A C340 PLT REQUESTED A GPS APCH IN ACTUAL IMC WITH LITTLE GPS APCH EXPERIENCE. THE AUTOMATION WAS PROGRAMMED TO PROCEED TO AN INCORRECT INITIAL APCH FIX.

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ON SEP/XA/04 A TWIN CESSNA DEPARTED PUB AT APPROX XA00 HRS FOR RNO ON A PART 91 PVT FLT TO REPOSITION THE ACFT. A DUATS WX BRIEFING WAS OBTAINED. THE DEP AND ENRTE PORTIONS OF THE FLT WAS UNREMARKABLE. BECAUSE OF STRONGER THAN PLANNED HEADWINDS AND THE REQUIREMENT TO VECTOR AROUND STORMS AND BUILDUPS A STOP WAS MADE AT TPH FOR FUEL WHERE AN IFR FLT PLAN TO RNO WAS FILED AND AN UPDATED WX BRIEFING WAS OBTAINED ALONG WITH AN ARR REGISTRATION RECEIVED BECAUSE OF AIR RACES THEN IN PROGRESS. THE FLT TO RNO WAS UNREMARKABLE AND CONDUCTED MOSTLY IN IMC CONDITIONS. AFTER HDOF TO APCH CTL; I WAS VECTORED TO THE RWY 34L ILS BACK COURSE APCH. AFTER A SHORT PERIOD WE REQUESTED THE GPS APCH TO RWY 34R. I WAS CLRED TO THE IAF WHICH WAS DIALED INTO THE ACFT GPS SYS AS A DIRECT RTE. APCHING IAF I REQUESTED AND WAS CLRED FOR THE GPS RWY 34R APCH WHICH WAS LOADED INTO THE GPS SYS. THE ACFT'S GPS SYS THEN DIRECTED US TO FLY AN APPROX 225 DEG HDG TO #2 IAF WHICH WAS DONE. WE MUST ASSUME THE GPS SYS DID THIS BECAUSE THE ACFT WAS CLOSER TO #2 IAF THAN TO #1 IAF. PART WAY TO #2 IAF; RNO APCH CTL CALLED AND ASKED WHY WE WERE ON A 225 DEG APPROX HDG AND WE EXPLAINED WE WERE BEING DIRECTED BY THE GPS SYS (TO THE #2 IAF). WE THEN REALIZED WE WERE BACK TRACKING AND AT ABOUT THE SAME TIME WE BECAME CLR OF THE CLOUDS AND HAD THE ARPT AREA APPROX IN SIGHT. WE THEN ASKED FOR AND CANCELED THE GPS RWY 34R APCH AND REQUESTED AND RECEIVED RADAR VECTORS TO RWY 34R WHICH WAS RECEIVED. PROB: THE PLT IS RELATIVELY INEXPERIENCED IN FLYING GPS APCHS IN HEAVILY CTLED AIRSPACE DURING MARGINAL WX CONDITIONS AND SHOULD HAVE ASKED THE CTLR IF HE WANTED US TO BEGIN THE APCH FROM #1 IAF INSTEAD OF #2 IAF. IN RETROSPECT THIS IS LOGICAL AND APPROPRIATE. THE COMPOUNDING PROB WAS THE PLT'S USE OF A DIRECT TO #1 IAF PROC RATHER THAN LOADING THE FULL APCH AND THEN SELECTING AND FLYING THE GPS SEGMENT TO #1 IAF WHICH SHOULD HAVE BEEN DONE. NOTE: I THOUGHT I WAS CLRED TO #1 IAF AS A FIX AND WAS NOT CLRED FOR THE APCH AT THAT TIME. COMPOUNDING ISSUES WERE THE PLT'S LACK OF TOTAL FAMILIARITY WITH THE ACFT GPS SYS AND THE SHOWING OF AN AREA OF INTENSE RAIN AND SHOWERS ON THE ACFT NEXRAD SYS ALONG THE APCH PATH (PROVED TO BE FALSE) WHICH DIVERTED ATTN FROM THE APCH PROC. CLRLY; THE PLT SHOULD HAVE QUERIED THE CTLR AS TO HOW BEST TO PROCEED INSTEAD OF BLINDLY FOLLOWING THE GPS SYS WITHOUT QUESTION. THE PLT REALIZES THAT BACK TRACKING CAN CAUSE PROBS FOR THE CTLR IN HEAVILY CONGESTED AIRSPACE AND PERHAPS LEAD TO ALT VIOLATIONS. CORRECTIVE ACTIONS: PLT WILL GAIN FURTHER EXPERIENCE WITH THE ACFT GPS SYS BEFORE FLYING MORE GPS INST APCHS TO SHARPEN HIS SKILLS BOTH IN A SIMULATOR AND IN THE ACFT.