Narrative:

While working the boone sector at ZAU, I received a handoff on small transport X from aid to pwk descending to 100. This aircraft's mode C then showed 9700 and I asked pilot if he was maintaining 10,000. Pilot affirmed this and muttered something about pushing a button again. Mode C went to 9500 then back to show level at 10,000. The boone sector owns 9000 and below so I quick-looked the cgt sector which lies overhead boone to make sure the controller hadn't inadvertently dropped the data block on small transport X thinking this aircraft had descended below 10,000. As suspected the cgt sector had dropped this data block and I also observed air carrier Y about 15 NM behind the small transport X which showed descending to 11,000. I put a data block back on cgt scope of small transport X level at 10,000 so that the controller would not descend air carrier Y below 11,000. As it turns out the cgt controller had already descended air carrier Y to 10,000 at the same time he dropped the data block of small transport X but did not update the data block on air carrier Y to show descending to 10,000. So now small transport X reappears on the cgt scope level at 10,000 and this is observed by the cgt controller but he mistakenly thinks he only descended air carrier Y to 11,000 because that's what the data block shows. Traffic was called several times to air carrier Y for the small transport X at 10,000. These calls were rogered by air carrier Y with no remarks even though air carrier Y was going to 10,000 as well. Conflict alert didn't go off until air carrier Y went below 11,000 because of the incorrect data information. Aircraft concerned lost prescribed separation and the cgt controller asked air carrier Y to say altitude and was then made aware of what had happened. Air carrier Y then was climbed back to 11,000 to separate the 2 aircraft. Cgt controller was credited with the operational error, myself at boone sector was exonerated. Supplemental information from acn 113125: this incident occurred approximately 15-20 mi northeast of bvt VORTAC (lafayette, in). I was working the cgt sector at chicago center. I own airspace 10,000-23,000. The boone sector underlies cgt sector 9000 and below. Small transport X was handed off to me descending to 10,000. I took the handoff and initiated a handoff to boone as the aircraft had to be descended below 10,000. Communication transfer was from ZID to boone, I did not talk to aircraft. Air carrier Y was inbound ord descending to 19,000 checked on my frequency. At this time I observed the mode C on small transport X as 9700 then 9600. So I thought the aircraft was descending. I dropped the full data block as the aircraft was out of my altitudes (I thought) and cleared air carrier Y to descend to 10,000. The boone controller had not descended small transport X, either the aircraft deviated or a bad mode C readout. The boone controller then forced a data block on small transport X back on my scope. At the time I went to put the altitude on air carrier Y in computer. Seeing the 10,000 traffic I put 11,000 in the data block of air carrier Y thinking I only cleared air carrier Y to 11,000. Traffic was issued to air carrier Y as he was descending through 12,000, still thinking air carrier Y was cleared to 11,000. As air carrier Y mode C indicated 10,500 I asked him to say altitude and he reported leveling at 10,000. Air carrier Y then was issued clearance to 11,000.

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Original NASA ASRS Text

Title: CTLR INADVERTENTLY DESCENDED AN ACR TO AN ALT OCCUPIED BY ANOTHER ACFT WHILE SHOWING A DIFFERENT ALT IN THE DATA BLOCK.

Narrative: WHILE WORKING THE BOONE SECTOR AT ZAU, I RECEIVED A HANDOFF ON SMT X FROM AID TO PWK DESCENDING TO 100. THIS ACFT'S MODE C THEN SHOWED 9700 AND I ASKED PLT IF HE WAS MAINTAINING 10,000. PLT AFFIRMED THIS AND MUTTERED SOMETHING ABOUT PUSHING A BUTTON AGAIN. MODE C WENT TO 9500 THEN BACK TO SHOW LEVEL AT 10,000. THE BOONE SECTOR OWNS 9000 AND BELOW SO I QUICK-LOOKED THE CGT SECTOR WHICH LIES OVERHEAD BOONE TO MAKE SURE THE CTLR HADN'T INADVERTENTLY DROPPED THE DATA BLOCK ON SMT X THINKING THIS ACFT HAD DESCENDED BELOW 10,000. AS SUSPECTED THE CGT SECTOR HAD DROPPED THIS DATA BLOCK AND I ALSO OBSERVED ACR Y ABOUT 15 NM BEHIND THE SMT X WHICH SHOWED DESCENDING TO 11,000. I PUT A DATA BLOCK BACK ON CGT SCOPE OF SMT X LEVEL AT 10,000 SO THAT THE CTLR WOULD NOT DSND ACR Y BELOW 11,000. AS IT TURNS OUT THE CGT CTLR HAD ALREADY DESCENDED ACR Y TO 10,000 AT THE SAME TIME HE DROPPED THE DATA BLOCK OF SMT X BUT DID NOT UPDATE THE DATA BLOCK ON ACR Y TO SHOW DESCENDING TO 10,000. SO NOW SMT X REAPPEARS ON THE CGT SCOPE LEVEL AT 10,000 AND THIS IS OBSERVED BY THE CGT CTLR BUT HE MISTAKENLY THINKS HE ONLY DESCENDED ACR Y TO 11,000 BECAUSE THAT'S WHAT THE DATA BLOCK SHOWS. TFC WAS CALLED SEVERAL TIMES TO ACR Y FOR THE SMT X AT 10,000. THESE CALLS WERE ROGERED BY ACR Y WITH NO REMARKS EVEN THOUGH ACR Y WAS GOING TO 10,000 AS WELL. CONFLICT ALERT DIDN'T GO OFF UNTIL ACR Y WENT BELOW 11,000 BECAUSE OF THE INCORRECT DATA INFO. ACFT CONCERNED LOST PRESCRIBED SEPARATION AND THE CGT CTLR ASKED ACR Y TO SAY ALT AND WAS THEN MADE AWARE OF WHAT HAD HAPPENED. ACR Y THEN WAS CLIMBED BACK TO 11,000 TO SEPARATE THE 2 ACFT. CGT CTLR WAS CREDITED WITH THE OPERATIONAL ERROR, MYSELF AT BOONE SECTOR WAS EXONERATED. SUPPLEMENTAL INFORMATION FROM ACN 113125: THIS INCIDENT OCCURRED APPROX 15-20 MI NE OF BVT VORTAC (LAFAYETTE, IN). I WAS WORKING THE CGT SECTOR AT CHICAGO CENTER. I OWN AIRSPACE 10,000-23,000. THE BOONE SECTOR UNDERLIES CGT SECTOR 9000 AND BELOW. SMT X WAS HANDED OFF TO ME DESCENDING TO 10,000. I TOOK THE HANDOFF AND INITIATED A HANDOFF TO BOONE AS THE ACFT HAD TO BE DESCENDED BELOW 10,000. COM TRANSFER WAS FROM ZID TO BOONE, I DID NOT TALK TO ACFT. ACR Y WAS INBND ORD DESCENDING TO 19,000 CHECKED ON MY FREQ. AT THIS TIME I OBSERVED THE MODE C ON SMT X AS 9700 THEN 9600. SO I THOUGHT THE ACFT WAS DESCENDING. I DROPPED THE FULL DATA BLOCK AS THE ACFT WAS OUT OF MY ALTS (I THOUGHT) AND CLRED ACR Y TO DSND TO 10,000. THE BOONE CTLR HAD NOT DESCENDED SMT X, EITHER THE ACFT DEVIATED OR A BAD MODE C READOUT. THE BOONE CTLR THEN FORCED A DATA BLOCK ON SMT X BACK ON MY SCOPE. AT THE TIME I WENT TO PUT THE ALT ON ACR Y IN COMPUTER. SEEING THE 10,000 TFC I PUT 11,000 IN THE DATA BLOCK OF ACR Y THINKING I ONLY CLRED ACR Y TO 11,000. TFC WAS ISSUED TO ACR Y AS HE WAS DESCENDING THROUGH 12,000, STILL THINKING ACR Y WAS CLRED TO 11,000. AS ACR Y MODE C INDICATED 10,500 I ASKED HIM TO SAY ALT AND HE REPORTED LEVELING AT 10,000. ACR Y THEN WAS ISSUED CLRNC TO 11,000.

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.