INSTRUCTIONAL C172 PLT DEPARTS BWG WHEN ARPT ASOS XMITS IFR CONDITIONS. PLT DEPARTS WHEN HE DETERMINES VISIBILITY IS 3 MI OR GREATER. FSDO CORRECTS PLT PERCEPTION.

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MY STUDENT CALLED FSS AND GOT A BRIEFING PRIOR TO XA00. RPTED WX WAS CLR; 4 MI IN MIST. A VFR FLT PLAN WAS FILED. WHILE DRIVING TO THE ARPT; VISIBILITY LOOKED LIKE 4 MI IN MIST. WE ARRIVED AT THE ARPT AT XA35. THERE WAS FROST THIS MORNING; BUT WE HAD WING COVERS AND WERE ABLE TO QUICKLY PREFLT (WITH 3 OF US) AND STARTED THE ENG AT XA55. SHORTLY AFTER START-UP; WE LISTENED TO ASOS AND IT RPTED THE VISIBILITY AS 3/4 MI. WE COULD SEE THAT THE VISIBILITY WAS OBVIOUSLY BETTER THAN THAT; AND I MENTALLY CHKED THE ASOS OFF AS INACCURATE; JUST LIKE OUR AWOS ON OUR HOME FIELD. THERE WERE 4 COMPANY ACFT ON THIS TRIP; OVERNIGHTING IN BWG. A COMPANY C172 TAXIED OUT PRIOR TO OUR DEP. AS THEY LIFTED OFF; ANOTHER CREW ASKED THEM FOR A PIREP ON THE COMPANY FREQ. THEY RPTED THAT THEY HAD DEPARTED ON AN SVFR CLRNC AND INFLT VISIBILITY WAS 3-5 MI. WE TAXIED TO THE RWY AND PREPARED FOR TKOF. WE LISTENED TO ASOS AGAIN AND IT RPTED 2 1/2 MI; STILL BELOW WHAT WE COULD SEE AND WHAT OTHER ACFT RPTED. I THOUGHT IT WOULD BE GOOD TO GO THROUGH PICKING UP AN SVFR CLRNC FOR THE LEARNING EXPERIENCE FOR MY STUDENTS; NOT BECAUSE THE FIELD WAS RPTING IFR. WE CALLED LOUISVILLE FSS TO TRY TO PICK UP AN SVFR CLRNC AND OPEN OUR VFR FLT PLAN. AFTER SEVERAL CALLS; THEY FINALLY RESPONDED. WE GAVE THEM OUR REQUEST AND THAT'S THE LAST WE HEARD FROM THEM ON THE GND. I DIDN'T NOW IF THEY WERE LOOKING FOR OUR CLRNC FROM MEMPHIS OR LOOKING FOR OUR FLT PLAN. I CALLED AGAIN; ASKING HOW THINGS WERE GOING; BUT GOT NO RESPONSE. WHILE MONITORING FSS FREQ; WE HEARD THE FIRST ACFT OPEN THEIR FLT PLAN. LOUISVILLE ASKED HOW THEY DEPARTED SINCE THE FIELD WAS IFR. THEY RESPONDED THAT THEY DEPARTED ON AN SVFR CLRNC. THIS COMMENT FROM FSS DIDN'T TRIGGER ANYTHING WITH ME BECAUSE I COULD SEE THAT THE ASOS WAS WRONG AND THE FIELD WAS ACTUALLY VFR. WHILE WE WAITED FOR SOME RESPONSE FROM FSS; A SECOND COMPANY C172 DEPARTED. THEY GAVE US A PIREP THAT FLT VISIBILITY WAS 3-5 MI AND THEY DIDN'T TAKE OFF WITH AN SVFR CLRNC. HAVING GIVEN UP ON WAITING ANY LONGER FOR FSS TO HELP MY STUDENTS WITH THE LEARNING EXPERIENCE OF GETTING AN SVFR CLRNC; WE DEPARTED AT XB30. UPON OPENING OUR VFR FLT PLAN ALOFT; LOUISVILLE SAID BWG VISIBILITY WAS 1 3/4 MI AND THE FIELD WAS IFR. WE RPTED THAT SURFACE AND FLT VISIBILITY WAS 3-5 MI. LOUISVILLE OPENED OUR FLT PLAN AND THAT WAS THE END OF IT UNTIL THE FAA CALLED THE NEXT MORNING. THE FAA INFORMED US OF OUR ERROR; THAT ASOS IS CONSIDERED THE OFFICIAL WX AND THAT WE HAD DEPARTED VFR UNDER IFR CONDITIONS (WITH THE EXCEPTION OF THE PLANE THAT HAD AN SVFR CLRNC). MY UNDERSTANDING WAS IF I HAD THE REQUIRED VISIBILITY I COULD MAKE THE 'GO' DECISION; MUCH LIKE THE PIC MAKES THE DECISION ON LNDG UNDER IFR WHETHER OR NOT HE HAS THE REQUIRED FLT VISIBILITY FOR LNDG PER FAR PART 91; 175.C.2. I FOUND OUT I WAS MISTAKEN. THE FAA SAID ASOS IN THE OFFICIAL WX AND IS NOT OPEN TO PLT INTERP. WE WERE ALSO TOLD THAT IF WE NOTE A DISCREPANCY BTWN WHAT WE SEE AND WHAT ASOS RPTS; WE CAN CALL THE HUMAN OBSERVER AT THE FACILITY AND HE IS ABLE TO OVERRIDE THE ASOS BASED ON HIS OBSERVATIONS. IN TALKING FURTHER WITH AN FSS SPECIALIST I FOUND THIS IS NOT TRUE. AS A PLT; I HAVE NO WAY TO DIRECTLY CONTACT THE HUMAN OBSERVER. IF I SEE A DISCREPANCY; I CAN CALL FSS AND THEY CAN PHONE THE OBSERVER AND KINDLY ASK IF HE WOULD MIND CHKING THE VALIDITY OF THE ASOS. ONLY THEN COULD THE ASOS BE OVERRIDDEN AT THE DISCRETION OF THE HUMAN OBSERVER. HOWEVER; THE FSS SPECIALIST ASKED THAT I NOT MAKE A HABIT OF DOING THIS. CONTRIBUTING FACTORS: MY MISUNDERSTANDING OF THE OFFICIAL POS OF ASOS READINGS AND MY MISUNDERSTANDING OF THE FARS. CORRECTIVE ACTION: HAD I KNOWN THE ABOVE; I SHOULD HAVE WAITED TO GET AN SVFR CLRNC RELAYED THROUGH LOUISVILLE FSS. I DON'T KNOW IF I COULD HAVE CONTACTED ZME DIRECTLY ON THE GND AT BWG OR IF I WOULD HAVE HAD TO PHONE. I STILL DON'T KNOW WHY LOUISVILLE NEVER RESPONDED TO ME AFTER MY FIRST CONTACT WITH THEM ON THE GND.