PA32 INSTRUCTOR PLT RPTS EARLY DSCNT DURING APCH TO STC UTILIZING DUAL GPS EQUIP.

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AS WE WERE APCHING STC VOR ON AN IFR FLT PLAN; WE REQUESTED 2 FULL VOR DME RWY 13 PRACTICE APCHS INTO STC. THE ACFT WAS EQUIPPED WITH DUAL GARMIN 530 AND 43C GPS UNITS; BUT WITHOUT DME. WE WERE INITIALLY CLRED DIRECT TO THE STC VOR AND IN PREPARATION FOR THE VOR DME RWY 13 APCH WE PROGRAMMED THE #2 GPS TO THE STC VOR TO SUBSTITUTE FOR THE DME. WE BRIEFED THE APCH; SET THE FREQS; AND BACKED UP THE APCH WITH THE #1 GARMIN 43C GPS FOR GENERAL REF. FOR SOME REASON ZMP DID NOT CLR US FOR THE VOR DME RWY 13 APCH AS WE WERE PASSING THE VOR OUTBOUND AND ASKED US TO RPT PROC TURN INBOUND. I QUERIED THE CTLR REGARDING THIS UNUSUAL PROC; SINCE WE WERE USED TO BEING CLRED FOR A FULL VOR APCH AS WE PROCEEDED OUTBOUND TO THE PROC TURN. HE ONLY INDICTED THAT WE WERE NOT CLRED FOR THE APCH. WE CONTINUED OUTBOUND AT THE ASSIGNED ALT AND STARTED OUR PROC TURN. AS WE BEGAN TO TURN INBOUND; WE MADE OUR RPT TO CTR AND WE WERE CLRED FOR THE VOR DME RWY 13 APCH AT STC. ESTABLISHED ON THE INBOUND COURSE; WE DSNDED TO 2800 FT. THE #1 GPS SHOWED US OFF COURSE AND PARALLEL TO THE INBOUND COURSE. WE CHKED BOTH VORS; AND THEY BOTH WERE WELL WITHIN TOLERANCES FOR A DUAL VOR CHK AND BOTH WERE CORRECTLY IDENTED. WE DECIDED TO RELY ON THE VORS AND CHKED THE INDICATION OF THE #2 GPS RECEIVER; WHICH WE HAD SET TO READ AND SUBSTITUTE FOR THE DME FROM STC VOR. THE #2 GPS INDICATED THAT WE WERE WELL INSIDE FIMKO (FAF); SO WE BEGAN OUR DSCNT TO 1800 FT UNTIL PASSING JATSA (A STEPDOWN FIX). THEN THE DISTANCE INDICATED THAT WE HAD PASSED JATSA. WE WERE BREAKING OUT OF THE OVCST WELL ABOVE MINIMUMS WHEN IT BECAME CLR THAT WE WERE FURTHER OUT FROM THE ARPT/VOR THAN THE DISTANCE ON THE #2 GPS INDICATED. WE STOPPED OUR DSCNT AND SINCE WE WERE IN VISUAL CONDITIONS TRIED TO IDENT THE PROB. AS IT TURNS OUT; THE GPS EQUIP WAS PRESET TO AUTOMATICALLY XFILL FROM THE #1 GPS TO THE #2 GPS. THIS RESULTED IN THE #2 GPS NOT INDICATING THE DISTANCE TO THE STC VOR AS IT HAD BEEN SET; BUT INSTEAD INDICATING XFILLED DATA FROM THE #1 GPS SHOWING THE DISTANCE FROM FIMKO; THEN JATSA AND FINALLY THE STC VOR AS IT CYCLED THROUGH THE APCH FIXES. THE CONSEQUENCE WAS POSSIBLY A SOMEWHAT PREMATURE DSCNT THAT WE ARRESTED AS SOON AS THE PROB BECAME APPARENT. WE EXECUTED A MISSED APCH AND FLEW ANOTHER APCH WITH THE AUTO XFILL OFF WITHOUT FURTHER DIFFICULTIES. THIS SHOWS THAT THE RELIANCE ON EVER MORE COMPLICATED AND NON STANDARDIZED AVIONICS CREATES MANY ISSUES TO BE AWARE OF. FORTUNATELY THE WX CONDITIONS WERE REASONABLY GOOD; WE NOTICED THE INCORRECT INDICATIONS EARLY AND WE DID NOT FACE HIGH OBSTACLES OR MOUNTAINOUS TERRAIN. WHEN AUTO XFILL IS PRESENT ON GPS EQUIP; THE PLT SHOULD AT LEAST BE PROMPTED TO ACKNOWLEDGE THIS BEFORE THE AUTO XFILL OCCURS; SIMILARLY TO HOW THE PLT HAS TO ACKNOWLEDGE WHEN SELECTING NON GPS APCHS THAT THE DATA IS TO BE USED FOR REF ONLY AND NOT FOR NAV GUIDANCE.