Narrative:

Air carrier X, a cvg departure, was climbed to FL250 for traffic at FL260 swbnd. A higher altitude would not be available for air carrier X until approximately ZOB/ZID boundary. I told rav sector that air carrier X was at FL250 and would be their control for climb. At the same time I requested small transport Y stopped at FL240. Small transport Y altitude readout showed the aircraft at FL230. This coordination took place while air carrier X was 15 mi from ZOB boundary. A short time later I noticed small transport Y mode C at FL260 descending. Having released control for rav sector to climb air carrier X and not being sure what was happening with small transport Y, I did not switch air carrier X until I could ensure some sep. Realizing the altitude readout was not in error and that small transport Y was actually descending back to FL240 through the altitude of air carrier X at FL250, I turned air carrier X to a 090 degree heading. I did not talk to small transport Y until the aircraft was in the day high sector. I asked his altitude and he reported level at FL260.

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Original NASA ASRS Text

Title: SMT WAS DESCENDED THROUGH ALT OF AN MLG.

Narrative: ACR X, A CVG DEP, WAS CLBED TO FL250 FOR TFC AT FL260 SWBND. A HIGHER ALT WOULD NOT BE AVAILABLE FOR ACR X UNTIL APPROX ZOB/ZID BOUNDARY. I TOLD RAV SECTOR THAT ACR X WAS AT FL250 AND WOULD BE THEIR CTL FOR CLB. AT THE SAME TIME I REQUESTED SMT Y STOPPED AT FL240. SMT Y ALT READOUT SHOWED THE ACFT AT FL230. THIS COORD TOOK PLACE WHILE ACR X WAS 15 MI FROM ZOB BOUNDARY. A SHORT TIME LATER I NOTICED SMT Y MODE C AT FL260 DSNDING. HAVING RELEASED CTL FOR RAV SECTOR TO CLB ACR X AND NOT BEING SURE WHAT WAS HAPPENING WITH SMT Y, I DID NOT SWITCH ACR X UNTIL I COULD ENSURE SOME SEP. REALIZING THE ALT READOUT WAS NOT IN ERROR AND THAT SMT Y WAS ACTUALLY DSNDING BACK TO FL240 THROUGH THE ALT OF ACR X AT FL250, I TURNED ACR X TO A 090 DEG HDG. I DID NOT TALK TO SMT Y UNTIL THE ACFT WAS IN THE DAY HIGH SECTOR. I ASKED HIS ALT AND HE RPTED LEVEL AT FL260.

Data retrieved from NASA's ASRS site as of August 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.