ACR CREW DSNDED BELOW PROC ALT ON THE ARC TO THE ILS TO CPR; WY. BOTH CTLR AND JUMP SEAT RIDER NOTICED THE CFTT.

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CAPT AND FO LOST POSITIONAL AWARENESS DURING APCH (ILS RWY 3 CPR). DSNDED TO 8500 FT MSL WELL PRIOR TO PASSING 193 DEG RADIAL ON 29 DME ARC. FO WAS DISTR WITH CASPER RADIO ON VHF #1 AND CTR ON VHF #2. JUMP SEATER QUERIED THE ALT AND CAPT CLBED BACK TO 10400 FT. SUBSEQUENTLY CLRED FOR VISUAL APCH. SUPPLEMENTAL INFO FROM ACN 404310: ARR FROM THE E; CLRED TO INTERCEPT 29 NM DME ARC FOR THE ILS RWY 3 AT CPR. CLRED FOR APCH. CAPT INTERCEPTED AND TURNED ONTO ARC; DSNDED TO 10400 FT. I BECAME INVOLVED IN TALKING TO CPR RADIO AND ZDV. FSS GAVE A QUITE LENGTHY LIST OF NOTAMS AND FIELD CONDITIONS. AT SOME POINT I DROPPED OUT OF MONITORING APCH AND CONCENTRATED ON RADIOS. CAPT ASKED FOR MY VOR RADIO TO IDENT DDY 193 DEG RADIAL FOR STEPDOWN FIX. I COMPLIED AND CONTINUED WITH RADIOS. AT SOME POINT; CTR WANTED TO KNOW IF WE WERE COMPLYING WITH ILS RWY 3 PROCS. AT THIS POINT; I WAS UNSURE WHERE WE WERE. WE ENDED UP AT 8500 FT MSL; E OF 193 DEG RADIAL; AT ABOUT 25 NM. WE CLBED BACK TO 10400 FT; HEADED FOR THE 29 NM ARC; I CALLED ARPT IN SIGHT; CLRED FOR VISUAL. WE HAD BRIEFED FOR STRAIGHT-IN RWY 3 NOT FULL PROC. CAPT THOUGHT WE WERE W OF 193 DEG RADIAL WHEN WE WEREN'T. CAPT PROBABLY STARTED TOWARDS ARPT; THUS AT 25 NM; NOT 29 NM. A FULL BRIEFING; AND MY ATTN TO APCH; AND MY BACKING UP CAPT WOULD'VE AVOIDED THIS INCIDENT. COM WITH FSS SHOULD'VE CEASED. CONCURRENCE ON MY PART SHOULD HAVE PROCEEDED DSCNT TO 8500 FT MSL.