B737 FLT CREW CONTINUES A CAT III APCH AND LANDS WHEN THE RVR IS RPTR BELOW PUBLISH MINIMUMS AT SEA.

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ARRIVING FROM DEN; WX WAS GOOD ENOUGH FOR CAT 1 AT TOP OF DSCNT; BUT WE ELECTED TO BRIEF AND DO CAT III APCH; JUST IN CASE. WX DETERIORATED ON DSCNT; BUT RVR VARIED BTWN 800 AND 1600. ON FINAL; CONFIGURED AND JUST INSIDE THE FAF; TWR ADVISED RVR; AND MIDFIELD WAS CALLED 600. I MENTIONED I THOUGHT WE NEEDED THE MIDFIELD TO BE 700. THE CAPT REFERRED TO A CARD HE KEEPS IN HIS POCKET; AND SAID ALL WE NEED IS 600. I WAS CONFUSED; BUT DIDN'T THINK THAT WAS THE TIME FOR A DETAILED DISCUSSION. WE CONTINUED THE APCH AND LANDED WITHOUT INCIDENT. AFTER SHUTDOWN; WE TALKED ABOUT THE SIT. HE REFERRED TO THE FOM WHERE IT SAYS OUR COMPANY MINS ARE 600; BUT WE EVENTUALLY AGREED THAT THE SPECIFIC APCH MINS OVERRULE THAT; AND WE SHOULD HAVE GONE AROUND UNLESS WE COULD GET THE CTLR TO ADVISE OF IMPROVEMENT PRIOR TO DH. SUPPLEMENTAL INFO FROM ACN 595455: THE FO BROUGHT UP THE QUESTION OF 700 RVR ON THE SEATTLE APCH CHART. I LOOKED AT IT AGAIN; AND AGREED THAT I PROBABLY MADE A MISTAKE; BUT IT WAS A CLASSIC 'SIMULATOR' QUESTION THAT HAPPENED QUICKLY AND IN REAL TIME; AND I DID KNOW THAT WE WERE APPROVED FOR 600 RVR; AND OF COURSE; SUBSEQUENTLY; THE RVR WENT RIGHT BACK UP AGAIN. IN RETROSPECT; AND HONEST MISTAKE AND A GOOD LESSON.