UNAUTH LNDG BY A PA46-310P CFI ON A CLOSED RWY AT A NON TWRED ARPT AT 8S1; MT.

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I AM A CFI SPECIALIZING IN INITIAL AND RECURRENT TRAINING IN HIGH-PERFORMANCE; PRESSURIZED ACFT. ON OCT/FRI/03; I WAS REPOSITIONING A CUSTOMER'S PA46-310P MALIBU FROM MISSOULA (MSO); MT; TO POLSON (8S1; UNCTLED); MT; TO PICK UP THE AIRPLANE OWNER AND THEN CONDUCT FLT TRAINING. EARLIER IN THE DAY; I HAD BEEN FLYING THE SAME AIRPLANE OUT OF MISSOULA WITH ANOTHER STUDENT. THE WX WAS GOOD VFR WITH ALMOST UNLIMITED CEILING AND VISIBILITY. FOR THE SHORT (47 NM) FLT TO POLSON; I DID NOT FILE ANY FLT PLAN. I ARRIVED AT POLSON AT APPROX XA30 LCL TIME AND ENTERED A L PATTERN FOR RWY 36. I ANNOUNCED MY POS AT THE DOWNWIND; BASE AND FINAL LEGS ON THE UNICOM FREQ OF 122.8; WHICH WAS NOT MONITORED. AFTER LNDG; THE OWNER OF THE AIRPLANE CAME OVER TO THE AIRPLANE AND INFORMED ME HE HAD JUST FOUND OUT THAT THE RWY WAS CLOSED DUE TO RESEALING OF SOME CRACKS. INCIDENTALLY; THE RWY LOOKED NORMAL AND DID NOT HAVE ANY DAMAGE. WHILE WE WERE TALKING; ANOTHER AIRPLANE (AN EXPERIMENTAL KIS HOMEBUILT) LANDED ON THE SAME RWY 36; WITHOUT MAKING ANY POS RPTS. AFTER TALKING TO THE AIRPLANE OWNER ABOUT THE RWY CLOSURE; I DECIDED TO LEAVE THE ARPT. I TOOK OFF FROM RWY 36 AND FLEW TO RONAN; A FEW MI AWAY; WHERE WE KEPT THE AIRPLANE FOR THE NIGHT. AFTER TAKING OFF FROM RWY 36 AT POLTON; I FLEW OVER THE ARPT TO SEE IF I COULD NOTICE ANY CROSSES INDICATING THE RWY CLOSURE. THERE WERE NO CROSSES ON THE PAVED RWY; BUT THERE WERE YELLOW PLASTIC CROSSES (REPOSITIONED SOMEWHAT BY THE WIND) IN THE GRASSY AREA IN FRONT OF THE THRESHOLD OF EACH RWY. I FOUND THIS METHOD OF INDICATING A CLOSED RWY CONFUSING AND; OBVIOUSLY; OTHER PLTS HAD MADE THE SAME MISTAKE. INDEED; THE FOLLOWING DAY; FLYING FROM RONAN; WE ADVISED A BONANZA PLT NOT TO LAND AT POLSON; AS HE HAD NOT NOTICED THE CROSSES NOR HEARD OF THE RWY CLOSURE. NOTWITHSTANDING ALL THE ATTENUATING CIRCUMSTANCES; I LANDED ON A RWY WHERE I SHOULD NOT HAVE LANDED. TO PREVENT A REOCCURRENCE IN THE FUTURE; I WILL CHK ALL THE NOTAMS EVEN FOR THE SHORTEST OF FLTS; AND FILE AT LEAST A VFR FLT PLAN. I LEARNED A VALUABLE LESSON AS WELL.