Narrative:

I started to clear air carrier Y to climb from FL290 to maintain FL310, but changed the clearance to maintain FL290 and expect a higher altitude after stl. Shortly thereafter, I noticed air carrier X at FL295. I asked him to verify level at FL290. Air carrier X replied that I had cleared him to FL310. I instructed air carrier X to expedite descent back to FL290. I also instructed air carrier Y wbound traffic at FL310 to turn 40 degrees right. Loss of separation occurred and was reestablished after the aircraft passed each other. I believed that air carrier X had accepted the clearance to maintain FL290 when he acknowledged with an 'ok.' what seems to have happened is that when air carrier X heard the clearance to maintain FL310 he immediately read back his acknowledgement. I never unkeyed my microphone, so that I was clearing the aircraft to maintain FL290 at the same time air carrier X was reading back FL310. When air carrier X unkeyed his microphone all he heard was 'expect higher altitude after stl.' in retrospect, I should have gotten conclusive confirmation that it was air carrier X that said 'ok' and that he definitely knew to maintain FL290. Air carrier X should not have been in such a hurry to get to altitude (requested FL330) as to allow me to unkey or finish speaking. Supplemental information from acn 243059: air carrier X copied and read back clearance from FL290 to FL310. Upon climbing through FL305 controller asked if we were level at FL290. I said no and he said we were to return to FL290. Immediate descent initiated. No heading change was issued. He then told us that we were not cleared to FL310. I replied that we copied and read back clearance to FL310 and could expect higher east of stl. Both pilots heard FL310 as cleared altitude. No evasive action was required. One air carrier Y flight was given a 40 degree heading change.

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Original NASA ASRS Text

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

Narrative: I STARTED TO CLR ACR Y TO CLB FROM FL290 TO MAINTAIN FL310, BUT CHANGED THE CLRNC TO MAINTAIN FL290 AND EXPECT A HIGHER ALT AFTER STL. SHORTLY THEREAFTER, I NOTICED ACR X AT FL295. I ASKED HIM TO VERIFY LEVEL AT FL290. ACR X REPLIED THAT I HAD CLRED HIM TO FL310. I INSTRUCTED ACR X TO EXPEDITE DSCNT BACK TO FL290. I ALSO INSTRUCTED ACR Y WBOUND TFC AT FL310 TO TURN 40 DEGS R. LOSS OF SEPARATION OCCURRED AND WAS REESTABLISHED AFTER THE ACFT PASSED EACH OTHER. I BELIEVED THAT ACR X HAD ACCEPTED THE CLRNC TO MAINTAIN FL290 WHEN HE ACKNOWLEDGED WITH AN 'OK.' WHAT SEEMS TO HAVE HAPPENED IS THAT WHEN ACR X HEARD THE CLRNC TO MAINTAIN FL310 HE IMMEDIATELY READ BACK HIS ACKNOWLEDGEMENT. I NEVER UNKEYED MY MIKE, SO THAT I WAS CLRING THE ACFT TO MAINTAIN FL290 AT THE SAME TIME ACR X WAS READING BACK FL310. WHEN ACR X UNKEYED HIS MIKE ALL HE HEARD WAS 'EXPECT HIGHER ALT AFTER STL.' IN RETROSPECT, I SHOULD HAVE GOTTEN CONCLUSIVE CONFIRMATION THAT IT WAS ACR X THAT SAID 'OK' AND THAT HE DEFINITELY KNEW TO MAINTAIN FL290. ACR X SHOULD NOT HAVE BEEN IN SUCH A HURRY TO GET TO ALT (REQUESTED FL330) AS TO ALLOW ME TO UNKEY OR FINISH SPEAKING. SUPPLEMENTAL INFO FROM ACN 243059: ACR X COPIED AND READ BACK CLRNC FROM FL290 TO FL310. UPON CLBING THROUGH FL305 CTLR ASKED IF WE WERE LEVEL AT FL290. I SAID NO AND HE SAID WE WERE TO RETURN TO FL290. IMMEDIATE DSCNT INITIATED. NO HDG CHANGE WAS ISSUED. HE THEN TOLD US THAT WE WERE NOT CLRED TO FL310. I REPLIED THAT WE COPIED AND READ BACK CLRNC TO FL310 AND COULD EXPECT HIGHER E OF STL. BOTH PLTS HEARD FL310 AS CLRED ALT. NO EVASIVE ACTION WAS REQUIRED. ONE ACR Y FLT WAS GIVEN A 40 DEG HDG CHANGE.

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.