AN A320 FLT CREW DEVIATED FROM CHARTED TRACK DURING APCH TO RWY 12 AT BZN WHEN THEY FOLLOWED FMS GUIDANCE THAT TOOK THEM BEYOND THE 10 MI LIMIT DURING PROC TURN MANEUVER.

Narrative

DEPARTED MSP-BZN ON FLT XXA; BZN WX SHOWED GOOD VMC WITH CALM WIND AND FORECASTED FOR VMC. IT HAD BEEN 2+ YRS SINCE I HAD LAST FLOWN INTO BZN SO WE THOROUGHLY REVIEWED THE BZN COMMERCIAL CHART INFO ENRTE ON THE PREVIOUS LEG AND HAD QUERIED DISPATCH REGARDING SNOWFALL AND WIND CONDITIONS DUE TO LNDG RESTRS. ENRTE TO BZN WE AGAIN REVIEWED BZN COMMERCIAL CHART PAGES; DISCUSSED HOW WE WOULD EXECUTE THE FULL APCH USING MANAGED NAV AND REGULARLY UPDATED BZN; GTF; AND SEA (LISTED ALTERNATES) WX WHICH ALL INDICATED VMC. I LOADED THE ILS RWY 12 VIA BZN INTO THE FMS FLT PLAN; AND WE ARMED OUR TERR ON NAV DISPLAY (EGPWS) SWITCHES. ON DSCNT I ATTEMPTED TO COPY THE BZN ATIS BUT WAS UNABLE AS THE LISTED ATIS FREQ WAS ERRONEOUS. SO WE RECEIVED THE LATEST 'SA' RPT WHICH STILL SHOWED VMC. APPROX 40 NM OUT ZLC ADVISED US THAT BZN WAS SUDDENLY 1/4 MI VISIBILITY WITH FZFG AND ASKED OUR INTENTIONS. WE RECEIVED HOLDING INSTRUCTIONS AT MANNI AND CONSULTED WITH DISPATCH. WE COLLECTIVELY AGREED TO DIVERT TO GTF IF NEEDED AND CALCULATED BINGO FUEL. AFTER ABOUT 1.5 TURNS IN HOLDING SLC ADVISED US THAT THE BZN VISIBILITY WAS 1/2 MI AND ASKED IF WE WANTED APCH CLRNC FOR THE ILS RWY 12. WE ACCEPTED THE APCH CLRNC AND WERE HANDED OFF TO BZN TWR WHO REQUESTED THAT WE RPT MANNI INBOUND. ONCE ESTABLISHED WE DSNDED TO THE 'PT' ALT. WE WONDERED WHY THE OUTBOUND SEGMENT SEEMED LONGER THAN EXPECTED BUT DECIDED TO CONTINUE ON THE FMS COURSE WITH CAUTION (NAV DISPLAY SHOWED A 30 KT HEADWIND ON THE OUTBOUND LEG). AS WE STARTED OUR 'PT' OUTBOUND BZN TWR INSTRUCTED US TO CONTACT ZLC WHO IMMEDIATELY TOLD US TO CLB TO 11000 FT DUE TO A LOW ALT WARNING AND TO PROCEED DIRECT TO BZN VOR; WE HAD FLOWN BEYOND 10 NM FROM MANNI BY FOLLOWING THE FMS APCH PATH. WE COMPLIED WITH THE CLB AND ROUTING. APCHING THE VOR THE BZN ARPT WAS NOW VMC; AND WE COULD EASILY SEE THE RWY. SLC ISSUED US CLRNC FOR THE VISUAL APCH WHICH WAS COMPLETED TO A LNDG AFTER WHICH WE CONTACTED SLC BY PHONE. CAUSE: LOSS OF SITUATIONAL AWARENESS BY THIS CREW WITH REGARD TO OUR DISTANCE FROM MANNI. CONTRIBUTING FACTORS: 1) UNFAMILIARITY WITH THE POTENTIAL FOR OUR FMS TO BUILD AN ELONGATED APCH COURSE. I ASSUMED THAT FMS APCHS WERE BUILT WITH 'HARD' (GEOGRAPHICALLY DEFINED) POINTS IN SPACE AND NOT SUBJECT TO KNOWN HOLDING PATTERN ANOMALIES. 2) FATIGUE DUE TO A SCHEDULED 12 HR DUTY DAY BECOMING ALMOST 13 HRS. 3) LONG TIME LAPSE SINCE LAST EXECUTING A FULL APCH. CORRECTIVE ACTIONS: 1) MONITOR DISTANCE FROM 'ANCHOR' FIXES USING THE PROG PAGE; THE NAV DISPLAY ON THE NAV DISPLAY; AND TIMING. 2) EMPHASIZE THE DISTANCE RESTR ON THE APCH BRIEFING. 3) AGGRESSIVELY AND IMMEDIATELY INVESTIGATE ANY GUT FEELINGS THAT SOMETHING IS AMISS.