AN ACR LGT LANDED BELOW MINIMUMS.

Narrative

OUR FLT WAS INTO JFK EARLY IN THE MORNING. WX AT JFK WAS EXPECTED TO REQUIRE A CATIII APCH; AS FOG WAS RPTED RESTRICTING VISIBILITY TO 1/4-1/8 MI. DISPATCH INFORMED THE CAPT PRIOR TO DEP THAT VISIBILITIES WOULD BE VARIABLE; RIGHT AROUND CATIII MINIMUMS; BUT SHOULD ALLOW A LNDG UPON ARR. WX OBSERVATION AT JFK AT DEP TIME INDICATED THE RVR FOR RWY 4R TO BE 600-800 FT. PRIOR TO DSCNT INTO THE TERMINAL AREA; SEVERAL SPECIAL OBSERVATIONS OBTAINED BY THE CREW SHOWED THE RVR TO BE AT VARIOUS RANGES; BUT NEVER BELOW 600 FT. THE CAPT ELECTED TO ACCEPT RADAR VECTORS TO CONTINUE THE APCH TO RWY 4R. PRIOR TO COMMENCING THE ACTUAL APCH; APCH CTL INFORMED THE CREW THAT THE RVR FOR RWY 4R WAS NOW 300 FT. THE FO LOOKED IN THE OPERATING MANUAL TO CHK THE REQUIREMENTS FOR A CATIII APCH; AND READ THE PUBLISHED ADDITIONAL ACFT SYS OPERATIONAL REQUIREMENTS FOR AN RVR 300 APCH. BOTH THE CAPT AND THE FO AGREED THAT THEY WERE LEGAL TO SHOOT THE APCH AS THEY HAD PRACTICED IT IN THE SIMULATOR IN PREVIOUS TRAINING AND IT WAS PUBLISHED IN THE OPERATING MANUAL. THE WX WAS CLR DOWN TO APPROX 300 FT. ABOVE TOUCHDOWN; WHERE THE APCH LIGHTS AND RWY THRESHOLD ENVIRONMENT WERE VISIBLE FROM THE COCKPIT. A CATIII AUTOLAND WAS ACCOMPLISHED UNEVENTFULLY. AFTER TAXI IN; THE ACFT MANUAL AND FLT MANUAL WERE REVIEWED; AND NO OTHER REF TO CATIII MINIMUMS BELOW 600 RVR WAS FOUND. ONLY AFTER CALLING DISPATCH AT DEST AND HIS SUBSEQUENT REVIEW OF ALL PUBS WAS IT DISCOVERED BY THE CREW THAT THE 300 RVR REFED IN THE ACFT MANUAL WAS NOT YET APPROVED AND THE CREW WAS TECHNICALLY NOT LEGAL TO FLY THE APCH TO RWY 4R UNDER EXISTING CONDITIONS. I WOULD LIKE TO SUGGEST THAT TENTATIVE APPROVAL FOR LOWER MINIMUMS OR NEW REQUIREMENTS NOT EVEN BE PUBLISHED IN THE OPERATING MANUAL UNTIL THEY ARE APPROVED PROCS SO THAT THIS CHAIN OF EVENTS CAN BE AVOIDED IN THE FUTURE. WHEN FLYING AN APCH INTO LOW VISIBILITIES OR ACCOMPLISHING A PROC INFLT (ESPECIALLY WITH A 2 PERSON CREW); IT IS OFTEN NOT POSSIBLE TO TAKE TIME OUT FROM REGULAR REQUIRED DUTIES TO CHK ON CHANGES IN PROCS OR LEGALITIES WITHOUT DIVERTING TOO MUCH ATTN FROM OUR PRIMARY RESPONSIBILITY -- SAFE FLYING.