Narrative:

Aircraft X inbound to ssi. Aircraft Y en route to udg at 7000 ft. I descended aircraft X to 8000 ft (other traffic at 7000 ft). Once aircraft X was clear I descended aircraft X to 6000 ft (traffic at 5000 ft). Aircraft X was faster than anticipated and descended slower than expected and conflict alert went off between aircraft X and aircraft Y. I called traffic to aircraft X. He did not have aircraft Y in sight so I turned to a 280 degree heading. I called traffic to aircraft Y and he reported aircraft X in sight and reported no factor. I advised aircraft Y to provide visual but vertical was already lost. I had turned aircraft Y to a 080 degree heading and that is when he advised he will provide visual separation. I was controling approximately 14 aircraft at combined (R53; R54) sectors with a radar qualified radar associate; and a tracker which has been plugged in about 5 mins. I was controling 2 arrs into jax which was requiring speed and altitude control for sequencing. 5 aircraft into ssi; bqk (located within 5 mi of each other) ranging from C172 to BE20 to LR60. I had 2 VFR aircraft converging at 9500 ft which did not see each other yet (1 was NORDO) other wanted to climb to 10500 ft for avoidance but traffic sbound ahead at 10000 ft. I was giving VFR flight following to approximately 6 aircraft and numerous more VFR being displayed on radar around ssi. Possible remedies to prevent a recurrence: split the sectors; have arrs into ssi airport lower and handed off to sav approach to provide descent from the north; recognize speed of arrs (I thought the aircraft was slower than actually was). Limit VFR flight following to a minimum. Better traffic plan to ensure other separation rather than assuming visual separation.

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Original NASA ASRS Text

Title: ZJX CTLR EXPERIENCED OPERROR AT 7000 FT WHEN INCORRECTLY ANTICIPATING DSCNT RATE AND CONFLICT DEVELOPED WITH ENRTE ACFT.

Narrative: ACFT X INBOUND TO SSI. ACFT Y ENRTE TO UDG AT 7000 FT. I DSNDED ACFT X TO 8000 FT (OTHER TFC AT 7000 FT). ONCE ACFT X WAS CLR I DSNDED ACFT X TO 6000 FT (TFC AT 5000 FT). ACFT X WAS FASTER THAN ANTICIPATED AND DSNDED SLOWER THAN EXPECTED AND CONFLICT ALERT WENT OFF BTWN ACFT X AND ACFT Y. I CALLED TFC TO ACFT X. HE DID NOT HAVE ACFT Y IN SIGHT SO I TURNED TO A 280 DEG HDG. I CALLED TFC TO ACFT Y AND HE RPTED ACFT X IN SIGHT AND RPTED NO FACTOR. I ADVISED ACFT Y TO PROVIDE VISUAL BUT VERT WAS ALREADY LOST. I HAD TURNED ACFT Y TO A 080 DEG HDG AND THAT IS WHEN HE ADVISED HE WILL PROVIDE VISUAL SEPARATION. I WAS CTLING APPROX 14 ACFT AT COMBINED (R53; R54) SECTORS WITH A RADAR QUALIFIED RADAR ASSOCIATE; AND A TRACKER WHICH HAS BEEN PLUGGED IN ABOUT 5 MINS. I WAS CTLING 2 ARRS INTO JAX WHICH WAS REQUIRING SPD AND ALT CTL FOR SEQUENCING. 5 ACFT INTO SSI; BQK (LOCATED WITHIN 5 MI OF EACH OTHER) RANGING FROM C172 TO BE20 TO LR60. I HAD 2 VFR ACFT CONVERGING AT 9500 FT WHICH DID NOT SEE EACH OTHER YET (1 WAS NORDO) OTHER WANTED TO CLB TO 10500 FT FOR AVOIDANCE BUT TFC SBOUND AHEAD AT 10000 FT. I WAS GIVING VFR FLT FOLLOWING TO APPROX 6 ACFT AND NUMEROUS MORE VFR BEING DISPLAYED ON RADAR AROUND SSI. POSSIBLE REMEDIES TO PREVENT A RECURRENCE: SPLIT THE SECTORS; HAVE ARRS INTO SSI ARPT LOWER AND HANDED OFF TO SAV APCH TO PROVIDE DSCNT FROM THE N; RECOGNIZE SPD OF ARRS (I THOUGHT THE ACFT WAS SLOWER THAN ACTUALLY WAS). LIMIT VFR FLT FOLLOWING TO A MINIMUM. BETTER TFC PLAN TO ENSURE OTHER SEPARATION RATHER THAN ASSUMING VISUAL SEPARATION.

Data retrieved from NASA's ASRS site as of January 2009 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.