Narrative:

After receiving a thorough briefing from the controller being relieved I assumed the arrival radar position. At the time traffic was moderate with 6 aircraft on frequency. Air carrier X was at 4000' heading 050 degrees on the right downwind for runway 22 and air carrier Y was on a modified left base at 4000' heading 270 degrees. Air carrier X was approximately 2-3 mi left of final on the downwind when I turned air carrier Y on the localizer. After turning air carrier Y on the localizer,I issued traffic between the 2 (air carrier Y and air carrier X, 1 O'clock 5 mi) and the same for air carrier X to air carrier Y. When neither had each other in sight and still at 4000', I descended air carrier Y on the approach and issued a heading to the left for air carrier X. After that sep appeared to have been less than 3 mi with 100-200' vertical sep. I believe what really happened was #1) I hurried myself to take the position assuming the controller being relieved was due to go home. I had glanced at the schedule and saw he was an 8-4, but in actuality was working later. #2) no attention or lack of attention to detail. I knew both were at 4000' and assumed they would see each other visually. #3) this occurred within the first mins of an assigned shift just off of a weekend. #4) my attention had been diverted by an air carrier making a right 270 degree back to final in front of these other aircraft. As far as what can be done to keep this from happening again, just be more prepared for the shift, traffic, etc and not assume something (visibility) when it should be already been taken care of (radar or vertical)! Supplemental information from acn 111548: possibly that I am unsure as to what my responsibilities are during this overlap.

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Original NASA ASRS Text

Title: 2 ACFT ON RIGHT AND LEFT BASE LEGS VECTOR TOWARD EACH OTHER. LESS THAN STANDARD SEPARATION OCCURRED.

Narrative: AFTER RECEIVING A THOROUGH BRIEFING FROM THE CTLR BEING RELIEVED I ASSUMED THE ARR RADAR POS. AT THE TIME TFC WAS MODERATE WITH 6 ACFT ON FREQ. ACR X WAS AT 4000' HDG 050 DEGS ON THE RIGHT DOWNWIND FOR RWY 22 AND ACR Y WAS ON A MODIFIED LEFT BASE AT 4000' HDG 270 DEGS. ACR X WAS APPROX 2-3 MI LEFT OF FINAL ON THE DOWNWIND WHEN I TURNED ACR Y ON THE LOC. AFTER TURNING ACR Y ON THE LOC,I ISSUED TFC BTWN THE 2 (ACR Y AND ACR X, 1 O'CLOCK 5 MI) AND THE SAME FOR ACR X TO ACR Y. WHEN NEITHER HAD EACH OTHER IN SIGHT AND STILL AT 4000', I DSNDED ACR Y ON THE APCH AND ISSUED A HDG TO THE LEFT FOR ACR X. AFTER THAT SEP APPEARED TO HAVE BEEN LESS THAN 3 MI WITH 100-200' VERT SEP. I BELIEVE WHAT REALLY HAPPENED WAS #1) I HURRIED MYSELF TO TAKE THE POS ASSUMING THE CTLR BEING RELIEVED WAS DUE TO GO HOME. I HAD GLANCED AT THE SCHEDULE AND SAW HE WAS AN 8-4, BUT IN ACTUALITY WAS WORKING LATER. #2) NO ATTN OR LACK OF ATTN TO DETAIL. I KNEW BOTH WERE AT 4000' AND ASSUMED THEY WOULD SEE EACH OTHER VISUALLY. #3) THIS OCCURRED WITHIN THE FIRST MINS OF AN ASSIGNED SHIFT JUST OFF OF A WEEKEND. #4) MY ATTN HAD BEEN DIVERTED BY AN ACR MAKING A RIGHT 270 DEG BACK TO FINAL IN FRONT OF THESE OTHER ACFT. AS FAR AS WHAT CAN BE DONE TO KEEP THIS FROM HAPPENING AGAIN, JUST BE MORE PREPARED FOR THE SHIFT, TFC, ETC AND NOT ASSUME SOMETHING (VIS) WHEN IT SHOULD BE ALREADY BEEN TAKEN CARE OF (RADAR OR VERT)! SUPPLEMENTAL INFO FROM ACN 111548: POSSIBLY THAT I AM UNSURE AS TO WHAT MY RESPONSIBILITIES ARE DURING THIS OVERLAP.

Data retrieved from NASA's ASRS site as of August 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.