A DH8D CREW; ON APCH TO LWS; BECAME DISORIENTED; REQUIRING VECTORS AND ALT ASSIGNMENT FROM ATC.

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WE WERE PLANNING A VISUAL APCH AT LEWISTON; BUT HAD LOADED THE VOR RWY 8 APCH IN THE FMS'S FOR A LNDG ON EITHER RWY 11 OR RWY 8. THIS LEG IS NORMALLY 8-10 MINS IN LENGTH AND BY USING RWY 26 WOULD SHAVE ROUGHLY 2 MINS OFF THE ENRTE TIME. (THE PNF IS VERY BUSY DURING THIS LEG WITH CHKLISTS; OBTAINING WX; RPTING ON; OFF AND ARR TIMES TO 2 DIFFERENT STATIONS.) AFTER DEPARTING PULLMAN AND XING THE PULLMAN VOR; WE CHKED ON WITH ZSE AND WERE CLRED TO 6000 FT. A FEW MINS LATER CTR INFORMED ME THAT THE WINDS WERE NOW FAVORING RWYS 26 OR 29. I REQUESTED THE ILS RWY 26 AND WAS CLRED AS REQUESTED VIA THE 10 DME ARC. WE WERE AT 13 DME WHEN I RECEIVED THIS CLRNC. I QUICKLY TURNED E ON THE ARC; HELD THE DME ON THE MQG VOR; DIALED IN THE LOC FREQ FOR THE ILS AND BEGAN DSNDING TO 5300 FT WHEN SOMETHING SEEMED OUT OF PLACE. AT FIRST I THOUGHT I HAD TURNED THE WRONG DIRECTION ON THE ARC. MY POOR FO WAS JUST FINISHING COMPANY RADIO COMS WHEN I TOLD HER WHAT I THOUGHT I HAD DONE. I TOLD HER HOW WE HAD BEEN CLRED AND SHE WAS SUDDENLY THRUST INTO TRYING TO SET UP THE APCH AND HELP ME FIGURE OUT WHERE WE WERE ON THE ARC. I THEN REALIZED WHAT WAS OUT OF PLACE. BY HOLDING THE DME ON THE VOR AND SWITCHING FREQS; I HAD INADVERTENTLY REMOVED THE RMI NEEDLE FROM THE HSI WHICH I NEEDED TO CONDUCT THE ARC. WITH THE FMS SETUP FOR THE VOR APCH I HAD NO USEFUL SECONDARY INFO TO HELP KEEP ME ESTABLISHED ON THE ARC. WHEN I DISCOVERED THIS WE WERE NOW CLOSE TO THE 13 OR 14 DME ARC. CTR CALLED AND STATED THE SAME. WE ADVISED CTR WE WERE HAVING TROUBLE INTERCEPTING THE ARC; AT WHICH TIME WE WERE CLRED BACK TO 6000 FT AND GIVEN VECTORS TO REINTERCEPT THE ARC AND LANDED IN LEWISTON WITHOUT FURTHER INCIDENT. AFTER REVIEWING THIS EVENT; SEVERAL THINGS BECAME CLR. FIRST; BY GETTING BEHIND AND TRYING TO 'SAVE' A FEW MINS; I CAUSED MYSELF AND MY CREW TO BECOME OVERLY RUSHED. BY CHOOSING TO DEPART DIRECTLY TO THE PULLMAN VOR VERSUS DEPARTING USING THE DEP PROC; THE FEW MINS THAT WERE SAVED IN REALITY MADE MATTERS WORSE. THIS DECISION ALSO PUT MY FO IN A TIME CRUNCH; WHICH KEPT HER OUT OF THE APCH PROCESS ENRTE TO LEWISTON. SECOND; I WAS NOT ADEQUATELY PREPARED FOR THE APCH INTO LEWISTON. I SHOULD HAVE PLANNED BETTER THAN TO DEPART ON THAT SHORT OF A FLT WITHOUT BETTER PREPARATION AND PLANNING FOR CONTINGENCIES. THIRD; BOTH THE FO AND I WERE A LOT MORE TIRED THAN I REALIZED WHICH ALSO CONTRIBUTED TO HOW DECISIONS WERE BEING MADE AT THE TIME.