Narrative:

I was vectoring aircraft out of holding pattern airspace southeast of chicago O'hare for 20 mi intrail spacing. Air carrier medium large transport was 15 NM south of the holding pattern at 12000 ft MSL. I turned medium large transport right to a heading of 350 degrees and descended the aircraft to 11000 ft MSL. Medium large transport read the clearance back correctly. I observed medium large transport at 11900 ft indicating to me that the aircraft had started its descent. Large transport in the holding pattern was then descended from FL140 to FL120. I went on to some coordination with ord tower and surrounding sectors. When I looked back medium large transport was back up at FL120 along with large transport. I attempted to turn and descend medium large transport and climb large transport but the aircraft were already responding to TCASII RA's. Separation was lost, the aircraft passed with 1.6 NM and 700 ft at their closest point. Supplemental information from acn 236215: we received an RA from TCASII indicating a 'monitor vertical speed' at which time we complied with a climb and also initiated a left turn as the offending large transport was in sight visually ahead and to the right. The controller asked us if the previous controller had cleared our flight to 11000 ft. We responded that we had not received any such clearance and that the last altitude clearance was to 12000 ft.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN ACR MLG CLBED AND TURNED IN RESPONSE TO A TCASII COMMAND.

Narrative: I WAS VECTORING ACFT OUT OF HOLDING PATTERN AIRSPACE SE OF CHICAGO O'HARE FOR 20 MI INTRAIL SPACING. ACR MLG WAS 15 NM S OF THE HOLDING PATTERN AT 12000 FT MSL. I TURNED MLG R TO A HDG OF 350 DEGS AND DSNDED THE ACFT TO 11000 FT MSL. MLG READ THE CLRNC BACK CORRECTLY. I OBSERVED MLG AT 11900 FT INDICATING TO ME THAT THE ACFT HAD STARTED ITS DSCNT. LGT IN THE HOLDING PATTERN WAS THEN DSNDED FROM FL140 TO FL120. I WENT ON TO SOME COORD WITH ORD TWR AND SURROUNDING SECTORS. WHEN I LOOKED BACK MLG WAS BACK UP AT FL120 ALONG WITH LGT. I ATTEMPTED TO TURN AND DSND MLG AND CLB LGT BUT THE ACFT WERE ALREADY RESPONDING TO TCASII RA'S. SEPARATION WAS LOST, THE ACFT PASSED WITH 1.6 NM AND 700 FT AT THEIR CLOSEST POINT. SUPPLEMENTAL INFO FROM ACN 236215: WE RECEIVED AN RA FROM TCASII INDICATING A 'MONITOR VERT SPD' AT WHICH TIME WE COMPLIED WITH A CLB AND ALSO INITIATED A L TURN AS THE OFFENDING LGT WAS IN SIGHT VISUALLY AHEAD AND TO THE R. THE CTLR ASKED US IF THE PREVIOUS CTLR HAD CLRED OUR FLT TO 11000 FT. WE RESPONDED THAT WE HAD NOT RECEIVED ANY SUCH CLRNC AND THAT THE LAST ALT CLRNC WAS TO 12000 FT.

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.