A CPR WESTWIND CAPT WANTS TO KNOW IF HE WAS LEGAL TO CONTINUE THE APCH AND LAND WITH AN RVR READOUT OF LESS THAN 1800 FT. HE WAS INSIDE THE FINAL APCH FIX WHEN THE CTLR ADVISED HIM OF THE LOWERED RVR.

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WE WERE INFORMED BY ATC THAT THE WX AT ORD WAS BELOW CAT I LNDG MINIMUMS AND INSTRUCTED TO HOLD AT BULLZ INTXN. ABOUT 45 MINS LATER THE RVR IMPROVED ABOVE 1800 FT AND WE WERE GIVEN VECTORS TOWARD THE ARPT. SHORTLY THEREAFTER THE RVR WENT BELOW 1800 FT AND ATC WAS INFORMED THAT WE NEEDED 1800 FT OR GREATER. ATC TOLD US TO CONTINUE THAT THE RVR WAS GOING UP AND DOWN. HE STATED THAT THE RVR WAS NOW ABOVE 1800 FT AND CLRED US FOR THE APCH BEHIND A COMMUTER. INSIDE THE OM; ATC (LCL) STATED THAT THE RVR WAS BELOW 1800 FT AND CLRED US TO LAND. THE XMISSION WAS GIVEN WHEN WE WERE ON ABOUT A 2 MI FINAL. AS WE APCHED THE DECISION HT NO RVR WAS GIVEN BY ATC. AT DECISION HT THE FLT VISIBILITY WAS PROBABLY AROUND 2000 FT AND WE LANDED WITHOUT INCIDENT. FAR 91.175(D) STATES 'NO PLT OPERATING AN ACFT MAY LAND THAT ACFT WHEN THE FLT VISIBILITY IS LESS THAN THE VISIBILITY PRESCRIBED IN THE STANDARD INST APCH PROC BEING USED.' RVR MEASURES VISIBILITY ON THE GND AT TOUCHDOWN; MIDPOINT; AND ROLLOUT POINTS. THERE IS NO INST TO MEASURE FLT VISIBILITY AT 200 FT AGL OTHER THAN THE FLC'S JUDGEMENT. THERE SEEMS TO BE SOME CONFUSION AMONG CREWS AS HOW TO INTERPRET THIS FAR BECAUSE INFLT VISIBILITY AND RVR CAN GREATLY DIFFER. YOU COULD HAVE A SIT WHERE THERE IS GND FOG AND THE RVR IS 1000 FT AND THE FLT VISIBILITY AT 200 FT AGL IS SIGNIFICANTLY GREATER. I BELIEVE IT WOULD BE IN EVERYBODY'S INTEREST IF THIS FAR WAS WORDED SO THAT THERE WOULD BE NO DISCREPANCY.