C175 PILOT REPORTS CONFLICT WITH LIGHT TWIN DURING VISUAL APPROACH TO LWS; AFTER BEING TOLD TRAFFIC NO FACTOR.

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I WAS ON A DAYTIME VFR FLT TO LEWISTON VIA RENGO AND V187. I WAS 10 MI OUT AND JUST REACHING FOR THE PTT SWITCH TO CONTACT THE TWR WHEN ANOTHER ACFT (IDENTING SELF AS 'ACFT Y') CALLED IN FROM 15 MI OUT AND A LOWER ALT ANNOUNCING INTENTION TO LAND. I THEN ANNOUNCED MYSELF AND EXCHANGED ALT AND LOCATION INFO WITH THE TWR WHO THEN CONTACTED THE ACFT Y WITH THIS INFO. THIS CONVERSATION RESULTED IN A STATEMENT BY THE TWR OF 'TFC NO FACTOR.' I WAS CLRED TO LAND AND TOLD TO ANNOUNCE MYSELF ON A R BASE FOR RWY 11. WHILE ON R BASE; OFF THE END OF RWY 8 AND ANNOUNCING MY POS THE TWR WARNED OF TFC IN AN URGENT TONE. I DON'T RECALL EXACTLY WHAT THE TWR SAID BUT IT WAS A TFC WARNING AND SOMETHING LIKE 'HALF MI' AND AN ALT. I RESPONDED 'NEGATIVE CONTACT' AND VERY SHORTLY AFTERWARD OBSERVED A TWIN ENG REGIONAL CARRIER TYPE ACFT PASSING DIRECTLY BENEATH ME. I DO NOT KNOW THE VERT SEPARATION WAS BTWN US. IT SEEMED ON THE ORDER OF 300-400-500 FT BUT I AM NOT AN EXPERIENCED JUDGE OF DISTANCE AND AS A STUDENT PLT FLYING INTO AN UNFAMILIAR CTLED FIELD PERCHED ON THE EDGE OF A CLIFF I WAS FOCUSED ON MAKING THE APCH AND LNDG; NOT TO MENTION STILL FEELING THE ADRENALINE FROM THE TWR'S TFC WARNING CALL. I DO NOT KNOW WHETHER THE OTHER ACFT HAD ME IN SIGHT. I DID NOT HEAR ANY INDICATION THAT THE OTHER ACFT HAD SEEN ME. MY MISTAKE WAS IN CEASING TO BE CONCERNED ABOUT THE OTHER ACFT WHEN I HEARD THE TWR PROCLAIM; 'TFC NOT A FACTOR.' I WAS 5 MI CLOSER THAN THE OTHER ACFT AND TRAVELING AT APPROX 135 MPH AND I ASSUMED THE OTHER ACFT MUST BE VERY MUCH FASTER THAN I AND WOULD GET TO THE ARPT WELL AHEAD OF ME. STUPID ASSUMPTION OF COURSE. THE R BASE I WAS CLRED FOR PUT ME SMACK IN THE PATH OF THE ACFT Y LNDG STRAIGHT-IN ON THE OTHER RWY. I SHOULD HAVE TAKEN A DETOUR OR NOT STARTED MY DSCNT UNTIL I KNEW FOR MYSELF THAT THE OTHER ACFT WAS NOT A FACTOR. PROBABLY SHOULD HAVE ALSO PLANNED MY R BASE TO STAY CLEAR OF THE APCH ON THE INTERSECTING RWY. I AM OLDER AND WISER NOW.