Narrative:

Aircraft #1 was issued a clearance to descend, after taking into account aircraft #2. It was believed that 5 mi lateral separation would be maintained and that with an eye towards monitoring the situation, it would not be a problem. There was, a short time later, considerable coordination with the sector bordering to the south. Returning to the situation previously described, a turn to the right was issued to aircraft #1 to 'pad' the separation. A measurement was taken of 5.6 mi when the 2 aircraft were nearing a point until they would pass. Conflict alert did not activate, it looked good, an insurance turn was issued. Aircraft 2's data block seemed to have 2/3 'jumps' to the left. At that time, I attempted to stop aircraft #1 1000 ft above. I believed the 'jumps' were invalid as it has been seen so many times before. The attempt to revert to altitude was a precautionary move. However, aircraft #1 reported already descending through the altitude. Conflict alert activated at that time. The aircraft passed with 4.5 mi. It is my understanding conflict alert went off after the aircraft had passed. Although conflict alert is not reliable as a separation tool, it should have provided advanced warning -- timely enough to make a difference. Conflict alert is used as a kind of assessment tool. If the clearance looks good, based on experience/training and conflict alert doesn't activate -- that non activation adds a sense of confirmation that the clearance is good.

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Original NASA ASRS Text

Title: ZAU CTLR INVOLVED IN AN OPERROR, QUESTIONS USABILITY OF CONFLICT PROBE FEATURE OF CONFLICT ALERT.

Narrative: ACFT #1 WAS ISSUED A CLRNC TO DSND, AFTER TAKING INTO ACCOUNT ACFT #2. IT WAS BELIEVED THAT 5 MI LATERAL SEPARATION WOULD BE MAINTAINED AND THAT WITH AN EYE TOWARDS MONITORING THE SIT, IT WOULD NOT BE A PROB. THERE WAS, A SHORT TIME LATER, CONSIDERABLE COORD WITH THE SECTOR BORDERING TO THE S. RETURNING TO THE SIT PREVIOUSLY DESCRIBED, A TURN TO THE R WAS ISSUED TO ACFT #1 TO 'PAD' THE SEPARATION. A MEASUREMENT WAS TAKEN OF 5.6 MI WHEN THE 2 ACFT WERE NEARING A POINT UNTIL THEY WOULD PASS. CONFLICT ALERT DID NOT ACTIVATE, IT LOOKED GOOD, AN INSURANCE TURN WAS ISSUED. ACFT 2'S DATA BLOCK SEEMED TO HAVE 2/3 'JUMPS' TO THE L. AT THAT TIME, I ATTEMPTED TO STOP ACFT #1 1000 FT ABOVE. I BELIEVED THE 'JUMPS' WERE INVALID AS IT HAS BEEN SEEN SO MANY TIMES BEFORE. THE ATTEMPT TO REVERT TO ALT WAS A PRECAUTIONARY MOVE. HOWEVER, ACFT #1 RPTED ALREADY DSNDING THROUGH THE ALT. CONFLICT ALERT ACTIVATED AT THAT TIME. THE ACFT PASSED WITH 4.5 MI. IT IS MY UNDERSTANDING CONFLICT ALERT WENT OFF AFTER THE ACFT HAD PASSED. ALTHOUGH CONFLICT ALERT IS NOT RELIABLE AS A SEPARATION TOOL, IT SHOULD HAVE PROVIDED ADVANCED WARNING -- TIMELY ENOUGH TO MAKE A DIFFERENCE. CONFLICT ALERT IS USED AS A KIND OF ASSESSMENT TOOL. IF THE CLRNC LOOKS GOOD, BASED ON EXPERIENCE/TRAINING AND CONFLICT ALERT DOESN'T ACTIVATE -- THAT NON ACTIVATION ADDS A SENSE OF CONFIRMATION THAT THE CLRNC IS GOOD.

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.