ACR Y HAD POSSIBLE NMAC WITH ACR X. SEE AND AVOID CONCEPT.

Narrative

ACR WAS ON IFR FLT PLAN DSNDING INTO REDMOND; OR; ACR Y HAD BEEN ISSUED CLRNC THROUGH FSS FOR DEP. ACR X RPTED THE ARPT IN SIGHT AND WAS INFORMED THERE WOULD BE A DELAY FOR ISSUANCE OF VISUAL APCH CLRNC DUE TO TFC WITH DEP CLRNC. ACR X CANCELLED IFR AND CONTINUED DSCNT WITH VFR FLT FOLLOWING. THIS IS STANDARD PROC AT RDM WHEN WX PERMITS. THE PLT OF ACR Y DID NOT CHK ON FREQ WHEN EXPECTED SO I CALLED TO SEE IF HE WAS THERE. THERE WAS NO REPLY. THE INBOUND ACFT RPTED THAT HE COULD HEAR THE OTHER ACFT (SAME AIRLINE AS THE DEPARTING ACFT) CALLING ON ADVISORY FREQ AND THAT HE WAS TAXIING. THE INBOUND ACFT WAS DSNDING THROUGH 074 AND I WOULD BE LOSING HIM ON RADAR OUT OF 065. I TOLD THE INBOUND TO CHANGE TO ADVISORY FREQ AS I NOW BELIEVED THE DEP WAS EITHER STILL ON THE GND OR JUST DEPARTING AND HE WOULD BE ABLE TO GET AN ACCURATE POS THROUGH ADVISORY. AS THE INBOUND WAS REPLYING TO THE FREQ CHANGE; THE TARGET OF THE DEP ACFT APPEARED DIRECTLY IN FRONT OF AND 200 FT BELOW THE INBOUND. I TRIED TO CALL THE INBOUND; BUT HE'D ALREADY LEFT FREQ. THE TARGETS MERGED AT THE SAME ALT. THE ACFT THE PLT HAD HEARD TAXIING WAS ANOTHER ACFT CALLING FSS FOR CLRNC. FSS HAD NOT YET CALLED CENTER SO WE DID NOT KNOW HE WAS ON ADVISORY FREQ. IT IS NORMAL FOR PLTS TO CHK ON AT ABOUT 5000-5500 FT ON DEP. THE DEPARTING PLT DID NOT CHK ON UNTIL 075 AND SAID HE HAD NOT SEEN THE ACR X.