Narrative:

At approximately XXXX local, I issued clearance to large transport X to descend now to FL310, then at pilot's discretion, maintain FL280, in order to stay above large transport Y at FL270. Large transport X read back only, half of the clearance to descend to FL310. I reissued the clearance to descend now to FL310, then at pilot's discretion maintain FL240, thinking all the time I had said FL280. I also issued traffic to large transport X on his company flight at 10 O'clock and 3 mi. Large transport X acknowledged, and out of FL294. Large transport X called his traffic in sight. At approximately XX32 I cleared large transport X to fly heading 080 degrees when able in order to get the aircraft away from the company flight at FL270, still thinking large transport X had clearance to FL280. At approximately XX33 local, company flight large transport Y requested right deviation for WX, and I approved. I cleared large transport X to fly heading 095 degrees when able to compensate for company deviation. At XX33 local, C/a activated between large transport Y and large transport X whose mode C indicated FL276. I immediately requested large transport X to verify at FL280 which he denied saying he recieved clearance to FL240, which I said negative a stated clearance was FL280. I told large transport X to fly heading 120 degrees immediatley, and large transport Y to fly heading 360 degrees to obtain sep which both aircraft did. Large transport X climbed back to FL280. Aircraft's closest proximity was 3.06 mi lateral, 500' vertical. I had been on the sector for 1 hour, 07 mins when the incident occurred.

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Original NASA ASRS Text

Title: AN ACFT DESCENDING TO ASSIGNED ALT LOST SEPARATION WITH ANOTHER ACFT.

Narrative: AT APPROX XXXX LCL, I ISSUED CLRNC TO LGT X TO DSND NOW TO FL310, THEN AT PLT'S DISCRETION, MAINTAIN FL280, IN ORDER TO STAY ABOVE LGT Y AT FL270. LGT X READ BACK ONLY, HALF OF THE CLRNC TO DSND TO FL310. I REISSUED THE CLRNC TO DSND NOW TO FL310, THEN AT PLT'S DISCRETION MAINTAIN FL240, THINKING ALL THE TIME I HAD SAID FL280. I ALSO ISSUED TFC TO LGT X ON HIS COMPANY FLT AT 10 O'CLOCK AND 3 MI. LGT X ACKNOWLEDGED, AND OUT OF FL294. LGT X CALLED HIS TFC IN SIGHT. AT APPROX XX32 I CLRED LGT X TO FLY HDG 080 DEGS WHEN ABLE IN ORDER TO GET THE ACFT AWAY FROM THE COMPANY FLT AT FL270, STILL THINKING LGT X HAD CLRNC TO FL280. AT APPROX XX33 LCL, COMPANY FLT LGT Y REQUESTED RIGHT DEVIATION FOR WX, AND I APPROVED. I CLRED LGT X TO FLY HDG 095 DEGS WHEN ABLE TO COMPENSATE FOR COMPANY DEVIATION. AT XX33 LCL, C/A ACTIVATED BTWN LGT Y AND LGT X WHOSE MODE C INDICATED FL276. I IMMEDIATELY REQUESTED LGT X TO VERIFY AT FL280 WHICH HE DENIED SAYING HE RECIEVED CLRNC TO FL240, WHICH I SAID NEGATIVE A STATED CLRNC WAS FL280. I TOLD LGT X TO FLY HDG 120 DEGS IMMEDIATLEY, AND LGT Y TO FLY HDG 360 DEGS TO OBTAIN SEP WHICH BOTH ACFT DID. LGT X CLBED BACK TO FL280. ACFT'S CLOSEST PROX WAS 3.06 MI LATERAL, 500' VERT. I HAD BEEN ON THE SECTOR FOR 1 HR, 07 MINS WHEN THE INCIDENT OCCURRED.

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.