Narrative:

I was working a combined sector without a handoff controller. Workload was moderate to busy. Air carrier X contacted me level at 7000, turning to join V64 eastbound. I already had small aircraft Y on the airway eastbound at 11000. Knowing air carrier X could not out climb small aircraft Y (he was in trail to the small aircraft), I had planned to stop air carrier X at 10000 and go underneath small aircraft Y prior to climbing air carrier X to 14000. When I issued the altitude instead of issuing 10000 as intended I issued 11000 which the pilot read back. Thinking air carrier X was climbing to 10000, I issued traffic to air carrier X as 'traffic, 12 O'clock, 6 mi, eastbound, small aircraft Y at 11000' which the pilot rogered for. Shortly thereafter I turned small aircraft Y nebound off the airway to allow air carrier X to continue to climb. While waiting for small aircraft Y to turn out of the way conflict alert was activated and I saw air carrier X at 11000 and in close proximity to small aircraft Y. I immediately issued traffic to air carrier X which the pilot reported in sight. I instructed air carrier X to maintain visual separation from the small aircraft Y and to continue climb. Upon questioning the pilot of air carrier X he stated he was issued a climb to 11000 which was confusing to me as I thought I had issued a climb to 10000. A subsequent review of the tapes showed that I had in fact issued a climb to 11000. I feel that the causal factors in this error were workload at the time and not really listening to what I actually said or the pilot readback. I already had small aircraft Y at 11000 and another aircraft climbing to 11000 and in thinking 10000 I inadvertently issued 11000. A stupid mistake! In the future I plan to call for help sooner and to strive to improve my listening skills to avoid this sort of incident.

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Original NASA ASRS Text

Title: ACR X CLB TO OCCUPIED ALT HAD LTSS FROM SMA Y. SYS ERROR.

Narrative: I WAS WORKING A COMBINED SECTOR WITHOUT A HDOF CTLR. WORKLOAD WAS MODERATE TO BUSY. ACR X CONTACTED ME LEVEL AT 7000, TURNING TO JOIN V64 EBOUND. I ALREADY HAD SMA Y ON THE AIRWAY EBOUND AT 11000. KNOWING ACR X COULD NOT OUT CLB SMA Y (HE WAS IN TRAIL TO THE SMA), I HAD PLANNED TO STOP ACR X AT 10000 AND GO UNDERNEATH SMA Y PRIOR TO CLBING ACR X TO 14000. WHEN I ISSUED THE ALT INSTEAD OF ISSUING 10000 AS INTENDED I ISSUED 11000 WHICH THE PLT READ BACK. THINKING ACR X WAS CLBING TO 10000, I ISSUED TFC TO ACR X AS 'TFC, 12 O'CLOCK, 6 MI, EBOUND, SMA Y AT 11000' WHICH THE PLT ROGERED FOR. SHORTLY THEREAFTER I TURNED SMA Y NEBOUND OFF THE AIRWAY TO ALLOW ACR X TO CONTINUE TO CLB. WHILE WAITING FOR SMA Y TO TURN OUT OF THE WAY CONFLICT ALERT WAS ACTIVATED AND I SAW ACR X AT 11000 AND IN CLOSE PROX TO SMA Y. I IMMEDIATELY ISSUED TFC TO ACR X WHICH THE PLT RPTED IN SIGHT. I INSTRUCTED ACR X TO MAINTAIN VISUAL SEPARATION FROM THE SMA Y AND TO CONTINUE CLB. UPON QUESTIONING THE PLT OF ACR X HE STATED HE WAS ISSUED A CLB TO 11000 WHICH WAS CONFUSING TO ME AS I THOUGHT I HAD ISSUED A CLB TO 10000. A SUBSEQUENT REVIEW OF THE TAPES SHOWED THAT I HAD IN FACT ISSUED A CLB TO 11000. I FEEL THAT THE CAUSAL FACTORS IN THIS ERROR WERE WORKLOAD AT THE TIME AND NOT REALLY LISTENING TO WHAT I ACTUALLY SAID OR THE PLT READBACK. I ALREADY HAD SMA Y AT 11000 AND ANOTHER ACFT CLBING TO 11000 AND IN THINKING 10000 I INADVERTENTLY ISSUED 11000. A STUPID MISTAKE! IN THE FUTURE I PLAN TO CALL FOR HELP SOONER AND TO STRIVE TO IMPROVE MY LISTENING SKILLS TO AVOID THIS SORT OF INCIDENT.

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.