A B737-300 CREW AND A C170 PLT HAD LESS THAN LEGAL SEPARATION IN HLN CLASS D AIRSPACE.

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DURING OUR DSCNT INTO HELENA FROM THE N ON V21; WE CONTACTED HELENA TWR AND WERE CLRED FOR A VISUAL APCH LNDG TO RWY 27. THE TWR ASKED US TO RPT 10 MI N; WHICH WE DID. THEY GAVE US A TA ON A CESSNA WHICH HAD DEPARTED RWY 34 AND WAS HEADING E. THE ADVISORY CONTAINED ONLY A RELATIVE POS (12 O'CLOCK POS); SO WE ASKED FOR HIS ALT. SINCE THE CESSNA WAS NOT OPERATING A XPONDER; HE WAS NOT ON OUR TCASII SCREEN AND THE TWR ASKED SEVERAL TIMES FOR MORE SPECIFIC POS AND ALT INFO. HE DID NOT RESPOND TO SEVERAL OF THESE TWR XMISSIONS EXCEPT TO SAY HIS ALT WAS 5300 FT. SO WE LEVELED AT 6000 FT AND CONTINUED TO SEARCH FOR HIM. THE TWR BEGAN TO ASK FOR HIS POS RELATIVE TO THE SMOKE STACKS 3 MI E OF THE RWY 27 THRESHOLD. THE CESSNA FAILED TO RESPOND TO SEVERAL OF THESE REQUESTS; BUT THEN DID GIVE HIS POS AS '3 MI FROM THE RIDGE LINE.' THIS RIDGE LINE IS WELL E OF THE RWY; SO MY COPLT AND I DECIDED TO CONTINUE OUR DSCNT OUT OF 6000 FT AND BEGIN OUR TURN FROM A R BASE LEG TO FINAL. AS WE DID THIS; THE COPLT; WHO WAS FLYING; SAW THE CESSNA DIRECTLY AHEAD APPROX 1 MI; MOVING FROM R TO L AT OR CLOSE TO OUR ALT. HE INCREASED OUR ANGLE OF BANK AND OUR DSCNT RATE. WE PASSED APPROX 1/2 MI BEHIND AND 200 FT BELOW THE CESSNA AND LANDED ON RWY 27. SUPPLEMENTAL INFO FROM ACN 567257: I WAS ON A RETURN FLT HLN TO M46. I RECEIVED ATIS; THEN ASKED TWR TO TAXI TO ACTIVE RWY. WAS TOLD BY TWR THAT THERE WAS NO WIND OR TFC AND THAT I COULD TAKE OFF ON RWY 34. I ASKED FOR A R TURN AFTER CLBOUT; AS I WAS EBOUND. AFTER CLBING FOR 3 OR 4 MINS; I BEGAN GETTING A LOT OF POPPING; STATIC; AND PARTS OF VOICE XMISSION FROM TWR AND OTHER TFC; THROUGH MY HEAD PHONES. I WIGGLED THE HEADPHONE CORD WHERE IT GOES INTO THE EAR CUP AND BEGAN TO PICK UP PARTS OF A CONVERSATION BTWN HLN TWR AND OTHER TFC. TWR ASKED FOR MY HDG AND ELEVATION. I RESPONDED EBOUND AT 5300 FT. TWR THEN ASKED IF I WAS N OR S OF EXTENDED CTRLINE OF RWY 27. I RESPONDED THAT I WAS ABOUT ON CTRLINE; ABOUT 5 OR 6 MI E OF ARPT. TWR THEN SAID 'ACFT Y; TURN R TOWARD THE S;' WHICH I DID. WHILE IN A R BANKED TURN TO THE S; I FIRST SAW THE ACR FLT; WHICH WAS BELOW AND TO THE W OF MY POS. THE ACR FLT WAS IN A R-HAND DSNDING TURN TO LINE UP ON FINAL FOR RWY 27. IT SHOULD BE NOTED THAT THERE WAS MIST/HAZE IN THE AREA. ALSO; THERE WAS SNOW COVER ON THE MOUNTAINS AND IN THE VALLEY AT THE TIME. I DID NOT; BUT SHOULD HAVE HAD MY XPONDER TURNED ON SO THAT THE COLLISION AVOIDANCE EQUIP ON BOARD THE ACR PLANE COULD HAVE PICKED UP MY POS. FOR CORRECTIVE ACTION; I INTEND TO UPGRADE MY RADIO GEAR BY HAVING A BUILT-IN INTERCOM INSTALLED. ALSO; TO PURCHASE A NEW HEADSET THAT HAS THE LATEST ACTIVE NOISE REDUCTION FEATURES. I AM NOT GOING TO FLY INTO ANY AREA REQUIRING RADIO COM UNTIL THE UPGRADES ARE COMPLETE.