Narrative:

B737 incorrectly read back assigned altitude. Even after issuing traffic; the B737 continued to climb. B737 reported on frequency and asked about ride conditions and requested FL400. I told air carrier X it would be 1 1/2 mins before a climb could be issued and to expect higher. During this time I decombined sectors and was in the process of setting my map for high altitude only when I issued a clearance for air carrier X to climb to FL320. My d-side was off on the line coordinating a pointout which also captured my attention while air carrier X was reading back their clearance. I applied merging target procedures by issuing traffic to both MD80 and to air carrier X. While the aircraft were still vertically separated and 20 mi apart; air carrier X questioned nothing. Air carrier X reported air carrier Y in sight. They were still vertically separated. I told air carrier X to expect a higher altitude when clear of traffic. An aircraft asked to verify climb clearance to FL380. The voice seemed similar to that of air carrier X. I asked air carrier X to say again. An aircraft without identing themselves again asked to verify climb clearance to FL380. I told air carrier X to maintain FL320. Separation was lost after the 2 aircraft passed. Closest proximity 2.5 mi 800 ft.

Related reports:

Google
 

Original NASA ASRS Text

Title: ZFW CTLR EXPERIENCED OPERROR AT APPROX FL330 WHEN ACFT INCORRECT READBACK WAS MISSED AND CLB THROUGH OCCUPIED ALT INITIATED.

Narrative: B737 INCORRECTLY READ BACK ASSIGNED ALT. EVEN AFTER ISSUING TFC; THE B737 CONTINUED TO CLB. B737 RPTED ON FREQ AND ASKED ABOUT RIDE CONDITIONS AND REQUESTED FL400. I TOLD ACR X IT WOULD BE 1 1/2 MINS BEFORE A CLB COULD BE ISSUED AND TO EXPECT HIGHER. DURING THIS TIME I DECOMBINED SECTORS AND WAS IN THE PROCESS OF SETTING MY MAP FOR HIGH ALT ONLY WHEN I ISSUED A CLRNC FOR ACR X TO CLB TO FL320. MY D-SIDE WAS OFF ON THE LINE COORDINATING A POINTOUT WHICH ALSO CAPTURED MY ATTN WHILE ACR X WAS READING BACK THEIR CLRNC. I APPLIED MERGING TARGET PROCS BY ISSUING TFC TO BOTH MD80 AND TO ACR X. WHILE THE ACFT WERE STILL VERTICALLY SEPARATED AND 20 MI APART; ACR X QUESTIONED NOTHING. ACR X RPTED ACR Y IN SIGHT. THEY WERE STILL VERTICALLY SEPARATED. I TOLD ACR X TO EXPECT A HIGHER ALT WHEN CLR OF TFC. AN ACFT ASKED TO VERIFY CLB CLRNC TO FL380. THE VOICE SEEMED SIMILAR TO THAT OF ACR X. I ASKED ACR X TO SAY AGAIN. AN ACFT WITHOUT IDENTING THEMSELVES AGAIN ASKED TO VERIFY CLB CLRNC TO FL380. I TOLD ACR X TO MAINTAIN FL320. SEPARATION WAS LOST AFTER THE 2 ACFT PASSED. CLOSEST PROX 2.5 MI 800 FT.

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.