FLC OF ACR LGT ACFT BECAME DISORIENTED DURING AN APCH DME ARC RESULTING IN THE ABANDONMENT OF THE APCH AND CLBING TO A HIGHER ALT THEN CLRED FOR TERRAIN CLRNC.

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PROCEEDING DIRECT WHILE ENRTE FROM KALISPELL TO HELENA; THE CAPT MADE CONTACT WITH HELENA APCH AT APPROX HLN 300/17 DME AND AT 12000 FT. APCH GAVE US THE OPTION OF LOC DME (BACK COURSE) C OR TO BE SENT WELL E OF HLN WHILE HE CLRED OUT OTHER TFC. I CHOSE THE LOC DME (BACK COURSE) C; WHICH WAS A MISTAKE DUE TO PREVIOUS EXPECTATIONS OF THE ILS RWY 27 APCH AND OUR CURRENT HIGH AIRSPD AND PROX TO THE 13 DME ARC. AT 14 DME; APCH CLRED US TO JOIN THE 13 DME ARC NW OF HLN FOR THE LOC DME (BACK COURSE) C AND TO DSND TO 10000 FT. I COMPLETED THE 300 KIAS SBOUND TURN INSIDE THE ARC AT 9 DME AND CORRECTED BACK AS I DIALED IN THE FRONT COURSE OF 267 DEGS; ALL IN VMC. IN THE PROCESS OF DSNDING TO 10000 FT; TRYING TO ESTABLISH THE ARC; BRIEFING THE NEW APCH; AND TUNING IN THE LOC; I APPARENTLY FLEW THROUGH THE LOC COURSE. AS I LEVELED AT 10000 FT; NOW IN IMC; BOTH THE SO AND I REALIZED THE COURSE HAD BEEN MISSED. THE CAPT INFORMED APCH THAT WE WERE NO SW OF HLN AND REQUESTED CLRNC DIRECT TO HLN VOR TO SET UP FOR ANOTHER APCH. APCH CLRED US TO HLN VOR AND ASKED TWICE FOR OUR POS. THE CAPT REPLIED THAT WE WERE SW OF HLN EACH TIME AND ALSO THAT WE HAD CLBED TO 12000 FT DURING THE TURN (FROM APPROX HLN 210/15 DME TO THE VOR). APCH CLRED US TO 10000 FT AGAIN SO I DSNDED TO 10000 FT ENRTE TO THE VOR. APPROX 4 DME WE ENTERED VMC; SAW THE ARPT; AND CANCELLED IFR. APCH INSTRUCTED US TO SQUAWK VFR AND CONTACT TWR. THE CAPT ADVISED TWR THAT WE WERE OVERHEAD THE VOR; NBOUND; AND REQUESTED A R DOWNWIND TO RWY 27. TWR STATED THAT HE COULD NOT SEE US; BUT CLRED US FOR A L DOWNWIND TO RWY 27; WHICH WOULD HAVE TAKEN US S OF HLN VOR NEAR THE TERRAIN. I CONTINUED ON APPROX A 030 DEG HDG; DSNDING FROM 10000 FT TO 7000 FT WHILE THE CAPT EXPLAINED TO TWR OUR POS; NOW N OF HLN VOR; AND THAT WE WERE REQUESTING A R BASE TO RWY 27. TWR CLRED ANOTHER ACFT TO TKOF RWY 9; THEN FINALLY SIGHTED US N OF THE FIELD AND CLRED US FOR A R BASE TO RWY 27. I MADE A NORMAL LNDG ON RWY 27. A VERY CONFUSING TIME FOR EVERYONE; ALL BECAUSE I ACCEPTED A 'LAST MIN' CLRNC TO DO SOMETHING THAT WAS NEITHER PREPARED FOR; NOR EVEN EXPECTED.