ACR X HAD NMAC WITH SMA. SEE AND AVOID CONCEPT.

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ACR X APCHED HAILEY NDB FROM THE W AT 15000 FT MSL. WE SIGHTED THE HAILEY ARPT (10 NM NW OF HLE NDB) IN CAVU WX AND REQUESTED A VISUAL APCH. SLC ARTCC WAS UNABLE TO CLR US FOR THE VISUAL BECAUSE OF AN ACR; LTT TAXIING OUT FOR AN IFR DEP. I THEN CANCELLED IFR AND INSTRUCTED THE FO TO DSND RAPIDLY TO GET UNDER THE DEPARTING LTT ACFT. NORMAL VFR ARR PROCS ENTAIL PROCEEDING OVER THE NDB TO A POINT APPROX 4 NM E OF THE NDB WITH A L TURN UP THE E SIDE OF THE VALLEY FOR A STRAIGHT-IN APCH TO RWY 31 AT HAILEY. THIS KEEPS US OUT OF THE PATH OF DEPARTING IFR ACFT WHO NORMALLY PROCEED OUT TO THE NDB AND THEN ON COURSE. OUR COMPANY PROCS DO NOT SPECIFY XING ALTS FOR HLE NDB DURING DAY VFR OPS. DURING THE DSCNT WE CHANGED FREQ TO HAILEY TWR; AND I OVERHEARD AN SMA CALL TWR S OF HAILEY FOR FREQ CHANGE OUTBOUND. I MOMENTARILY CONSIDERED ASKING TWR FOR HIS ALT BUT WAS MORE CONCERNED ABOUT THE LTT. BESIDES THE SMA HAD CHANGED FREQ AND HAD SAID NOTHING ABOUT HIS ALT OR DISTANCE FROM HAILEY. I ASSUMED HE WOULD BE 1000-2000 FT AGL. THE FO THEN CALLED FOR THE APCH CHKLIST WHICH I BEGAN AS WE CROSSED THE HLE NDB AT 8500 FT MSL (+\- 3500 FT AGL). A SPLIT SECOND LATER HE YELLED 'LOOK!' AND THREW HIS ARM ACROSS MY FRONT TOWARD THE L. AS I JERKED AROUND; I SAW A BROWN/YELLOW SINGLE ENG SMA JUST AHEAD OF THE L WING ABOUT 20 FT HIGHER AND +\- 50 YARDS AWAY. WE HAD NO TIME FOR EVASIVE MANEUVER; BUT HE BEGAN A BREAK TO HIS R AS I ACQUIRED HIM. OUR TCASII WAS OPERATIONAL; BUT NO ALERT WAS GIVEN. HE EITHER HAD NO TRANSPONDER OR HAD FAILED TO TURN IT ON. TWR HAD NOT ADVISED US OF THE OUTBOUND TFC NOR WAS HE REQUIRED TO. THE REMAINDER OF THE APCH AND LNDG WAS COMPLETED WITHOUT FURTHER INCIDENT. TWR WAS INFORMED BOTH ON THE RADIO AND BY TELEPHONE AFTER ARR.