CAPT OF A DC9 CONTINUED APCH AFTER BEING ADVISED OF THE LATEST WX RPT WHICH INDICATED BELOW CAT I LNDG MINIMUMS; TOGETHER WITH THE RPTING FO'S CAUTION TO THE CAPT OF BELOW THEIR MINIMUMS FOR LNDG. A SUCCESSFUL LNDG WAS MADE IN GOOD VISIBILITY AND THE ARPT WX RPT WAS SUBSEQUENTLY UPDATED TO REFLECT SAME.

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WHEN WE LEFT DTW TO JAX THE WX WAS CLR WITH GOOD VISIBILITY. ENRTE; DISPATCH SENT AN ACARS THAT JAX WAS 1/4 IN FOG AND ADDED AN ALTERNATE. WE CHKED JAX WX 2-3 MORE TIMES ENRTE AND NOTICED IT WENT FROM BELOW MINIMUMS TO VFR EVERY FEW MINS. THE ATIS SAID THE VISIBILITY WAS 3 MI. WE COULD SEE THE ARPT FROM 20 MI AWAY AND NOTICED SOME PATCHY GND FOG IN THE AREA. WE COULD HAVE EASILY ACCEPTED A VISUAL APCH. WHEN APCH TURNED US ONTO THE LOC; IN THE SAME XMISSION AS THE ILS CLRNC; HE ADVISED US THAT THE TOUCHDOWN RVR WAS 1200 FT WITH MID AND ROLLOUT BETTER THAN 6000 FT. I ASKED THE CTLR TO REPEAT THE RVR; WHICH HE DID. I ADVISED THE CAPT THAT IT WAS BELOW CAT I MINIMUMS AND WE WOULD NEED 1800 FT RVR. HE SAID THAT HE COULD SEE THE RWY CLRLY. I AGAIN ADVISED HIM THAT RVR HAS PRIORITY OVER THE PREVAILING VISIBILITY AND IT WAS ILLEGAL TO CONDUCT THE APCH. HIS RESPONSE WAS 'GEAR DOWN.' A THIRD TIME I ADVISED HIM THAT 'TECHNICALLY' WE CAN'T SHOOT THIS APCH BECAUSE OF THE TOUCHDOWN RVR. I SUGGESTED WE CIRCLE TO A RWY WITHOUT RVR. HE DECLINED. AT THE OM; I ADVISED THE TWR WE HAD 'A GOOD VISUAL WITH THE RWY' AND WERE CLRED TO LAND. I COULD SEE FULL LENGTH AND WIDTH OF THE RWY AND IT WAS OBVIOUS THERE WAS A PATCH OF FOG OVER THE TRANSMISSOMETER SITE -- IT WAS VISIBLE. ALTHOUGH I KNEW WE WERE TECHNICALLY ILLEGAL TO CONDUCT THE APCH; IN MY JUDGEMENT WE DID NOT CONDUCT AN UNSAFE OP. WE CONTINUED VISUALLY AND MADE A VISUAL LNDG WITHOUT INCIDENT. ON ABOUT A 3 MI FINAL; THE TWR ADVISED THAT THE ATIS HAD CHANGED; OBVIOUSLY THE RVR HAD INCREASED. I'M NOT SURE WHAT ELSE I COULD HAVE DONE SHORT OF TAKING THE ACFT AND I REALLY DIDN'T FEEL THAT WAS WARRANTED. I SEE THE FO'S JOB AS THAT OF A SAFETY VALVE -- I PROVIDE INPUT AND IDEAS; AND TAKE OVER IF SAFETY IS COMPROMISED. I MADE A DECISION THAT THIS WAS SAFE; ALBEIT TECHNICALLY ILLEGAL. I HAVE GONE OVER IT 100 TIMES AND DECIDED IF IT HAPPENS AGAIN TO BE MORE FORCEFUL IN MY INPUT; BUT AGAIN; I DOUBT I WOULD TAKE THE ACFT UNLESS SAFETY WERE COMPROMISED.