PLT OF CPR SMT ACFT DSNDED BELOW APCH PROC PRESCRIBED ALT RESULTING IN ATC ADVISING THAT THE ACFT WAS BELOW MINS.

Narrative

I WAS READY TO GO HOME THIS AFTERNOON WHEN A COMPANY THAT OPERATES AN SMT ACFT AND AN SMT ACFT CALLED ME TO ASK IF COULD FLY A TRIP TO FORT DODGE; IA; FOR THEM THAT EVENING IN THEIR SMT?! THEIR PLT WAS ON VACATION. I HAD FLOWN EARLIER AND WAS TIRED; BUT I RELUCTANTLY ACCEPTED. I HAD NOT FLOWN THIS PARTICULAR ACFT MODEL BEFORE BUT I HAVE MANY HRS IN TYPE. I CURRENTLY FLY ACFT MODEL MODELS AND ALTHOUGH I HAVE MANY HRS IN THE 'B' MODEL; I HADN'T FLOWN ONE IN MANY YRS. I TOOK OFF FOR 3CK TO PICK UP MY PAX. I WAS CLRED FOR THE VOR 26 APCH TO THE ARPT. I WAS INBOUND WHEN APCH CALLED ME AND SAID THEY SHOWED ME BELOW MINS. I IMMEDIATELY INITIATED A CLB AND TOLD APCH I WAS DOING SO; BACK TO 2300 FT. I WAS DOWN TO 1400 FT WHEN HE CALLED! I CHKED THE KNS-80 DME AND IMMEDIATELY SAW MY PROBLEM. I HAD THE KNS-80 IN THE RNAV MODE WHEN I THOUGHT IT WAS IN THE VOR MODE! I HAD STARTED MY DSCNT TO MDA 11.0 DME OUT FROM THE ARPT; NOT FROM THE VOR LIKE I HAD ASSUMED! THIS CAUSED ME TO START MY DSCNT 5.6 MI TOO EARLY! I COMPLETED THE APCH WITH NO FURTHER PROBLEMS BUT I FELT AWFUL. I KNOW THAT UNFAMILIARITY WITH THIS ACFT HAD CAUSED ME TO DIVERT MY ATTN FROM SETTING UP THE APCH PROPERLY! ADDITIONALLY I WAS FATIGUED AND SHOULD NOT HAVE TAKEN THIS TRIP! AS YOU CAN SEE FROM THE FOLLOWING LETTER; MY TROUBLES WERE NOT OVER YET! IN THE FUTURE; IF I'M TIRED AND NOT UP TO FLYING; I WILL DECLINE ANY TRIPS THAT COME MY WAY! ADDITIONALLY; I DO NOT INTEND TO FLY UNFAMILIAR ACFT IN LOUSY IFR WX. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE LEARNED TO NOT HURRY TO FLY UNFAMILIAR ACFT IN WX CONDITIONS AND BELIEVES THAT ATC SAVED HIS LIFE BY THEIR INTERVENTION. HE FURTHER STATED THAT 'THE FOLLOWING LETTER' REF WAS REFERRING TO ANOTHER ASRS RPT ACN 227108 WHEREIN HE DEVIATED FROM ALT DUE TO ACCIDENTALLY 'KICKING OFF' THE AUTOPLT.