AN MLG ACR CREW IN VMC DSNDED BELOW THEIR ASSIGNED IFR ALT WHILE LOOKING FOR HLN ARPT. THIS WAS THEIR FIRST TRIP INTO HLN.

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AIRLINES FLT APCHING HELENA REGIONAL ARPT; HELENA; MT; ON A FLT FROM SALT LAKE CITY MOST PROBABLY VIOLATED AN ALT RESTRICTION DURING ITS APCH TO THE ARPT. THE FLT UNDER SLC CTR RADAR CTL AND ON AN IFR FLT PLAN; WAS CLRED TO DSND TO 12000 FT. UPON LEVELOFF AT 12000 FT THE FLT WAS HANDED OFF TO HLN APCH CTL (NON-RADAR). HLN APCH CLRED THE FLT TO 10000 FT AND ASKED CREW TO RPT ARPT IN SIGHT. NEITHER PLT HAD EVER LANDED AT HLN ARPT AND WERE ONLY FAMILIAR WITH THE REQUIRED SPECIAL ARPT QUALIFICATION VIDEO TAPE. EVEN THOUGH THE CREW HAD IDENTED THE APPROX AREA OF THE ARPT; THEY WERE UNABLE TO POSITIVELY IDENT THE RWYS UNTIL 4 NM S OF THE ARPT. DURING THIS PERIOD OF TIME; THE CREW CONTINUED TO DSND AND HAD REACHED AN ALT OF 6500 MSL WHEN THEY RPTED THE ARPT IN SIGHT. HLN APCH THEN CLRED FLT FOR A VISUAL APCH TO RWY 9 AND ASKED CREW TO RPT LEAVING 10000. CREW REPLIED WITH THEIR ACTUAL ALT OF 6500. HLN APCH RESPONDED WITH 'RPT TURN ON TO DOWNWIND'. HLN APCH DID NOT INDICATE THAT THERE WAS ANY PROBLEM WITH THIS ALT AND THERE WAS NO CONFLICT WITH ANY OTHER TFC. A NORMAL VISUAL APCH AND LNDG WAS MADE WITHOUT FURTHER INCIDENT. PRIMARY CAUSE OF INCIDENT: PLT'S UNFAMILIARITY WITH THE HLN ARPT. EVEN THOUGH THE ARPT VIDEO TAPES HAD BEEN RECENTLY REVIEWED AND THE APPROX LOCATION OF THE ARPT WAS KNOWN; THE ARPT RWYS ARE EXTREMELY DIFFICULT TO SEE AS THEY BLEND IN WITH THE SURROUNDING TERRAIN. CONTRIBUTING CAUSE: BOTH PLTS WERE DISTRACTED NOT ONLY BY THEIR ATTEMPTS TO VISUALLY LOCATE THE RWYS BUT ALSO BY GND PROX 'TERRAIN' WARNINGS WHICH ARE NORMAL WHEN DSNDING INTO HLN. THE RESULT OF THE ABOVE WAS NEITHER PLT REALIZING THAT THEY WERE NOT AUTHORIZED TO DSND BELOW THEIR LAST ASSIGNED ALT UNTIL RPTING THE ARPT IN SIGHT AND RECEIVING CLRNC FOR THE VISUAL APCH. BOTH PLTS WERE FOCUSED ON CONTINUING A NORMAL DSCNT TO THE APPROX LOCATION OF THE ARPT IN ORDER TO SET UP FOR THE VISUAL TFC PATTERN AND LNDG. POSSIBLE CORRECTIVE ACTIONS: ALTHOUGH HLN IS ONLY A COMPANY DESIGNATED SPECIAL ARPT AND IS NOT PARTICULARLY HAZARDOUS; IT WOULD BE HELPFUL TO HAVE A LINE CHK AIRMAN ACCOMPANY A CREW ON THEIR FIRST ENTRY IN THE CASE WHERE NEITHER CREWMEMBER HAS HAD AN ACTUAL ENTRY PREVIOUSLY.