COMMUTER ACFT LANDED AT NON TWR ARPT WITH NO WX OBSERVER PRESENT.

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APCHING MHT FOR LNDG AFTER TWR HAS CLOSED. 121 OPS SPECS AND COMPANY OPS MANUAL REQUIRE IFR CAN BE CANCELLED IF LNDG SURFACE IS VISUALLY ESTABLISHED AND MAINTAINED -- CONDITION WAS EASILY MET. 121 OPS SPECS AND COMPANY OPS MANUAL REQUIRE IFR OPS INTO UNCTLED AIRSPACE MAY BE CONDUCTED IF THERE IS A WX OBSERVER AT THE FIELD -- CONDITION WAS NOT MET (NO WX OBSERVER WAS AVAILABLE). WHAT WE DID WAS CANCELLED IFR AND PROCEEDED IN UNDER VFR. OUR UNCERTAINTY COMES FROM NOT BEING CERTAIN THAT THE FLC CAN MAKE A LEGAL DETERMINATION OF THE WX CONDITIONS AT THE ARPT (INFLT CONDITIONS WERE EXCELLENT VFR). THE ARPT APPEARED TO BE (AND WAS) VFR; BUT NOBODY WAS AVAILABLE TO TELL US THAT. (THE FAA OPS SPECS ARE NOT CLR AS TO WHETHER THE CREW CAN MAKE A DETERMINATION.) LATE NIGHT AND LONG DAY MAKES FOR A DIFFICULT DECISION MAKING SIT. WE ARE STILL UNSURE WHETHER WE VIOLATED A REG -- BUT WE COULD SEE THAT THE CONDITIONS FOR A SAFE OP EXISTED. (CREW REST AND DUTY TIME ISSUES ENTER INTO THIS ONE TO SOME DEG.)