A SUPER KING AIR 200 PVT PLT IDENTS THE PAPI LIGHTS FOR THE REIL'S AND SUBSEQUENTLY LANDS OFF TO THE SIDE OF THE SNOW COVERED RWY 18 AT JAC; WY.

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I WAS PIC OF THE AIRPLANE THAT WAS ON AN IFR FLT PLAN FROM GRI TO JAC. UPON APCHING THE VICINITY OF JAC; I WAS IN IMC; AND I WAS #2 BEHIND A GULFSTREAM ACFT. I OBTAINED THE TRANSCRIBED WX ON THE AWOS. THE WX I RECEIVED WAS 1000 FT OVCST WITH 1 MI VISIBILITY. I WAS DIRECTED BY ZLC TO EXECUTE A HOLDING PATTERN WHILE THE GULFSTREAM AIRPLANE IN FRONT OF ME WAS CLRED FOR THE ILS APCH TO RWY 18 AT JACKSON; WY. AT SOME POINT IN THIS SEQUENCE OF EVENTS; ZLC GAVE UPDATED WX TO THE GULFSTREAM AND ASKED ME IF I HEARD THE WX RPT ALSO. I ACKNOWLEDGED THAT RADIO BROADCAST. AFTER MAKING 1 CIRCUIT OF THE HOLDING PATTERN; I WAS CLRED FOR THE ILS RWY 18 APCH INTO JACKSON; WY. I BROKE OUT JUST ABOVE DECISION HT. AFTER I BROKE OUT; I HAD BEFORE ME WHAT I BELIEVED WERE THE RWY THRESHOLD LIGHTS. IT WAS DUSK; AND THE AREA OF THE ARPT AND THE AREA SURROUNDING THE ARPT WAS COVERED IN SNOW. ALSO; THE RWY ITSELF WAS COVERED IN SNOW. UNLIKE MANY ARPTS; WHICH HAVE ILS APCHS; THIS ARPT DOES NOT HAVE AN APCH LIGHTING SYS WITH SEQUENCE FLASHERS. THE APCH PLATE INDICATES THAT IT DOES HAVE HIGH INTENSITY RWY LIGHTS AND REIL'S. I LINED UP MY AIRPLANE ON WHAT I BELIEVED WAS THE REIL'S. BECAUSE OF THE POS OF MY ACFT; 30 FT TO THE L OF THE RWY; THE BEST I CAN DETERMINE IS THAT I MUST HAVE MISTAKEN THE PAPI FOR REIL'S; AND THIS LED ME TO CONCLUDE THAT I WAS LINED UP WITH THE RWY AND RWY THRESHOLD IN A POOR LIGHTING AND VISIBILITY ENVIRONMENT WHERE THE GND AND RWY WERE COVERED IN SNOW. THE TOUCHDOWN ON THE SNOW COVERED GND TO THE L SIDE OF THE RWY WAS UNEVENTFUL; EXCEPT FOR THE ABRUPT DECELERATION. AFTER LNDG; I TURNED OFF ALL OF THE ACFT ELECTRICAL AND FUEL SYS AND HAD MY PAX DEPLANE THE AIRPLANE.