOLD; THRESHOLD MARKINGS; CENTERLINE LIGHTS AND MARKINGS; SEVERAL RWY EDGE LIGHTS (MUCH MORE THAN THE 3-4 WHICH WOULD EQUATE TO 700 RVR) AND; HAD THE TOUCHDOWN ZONE MARKINGS WHICH ARE 1000 FT DOWN THE RWY! THIS VISIBILITY WAS STABLE AND CONTINUED RELATIVE TO THE FORWARD PROGRESS OF OUR ACFT. I DETERMINED THAT A LNDG WOULD BE A SAFE COURSE OF ACTION SINCE: BOTH AUTOPLTS WERE PROGRAMMED TO 'AUTO LAND' AT A NORMAL RATE WELL WITHIN THE TOUCHDOWN ZONE AND ON CENTERLINE; THE FLT VISIBILITY WAS MUCH BETTER THAN THAT RPTED BY THE TWR AND GREATER THAN THAT REQUIRED FOR THE APCH; DISTINCTLY VISIBLE AND IDENTIFIABLE VISUAL REFS WERE AVAILABLE FOR THE RWY. THE AUTOLAND SEQUENCE CONTINUED TO AN UNEVENTFUL TOUCHDOWN AND WE CLRED THE RWY AT THE FIRST 'HIGHSPD' TAXIWAY. UPON REQUEST FROM TWR WE GAVE A PIREP OF 1000 RVR. THE DISTINCTION BTWN FLT VISIBILITY AND RPTED VISIBILITY IS A NEBULOUS SUBJECT AND NEEDS TO BE BETTER DEFINED IN LOW MIN APCHS. IF TWR INTENDS TO SUSPEND OPS ON A RWY DUE TO VISIBILITY; THEY SHOULD SAY SO RATHER THAN CONTINUING TO ISSUE APCH INSTRUCTIONS AND LNDG CLRNCS. NEITHER O'HARE TWR NOT MY COMPANY HAS CONTACTED ME CONCERNING MY DECISION TO LAND.