C172 ACT ON IFR FLT PLAN AND UNDER APCH CTLR COM. RPTR TOLD CTLR HE WOULD CANCEL IFR ON THE GND; BUT FORGOT TO CLOSE IT. AFTER BREAKING OUT OF CLOUDS AT ARPT NUMEROUS ACFT WERE LNDG AND DEPARTING. RPTR BECAME CONFUSED AND ENTERED PATTERN FOR WRONG RWYS AND HAD TO DEV ON AN NMAC.

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I WAS ENRTE TO A 'FLY-IN' BREAKFAST AT C91 FROM MY HOME BASE GRR ON AN INST FLT PLAN. THE FLT WAS CONDUCTED AT A CRUISE ALT OF 4000 FT MSL WITH A SINGLE PLT ON BOARD. SHORTLY BEFORE ARRIVING AT THE IAF; ELX VOR; FOR THE C91 VOR ALPHA APCH; I ENTERED IMC. SBN APCH ADVISED THAT I COULD EXPECT A DSCNT IN 5 MI. I SET MY #2 NAV RADIO TO IDENT THE FAF (SABAH) AND AS A BACKUP I REQUESTED SBN TO CALL SABAH WHEN I WAS OVER THE FAF. OVER SABAH; SBN APCH CALLED AND ALSO ASKED IF I WISHED TO CANCEL IFR NOW OR ON THE GND. I ADVISED THEM THAT I WAS STILL IMC AND THAT I WOULD CANCEL ON THE GND. I WAS ADVISED OF TARGETS IN THE AREA AND ELECTED TO STAY WITH SBN APCH FOR ADVISORIES AND SEPARATION UNTIL I WAS VISUAL. I ENTERED VMC AT 2000 FT MSL 3 MI FROM THE ARPT WITH AN INFLT VISIBILITY OF 4-5 MI. DURING OUR DSCNT MY FRONT SEAT PAX QUICKLY POINTED OUT A NAVION; 100 FT ABOVE AND 500 FT TO THE L OF US. THE NAVION WAS CLBING OUT OF THE DOWAGIAC TFC PATTERN. I PERFORMED A QUICK DSCNT TO 1500 FT MSL AND MADE A SERIES OF S-TURNS TO AVOID THE TFC AND ENTER THE DOWNWIND FOR RWY 9. AT THE TIME THERE WRE NUMEROUS OTHER ACFT ENTERING AND DEPARTING THE C91 TFC PATTERN. BECAUSE OF THE CONFUSION I CALLED THE WRONG RWY NUMBERS ON THE CTAF TWICE; FURTHER AGGRAVATING THE SIT. (I CALLED RWY 22 AND THEN RWY 27 -- RWY 9 WAS IN USE AND WAS THE RWY I INTENDED TO USE.) I ALSO THOUGHT I CALLED SBN APCH AND CANCELED MY IFR FLT PLAN. ALL WAS CLRED UP IN SHORT ORDER AND THE REST OF THE FLT WAS UNEVENTFUL. I SPENT THE TIME ON THE GND THINKING OVER THE FLT AND COLLECTING MY THOUGHTS. BELIEVING THAT I HAD CLOSED MY IFR FLT PLAN WITH SBN APCH I DID NOT CALL FSS. LATER I RECEIVED A NOTE FROM AN EVENT ORGANIZER ASKING ME TO CALL SBN APCH. I TELEPHONED SBN APCH AND I WAS ADVISED; BY THE CTLR; THAT I HAD NOT CLOSED MY FLT PLAN. I EXPLAINED TO THE CTLR THAT I THOUGHT I HAD CLOSED THE FLT PLAN IN THE AIR; BUT THAT THERE HAD BEEN A LOT OF CONFUSION AT THE TIME. WHAT DECISIONS WILL I MAKE NEXT TIME? I WILL NOT FLY TO A 'FLY-IN' AT AN UNCTLED FIELD IN MARGINAL WX; WHERE THE LIKELIHOOD EXISTS THAT THE APCH WILL REQUIRE ACTUAL IMC FLT. I WILL ALSO BE MORE ASSERTIVE WITH APCH CTL REGARDING THE LOCATIONS OF OTHER TARGETS NEAR A FAF. IF I HAPPEN TO ENTER IMC DURING AN APCH AT A 'FLY-IN;' I WILL REQUEST AN IMMEDIATE MISSED APCH IF I AM NOT VMC BY 2500 FT AGL AT LEAST 5 MI FROM THE ARPT WITH AN INFLT VISIBILITY OF AT LEAST 5 MI.