ACR SF340A FLC EXECUTE EVASIVE ACTIONS DUE TO NMAC WITH C172 WHILE ON STRAIGHT-IN APCH TO STC ARPT. FO ADMITS TO FAST APCH SPD WITHIN 20 MI OF A BUSY; NON TWR CTL ARPT.

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I WAS THE FO ON A SAAB 340A FROM MSP TO STC AND WAS THE PF. AFTER DEP FROM MSP WE WERE CLRED DIRECT TO STC AT 4000 FT MSL BY MSP. WE WERE ALIGNED WITH THE EXTENDED LOC COURSE; SO I STARTED TRACKING THE LOC FOR THE ILS RWY 31 AT APPROX 30 NM OUT. WE COULD SEE PROMINENT STC LANDMARKS FROM 30 NM OUT. AT APPROX 20 NM OUT OF STC; ATC CLRED US FOR THE VISUAL APCH TO RWY 31 AND ADVISED US OF NUMEROUS TARGETS IN THE VICINITY OF STC. WE HAD THE TCASII SET TO THE 20 NM RANGE AND COULD SEE SEVERAL TARGETS IN THE AREA AT VARIOUS ALTS. THE CAPT WAS XMITTING OUR INTENTIONS ON CTAF AND WE HEARD AT LEAST ONE ACFT SAY HE WAS HOLDING AT THE OM. AT THIS TIME WE WERE INDICATING 240 KIAS AND AT 4000 FT. THE CTAF FREQ WAS BUSY WITH STC TFC CALLS. THE CAPT ASKED THE ACFT AT THE OM WHAT HIS INTENTIONS WERE: HIS REPLY WAS TO THE EFFECT THAT HE WOULD CONTINUE HOLDING. AT APPROX 10 NM FROM THE OM; THE CAPT DECIDED TO CANCEL IFR. I TURNED THE AUTOPLT OFF AND WAS FLYING. BY NOW THE TCASII WAS DISPLAYING MULTIPLE TARGETS CLOSE IN AND SO CLOSE TO EACH OTHER; IT WAS HARD TO PICK OUT AT WHAT ALT THEY WERE AT. WE WERE FLYING INTO THE SUN. AT APPROX 5 NM FROM THE OM; WE GOT A TA FOR THE ACFT OFF OUR L AND BELOW US. THE CAPT SAID TO LEVEL OFF UNTIL WE SEE THE TFC. WHILE THE TA OCCURRED; THE CAPT WAS ASKING THE CESSNA TFC WHAT HE WAS GOING TO DO; WHICH HE RESPONDED HE WOULD CLB TO GET OUT OF OUR WAY. THE CAPT TOLD THE CESSNA TO TURN S; AWAY FROM US AND NOT TO CLB AS WE WERE ABOVE HIM. IMMEDIATELY FOLLOWING HIS XMISSION; WE GOT THE RA 'CLB.' I WAS LOOKING OUTSIDE AND PITCHED UP TO 10 DEGS AND ADDED PWR. SOON AFTER; I PITCHED TO 20 DEGS UP AND CHKED MY PWR AT 100%. I SEE THE C172 PASS UNDER THE L SIDE SLIGHTLY BELOW US AND -300 FT SHOWED ON THE TCASII; ABOUT 1/4 MI; IF THAT. AFTER SIGHTING THE TFC I REDUCED PWR AND LEVELED OFF. WE CONFIGURED THE ACFT FOR LNDG. JUST AFTER PASSING THE FIRST TFC THERE WAS A SECOND CESSNA SIGHTED AT THE OM; AND THEN WE STARTED TO SEE THE OTHERS IN THE PATTERN. NONE OF THE OTHER ACFT HAD LNDG LIGHTS ON. DUE TO OUR HIGH SPD; THE CLOSURE RATE WAS PROBABLY CLOSE TO 300 KTS WITH THE CESSNA HEADING TOWARDS US. I DON'T THINK HE REALIZED THAT WE WOULD COVER THE 10 MI IN LESS THAN 2 MINS. THIS WHOLE INCIDENT TOOK PLACE WITHIN 3 MINS. WE SHOULD HAVE SLOWED DOWN TO APCH SPD WITH THE AMOUNT OF TFC IN THE AREA TO GIVE US MORE TIME TO SEE THE OTHERS. I SHOULD HAVE WAITED TO START DOWN UNTIL AFTER WE CLRED THE CESSNA TFC INSTEAD OF FOLLOWING THE GS. WE ALSO HAD ALL EXTERIOR LIGHTS ON AND WE SHOULD HAVE BEEN VERY VISIBLE. I STRONGLY THINK THAT IF THEY HAD THE LNDG LIGHTS ON WE MAY HAVE SEEN THEM SOONER. WITHOUT TCASII; WE WOULD HAVE BEEN MUCH CLOSER OR EVEN IMPACTED WITH THE CESSNA. WITH TWO OF THEM THERE; IT CLUTTERED THE TCASII SO WE COULD NOT SEE CLRLY WHAT WAS GOING ON. THIS ARPT IS EXTREMELY BUSY DURING SUMMER DAYLIGHT HRS WITH A MIX OF AIRLINE TURBOPROPS AND NUMEROUS TRAINING ACFT AND COULD DEFINITELY BENEFIT FROM A CTL TWR THAT OPERATED DURING THESE PEAK USE PERIODS.