Narrative:

A B777 departed ord, climbed to FL230, cleared direct bae. A DC9 descending into msn was issued 11000 ft. The situation was being watched. The sector complexity was low. A coordination call from mke approach was answered at a time in the situation that it should have been ignored so control actions could be taken to ensure separation. After the coordination with mke approach was completed, the situation was re-evaluated and it was determined that interim altitudes should be used. I stopped the DC9 at FL230, then stopped the B777 at FL220. I thought in his readback he included the comment that he was level at FL220. The mode C continued to indicate a climb to FL225. I called the traffic for both aircraft and the B777 then stated 'we are starting down to FL220.' I advised my supervisor but indicated I had the previous report of 'level FL220.' separation was lost. The B777 then reported level again at FL220. After a review of the tapes, the initial report of 'level FL220' was not there. I had made the mistake of hearing what I wanted to hear. Other factors were the untimely coordination with mke approach that distraction me from my primary object of separation. Even when it appears you are not busy, it only takes a momentary distraction to cause an error.

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Original NASA ASRS Text

Title: ZAU CTLR ATTEMPTS TO AMEND B777 CLB CLRNC AFTER RE-EVALUATING OPPOSITE DIRECTION DSCNT DC9 CLRNC AND IS UNABLE TO ENSURE VERT SEPARATION.

Narrative: A B777 DEPARTED ORD, CLBED TO FL230, CLRED DIRECT BAE. A DC9 DSNDING INTO MSN WAS ISSUED 11000 FT. THE SIT WAS BEING WATCHED. THE SECTOR COMPLEXITY WAS LOW. A COORD CALL FROM MKE APCH WAS ANSWERED AT A TIME IN THE SIT THAT IT SHOULD HAVE BEEN IGNORED SO CTL ACTIONS COULD BE TAKEN TO ENSURE SEPARATION. AFTER THE COORD WITH MKE APCH WAS COMPLETED, THE SIT WAS RE-EVALUATED AND IT WAS DETERMINED THAT INTERIM ALTS SHOULD BE USED. I STOPPED THE DC9 AT FL230, THEN STOPPED THE B777 AT FL220. I THOUGHT IN HIS READBACK HE INCLUDED THE COMMENT THAT HE WAS LEVEL AT FL220. THE MODE C CONTINUED TO INDICATE A CLB TO FL225. I CALLED THE TFC FOR BOTH ACFT AND THE B777 THEN STATED 'WE ARE STARTING DOWN TO FL220.' I ADVISED MY SUPVR BUT INDICATED I HAD THE PREVIOUS RPT OF 'LEVEL FL220.' SEPARATION WAS LOST. THE B777 THEN RPTED LEVEL AGAIN AT FL220. AFTER A REVIEW OF THE TAPES, THE INITIAL RPT OF 'LEVEL FL220' WAS NOT THERE. I HAD MADE THE MISTAKE OF HEARING WHAT I WANTED TO HEAR. OTHER FACTORS WERE THE UNTIMELY COORD WITH MKE APCH THAT DISTR ME FROM MY PRIMARY OBJECT OF SEPARATION. EVEN WHEN IT APPEARS YOU ARE NOT BUSY, IT ONLY TAKES A MOMENTARY DISTR TO CAUSE AN ERROR.

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.