Narrative:

During an arrival push at atlanta's hartsfield airport, I was working the south final position to runway 27L. Our base leg traffic arrives descending to 5000 ft, our downwind is descending to 7000 ft. The md-88 air carrier X was flowed to the south side to balance the traffic. Air carrier Y E-120 was handed off to me on the base leg out of 6000 ft for 5000 ft. X was descending to 3500 ft (our normal turn-on altitude for the south final). X seemed to stop descending at around 5200 ft, so I issued traffic to Y with the intent of using visual separation. Y reported X 'in sight,' however I did not instruct Y to 'maintain visual separation,' and less than standard separation occurred. At the time of the occurrence I was sure I had issued the visual separation instructions but after listening to the tape I had not. I believe that had X continued a normal descent rate this situation would not have occurred, or if there was a better balance of the traffic I would not have had to put an aircraft on the other final, thus giving me 1 less distraction and allowing me to ensure I had issued the proper instructions.

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Original NASA ASRS Text

Title: ACR X DSCNT THROUGH OCCUPIED ALT HAD NMAC LTSS FROM ACR Y. SYS ERROR.

Narrative: DURING AN ARR PUSH AT ATLANTA'S HARTSFIELD ARPT, I WAS WORKING THE S FINAL POS TO RWY 27L. OUR BASE LEG TFC ARRIVES DSNDING TO 5000 FT, OUR DOWNWIND IS DSNDING TO 7000 FT. THE MD-88 ACR X WAS FLOWED TO THE S SIDE TO BAL THE TFC. ACR Y E-120 WAS HANDED OFF TO ME ON THE BASE LEG OUT OF 6000 FT FOR 5000 FT. X WAS DSNDING TO 3500 FT (OUR NORMAL TURN-ON ALT FOR THE S FINAL). X SEEMED TO STOP DSNDING AT AROUND 5200 FT, SO I ISSUED TFC TO Y WITH THE INTENT OF USING VISUAL SEPARATION. Y RPTED X 'IN SIGHT,' HOWEVER I DID NOT INSTRUCT Y TO 'MAINTAIN VISUAL SEPARATION,' AND LTSS OCCURRED. AT THE TIME OF THE OCCURRENCE I WAS SURE I HAD ISSUED THE VISUAL SEPARATION INSTRUCTIONS BUT AFTER LISTENING TO THE TAPE I HAD NOT. I BELIEVE THAT HAD X CONTINUED A NORMAL DSCNT RATE THIS SIT WOULD NOT HAVE OCCURRED, OR IF THERE WAS A BETTER BAL OF THE TFC I WOULD NOT HAVE HAD TO PUT AN ACFT ON THE OTHER FINAL, THUS GIVING ME 1 LESS DISTR AND ALLOWING ME TO ENSURE I HAD ISSUED THE PROPER INSTRUCTIONS.

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.