Narrative:

Air carrier X came on frequency (127.8) at FL240. He was told to maintain 300 KTS and was cleared direct bear 2, direct ord. Air carrier X was cleared to 16000 ft. I entered the interim altitude. At some point south of the oxi VORTAC, I entered an interim altitude of 1700 ft in air carrier X data block. I do not remember doing it or why. Air carrier Y was an SF34 at 16000 ft going direct bmi north of the oxi VORTAC. Air carrier Y was turned 20 degrees left for descending traffic into ord. Air carrier X was slowed to 250 KTS for in trail sequencing to ord. I gave air carrier Y traffic at 10 O'clock position and 3 mi, a B737 at 17000 ft. I gave air carrier X traffic at 2 O'clock and 2 mi, an SF34 at 16000 ft. I saw air carrier X's altitude readout go down to 16700 ft and told him to maintain 17000 ft. When air carrier X's readout showed 16400 ft, I told him to just descend to 11000 ft. Then I saw air carrier Y's readout go to 15600 ft and I asked if he was descending and he replied that he was, due to a TCASII resolution. Air carrier X said he was climbing to 17000 ft.

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Original NASA ASRS Text

Title: ZAU CTL MISTAKENLY ENTERS INCORRECT ALT DATA INTO A B737-500'S RADAR DATA BLOCK AND LATER DSNDS THE ACR INTO CONFLICT WITH AN SF34. THE SF34 INITIATES A TCASII MANEUVER.

Narrative: ACR X CAME ON FREQ (127.8) AT FL240. HE WAS TOLD TO MAINTAIN 300 KTS AND WAS CLRED DIRECT BEAR 2, DIRECT ORD. ACR X WAS CLRED TO 16000 FT. I ENTERED THE INTERIM ALT. AT SOME POINT S OF THE OXI VORTAC, I ENTERED AN INTERIM ALT OF 1700 FT IN ACR X DATA BLOCK. I DO NOT REMEMBER DOING IT OR WHY. ACR Y WAS AN SF34 AT 16000 FT GOING DIRECT BMI N OF THE OXI VORTAC. ACR Y WAS TURNED 20 DEGS L FOR DSNDING TFC INTO ORD. ACR X WAS SLOWED TO 250 KTS FOR IN TRAIL SEQUENCING TO ORD. I GAVE ACR Y TFC AT 10 O'CLOCK POS AND 3 MI, A B737 AT 17000 FT. I GAVE ACR X TFC AT 2 O'CLOCK AND 2 MI, AN SF34 AT 16000 FT. I SAW ACR X'S ALT READOUT GO DOWN TO 16700 FT AND TOLD HIM TO MAINTAIN 17000 FT. WHEN ACR X'S READOUT SHOWED 16400 FT, I TOLD HIM TO JUST DSND TO 11000 FT. THEN I SAW ACR Y'S READOUT GO TO 15600 FT AND I ASKED IF HE WAS DSNDING AND HE REPLIED THAT HE WAS, DUE TO A TCASII RESOLUTION. ACR X SAID HE WAS CLBING TO 17000 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.