RWY VISUAL RANGE AS A VARIABLE NUMBER (RVR) WITH REGARD TO THE LEGAL STATUS OF LNDG WX MINIMUMS VALUE.

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MYSELF AND THE CAPT DEPARTED IN OUR HOSPITAL OWNED BE200 ON AN AIR AMBULANCE MEDICAL FLT TO PICK UP A PATIENT IN MARSHALL; MN (MML) AND RETURN HIM TO SIOUX FALLS; SD (FSD). THE TWR HAD CLOSED AT XX00 AND THE CAPT CHKED WITH THE TWR AT THAT TIME TO GET THE LATEST RVR READING WHICH I BELIEVE WAS XX00. THE SA RPTED WX AS 1 (OVCST) 1/4 WITH FOG. SO; WITH THE RVR OF XX00; WE DEPARTED AND PROCEEDED TO AND LANDED AT MML. JUST MIN BEFORE DEPARTING MML. THE CAPT CHKED THE WX AT FSD; AND IT WAS 1 (OVCST) 1/4 FOG RVR 18V24. AFTER DISCUSSING THE FACT THE RVR WAS LISTED AS VARIABLE AND NOT A SET NUMBER; WE DECIDED TO DEPART TO FSD SINCE WE HAD 2400 RVR LISTED IN THE LATEST SA. ALSO; THE FSD FT'S WERE CALLING FOR MUCH BETTER CONDITIONS. WE NEVER RECEIVED ANY FURTHER WX RPTS FROM ZMP OTHER THAN AN UPDATED WIND (SURFACE) AND ALTIMETER SETTING. WE DID THE ILS 21 AT FSD AND UPON REACHING DECISION HT HAD THE REQUIRED VISIBILITY TO LAND AND WE DID LAND. THE PARAMEDIC NURSE ON BOARD WAS CONCERNED WITH THE HVY FOG AND WE HAD A MEETING WITH THE MEDICAL CREW AND OUR PROGRAM DIRECTOR TO DISCUSS THE TRIP. WE EXPLAINED RVR AND WE ALL THOUGHT THE MATTER WAS OVER. BUT NOW; SOMEONE HAS MADE MORE ACCUSATIONS AT US PLTS AND WE ARE NOW MAILING IN THE NASA FORM EVEN THOUGH IT HAS BEEN 13 DAYS. WE FEEL THIS IS GOOD CAUSE. WE ARE CONFUSED ABOUT THE 1800-2400 RVR AS LISTED IN THE LATEST SA THAT WE RECEIVED. WE DISCUSSED OUR SIT AND DID NOTHING BLATANT OR UNSAFE. WE HAD THE REQUIRED VISIBILITY AT DECISION HT. BUT PEOPLE ARE QUESTIONING IF THE RVR RPT IN THE SA WAS LEGAL TO START THE APCH. WE HAVE LOOKED AT THE FAR'S AND AIRMAN'S INFO MANUAL AND EVEN AC 97.1 BUT CAN FIND NO ANSWERS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT EVEN THOUGH THIS MATTER WAS RPTED TO THE FAA BY SOME UNKNOWN PERSON; THERE HAS NOT BEEN ANY REPLY FROM THE FAA TO DATE. IT WAS DISCUSSED WITH THE RPTR REGARDING THE LACK OF ANY GUIDANCE OR REG CLARIFYING THE VALUE LEGALLY USED WHEN THE VARIABLE RVR IS RPTED. THEREFORE; THE HIGHEST VALUE COULD THEN BE USED FOR THE PURPOSE OF THE MINIMUM RVR REQUIRED. HIS COMPANY HAS SINCE THAT DATE MADE A POLICY TO NOT USE THE HIGHEST VALUE IF IT IS NEEDED FOR THE MINIMUMS UNTIL RECEIVING SOMETHING FIRM IN WRITING FROM THE FAA.