Narrative:

Small transport X inbound from the south at 10000' MSL. I was holding him up for air carrier Y aircraft sebnd at 9000'. I was planning to apply visibility sep once I got the inbound in sight. At that time the inbound was cleared for a visibility approach. I perceived the outbnd traffic to be no longer a factor but issued traffic anyway, first to the inbound and then to the departure. After issuing traffic to the departure, he immediately advised he needed to take evasive maneuvers to miss the inbound who apparently descended quickly and was still south of him. I never verified with the inbound aircraft that he had the departure traffic in sight, and it looks like my perception of the departure being clear of traffic was incorrect. Supplemental information from acn 145578: after departure turned left to a heading to intercept V356 (non radar). Heard small transport X waiting for a clearance into cys. Then I heard ATC clearance small transport X for a visibility approach thought this might be strange considering that we were purposely held to 9000' because he was above us. Then ATC told us that there was an small transport cleared for a visibility approach. We saw him coming right at us at our altitude about 4 seconds before evasive action was taken. I don't think small transport X ever saw us, for he didn't make any apparently attempt to change flight path. Later ATC said that he cleared small transport X for visibility approach because he had both of us in sight, and thought there was plenty of sep--wrong. The evasive action that I took was a sharp descending left turn at about 1700 FPM down to 8100'.

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Original NASA ASRS Text

Title: ACR Y HAD LESS THAN STANDARD SEPARATION FROM SMT X. NMAC. SYSTEM ERROR.

Narrative: SMT X INBND FROM THE S AT 10000' MSL. I WAS HOLDING HIM UP FOR ACR Y ACFT SEBND AT 9000'. I WAS PLANNING TO APPLY VIS SEP ONCE I GOT THE INBND IN SIGHT. AT THAT TIME THE INBND WAS CLRED FOR A VIS APCH. I PERCEIVED THE OUTBND TFC TO BE NO LONGER A FACTOR BUT ISSUED TFC ANYWAY, FIRST TO THE INBND AND THEN TO THE DEP. AFTER ISSUING TFC TO THE DEP, HE IMMEDIATELY ADVISED HE NEEDED TO TAKE EVASIVE MANEUVERS TO MISS THE INBND WHO APPARENTLY DSNDED QUICKLY AND WAS STILL S OF HIM. I NEVER VERIFIED WITH THE INBND ACFT THAT HE HAD THE DEP TFC IN SIGHT, AND IT LOOKS LIKE MY PERCEPTION OF THE DEP BEING CLR OF TFC WAS INCORRECT. SUPPLEMENTAL INFO FROM ACN 145578: AFTER DEP TURNED LEFT TO A HDG TO INTERCEPT V356 (NON RADAR). HEARD SMT X WAITING FOR A CLRNC INTO CYS. THEN I HEARD ATC CLRNC SMT X FOR A VIS APCH THOUGHT THIS MIGHT BE STRANGE CONSIDERING THAT WE WERE PURPOSELY HELD TO 9000' BECAUSE HE WAS ABOVE US. THEN ATC TOLD US THAT THERE WAS AN SMT CLRED FOR A VIS APCH. WE SAW HIM COMING RIGHT AT US AT OUR ALT ABOUT 4 SECS BEFORE EVASIVE ACTION WAS TAKEN. I DON'T THINK SMT X EVER SAW US, FOR HE DIDN'T MAKE ANY APPARENTLY ATTEMPT TO CHANGE FLT PATH. LATER ATC SAID THAT HE CLRED SMT X FOR VIS APCH BECAUSE HE HAD BOTH OF US IN SIGHT, AND THOUGHT THERE WAS PLENTY OF SEP--WRONG. THE EVASIVE ACTION THAT I TOOK WAS A SHARP DSNDING LEFT TURN AT ABOUT 1700 FPM DOWN TO 8100'.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.