A CIRRUS SR22 PLT FAILS TO GET A WX BRIEF AND NOTAMS FOR A FLT TO 851 WHICH RESULTS IN A LNDG ON A CLOSED RWY. THE PLT NOTICED AN 'X' ON THE OPPOSITE END OF THE RWY.

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I FLEW FROM MSO (MISSOULA; MT) TO 851 (POLSUN; MT); A DISTANCE OF 55 NM; ON OCT/MON/03. WX WAS: VISIBILITY AND CEILING UNRESTR; SO I DIDN'T CHK WX (OR NOTAMS) WITH FLT SVC. IF I HAD; I WOULD HAVE FOUND THAT 851 WAS CLOSED FOR 3 DAYS FOR MAINT. ARRIVING AT 851; I SAW A YELLOW 'X' OFF THE APCH END OF RWY 36. SEEING NO PROBS WITH THE RWY; AND NOT SEEING AN 'X' ON THE APCH END OF RWY 18; WHERE I WAS PLANNING TO LAND; I THOUGHT THAT THERE MIGHT HAVE BEEN SOME TEMPORARY RESTR TO LNDG ON RWY 36. I HAD CALLED SEVERAL TIMES ON 122.8; THE CTAF; BUT RECEIVED NO XMISSIONS ABOUT THE AIRFIELD. A CESSNA 206 WAS TAXIING. SEEING WHAT APPEARED TO BE NORMAL ACTIVITY AND A CLR RWY; I LANDED ON RWY 18. AFTER LNDG; I SAW EQUIP OFF THE SIDE OF THE RWY AND LEARNED FROM A WORKER THAT THE RWY WAS TO BE CLOSED FOR 3 DAYS FOR REPAIR. THE CESSNA 206 WHICH WAS TAXIING WAS BEING ALLOWED TO DEPART WHILE THE RWY WAS STILL USABLE. I ASKED THE WORKER IF IT WOULD BE OK IF I DEPARTED; TOO; AND I LANDED AT NEARBY RONAN; MT. LATER IN THE DAY; I FLEW BACK OVER 851 TO SEE WHY I DID NOT SEE AN 'X' ON THE APCH END OF RWY 18 AND FOUND IT TO BE THERE; BUT DIFFICULT TO SEE; AS IT WAS YELLOW LAYING ON YELLOW MOWED GRASS. THERE WERE NO HUMAN FACTORS -- I WAS WELL RESTED AND FIT. I CERTAINLY LEARNED A LESSON ABOUT MAKING SURE I CHK NOTAMS AND OTHER PERTINENT PREFLT INFO BEFORE FLYING ANYWHERE; AND THE IMPORTANCE OF ASCERTAINING THE ACTUAL CONDITION OF THE AIRFIELD.