AN SR22 PILOT USING AN ADVANCED NAVIGATION SYSTEM FINDS THAT IT DOES NOT FUNCTION AS HE EXPECTED DURING A PRACTICE INSTRUMENT APPROACH.

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WE WERE ON A TRAINING/CURRENCY FLT PRACTICING INST APCHS IN A TECHNICALLY ADVANCED AIRPLANE. WE ACTIVATED ILS RWY 35R APCH TO CENTENNIAL ARPT ON GPS1 (A GARMIN 430) AND SWITCHED THE NAV SOURCE TO VLOC1. UPON BEING VECTORED ONTO FINAL; WE NOTICED THAT THE COURSE ON THE HSI (AN AVIDYNE ENTEGRA PFD) WAS 260 DEGS; NOT THE CORRECT COURSE OF 347 DEGS. THE SEPARATE HDI AND VDI PROVIDED VALID NAV INDICATIONS FOR THE LOC/GS AND GPS1 INDICATED THE CORRECT DTK TO THE NEXT WAYPOINT. HOWEVER; THE ERRONEOUS COURSE ON THE HSI WAS EXTREMELY CONFUSING TO THE PF. WE CHANGED THE HSI NAV SOURCE TO GPS1; AND THIS RESET THE HSI TO THE CORRECT FINAL APCH COURSE (347 DEGS). SWITCHING BACK TO VLOC1 MAINTAINED THE CORRECT COURSE ON THE HSI. WE THINK THE ERRANT FINAL APCH COURSE REMAINED FROM THE PREVIOUS APCH FLOWN (FTG ILS 26). SINCE WE WERE PRACTICING IN VFR CONDITIONS; THIS FLT WAS NOT IN DANGER. HOWEVER; WE ARE CONCERNED ABOUT THIS CONDITION OCCURRING IN IFR; AND WE DISCUSSED HOW WE WOULD HANDLE THAT SITUATION AFTER COMPLETING THE FLT. WE DID NOT RPT THE INCIDENT TO MAINT BECAUSE WE THOUGHT IT WAS A FREAK OCCURRENCE NOT EASILY REPRODUCED. WE SUBMIT THE RPT TO NASA IN THE EVENT OTHER SUCH SITUATIONS ARE OBSERVED AND AN ALERT TO OPERATORS IS DEEMED NECESSARY.