Narrative:

Frequency was extremely busy. Air carrier X (ord inbound) heading 230 degrees south level FL240. Air carrier Y (ord departure) wbound level FL230. Air carrier Z (ord inbound) eastbound descending FL250. Air carrier Z coming towards air carrier X still descending just leveling off at FL250 (air carrier Z might have went to FL248, data block never showed any altitude lower than FL250). With air carrier Z just off of air carrier X's right side, air carrier X received a TCASII instruction to descend. Pilot advised ZAU of his situation. At the same time, air carrier Y was right under air carrier X at FL230. Air carrier Y said the air carrier X was between 1 to 2 mi at the same altitude. After occurrence ZAU climbed air carrier X back to FL240.

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Original NASA ASRS Text

Title: ACR X TCASII RA UNAUTH DSCNT TO OCCUPIED ALT HAD LTSS FROM ACR Y. ACR Z POSSIBLE ALTDEV CAUSED ACR X TCASII RA. PLTDEV.

Narrative: FREQ WAS EXTREMELY BUSY. ACR X (ORD INBOUND) HDG 230 DEGS S LEVEL FL240. ACR Y (ORD DEP) WBOUND LEVEL FL230. ACR Z (ORD INBOUND) EBOUND DSNDING FL250. ACR Z COMING TOWARDS ACR X STILL DSNDING JUST LEVELING OFF AT FL250 (ACR Z MIGHT HAVE WENT TO FL248, DATA BLOCK NEVER SHOWED ANY ALT LOWER THAN FL250). WITH ACR Z JUST OFF OF ACR X'S R SIDE, ACR X RECEIVED A TCASII INSTRUCTION TO DSND. PLT ADVISED ZAU OF HIS SIT. AT THE SAME TIME, ACR Y WAS RIGHT UNDER ACR X AT FL230. ACR Y SAID THE ACR X WAS BTWN 1 TO 2 MI AT THE SAME ALT. AFTER OCCURRENCE ZAU CLBED ACR X BACK TO FL240.

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.