ACR ON DSCNT HAS NMAC WITH SMA.

Narrative

I WAS AN FO ON AIRLINES FLT DISPATCHED FORM SLC TO SUN VALLEY; ID (FRIEDMAN MEMORIAL ARPT). THE FLT WAS IFR AND UNEVENTFUL DURING THE DEP AND CRUISE PHASE. WE WERE GIVEN A DSCNT BY ZLC ABOUT 40-50 MI OUT AND WERE VMC SOON AFTER STARTING THE DSCNT. AT ABOUT 20 MI WE ACCEPTED A VISUAL APCH WITH THE ARPT IN SIGHT. WE IMMEDIATELY CONTACTED HAILEY TWR FOR ADVISORIES. APPROX 12-15 MI SSE OF THE ARPT AN OBJECT ENTERED MY PERIPHERAL VISION FROM THE R. I TURNED TO LOOK DIRECTLY AT THE OBJECT AND SAW A BLUE AND WHITE SINGLE ENG ACFT SMA; LEVEL AND WBOUND ON A COLLISION COURSE ON THE HORIZ PLANE. I QUICKLY POINTED THE ACFT OUT TO THE CAPT WHO WAS FLYING AND HE IMMEDIATELY SAW THE ACFT. OUR SINK RATE WAS OVER 1000 FPM AND CARRIED US UNDER THE APCHING ACFT WITH A SEPARATION OF 100 TO 200 FT. WE IMMEDIATELY ASKED HAILEY TWR IF THEY WERE IN CONTACT WITH THE ACFT AND THEY RESPONDED 'NEGATIVE.' WHILE THIS WAS IN CTLED AIRSPACE; CONDITIONS WERE VMC AND NO CONTACT WITH ATC WAS REQUIRED BY THE OTHER ACFT; I OBSERVED NO EVASIVE ACTION BY THE SINGLE ENG AIRPLANE. HE APPARENTLY WAS IN CRUISE FLT AT APPROX 8500 FT MSL. SEVERAL PAX IN OUR ACFT OBSERVED THE INCIDENT AND MENTIONED IT ON THE GND. DURING OUR DSCNT PRIOR TO THE INCIDENT; THE SMA WAS PATTERNED AGAINST THE MOUNTAINS NE OF OUR RTE. PATCHES OF SNOW ON THE MOUNTAINS MAY HAVE REDUCED THE CONTRAST OF THE SMA AGAINST THE BACKGND. NONETHELESS; THE ENVIRONMENT WAS 'SEE AND AVOID' FOR BOTH ACFT; AND I THINK THE REASON THAT THE ACFT PASSED SO CLOSE WAS A LACK OF PROPER SCAN AND VIGILANCE ON THE PART OF MYSELF AND THE SINGLE ENG PLT. MY CAPT WAS PREVENTED FROM OBSERVING THE OTHER ACFT BY THE LONG NOSE AND WINDSHIELD FRAME. THE ACFT WAS AN SMA. MY CAPT AND I AGREED THAT PRESENCE OF TCASII BACKUP (ASSUMING THE OTHER ACFT HAD A XPONDER) WOULD HAVE BEEN BENEFICIAL.