Narrative:

On J80 wbound at FL310, the ride became bumpy at this altitude so the PF asked me to request lower altitude. Upon our request, ZOB asked us to stand by a moment, then he issued us descent clearance to FL280. Just prior to leveling at FL280, the TCASII issued a TA. We observed a target on the TCASII screen at our 2 O'clock position, climbing from 600 ft below us and converging. We questioned ATC as to the traffic presence and he immediately issued an immediate 90 degree left turn. At no time did we alter our course to avoid the conflict either from a radar vector or an RA from the TCASII. The closest the 2 aircraft got appeared on the TCASII to be 4 mi horizontal and co-altitude converging at 90 degrees. The captain called to ask ATC how such a situation had occurred. The controller indicated that he had set up our descent so as to cross in front of the climbing traffic with the proper vertical separation based on his estimate of winds and each aircraft's speed. He also indicated that he became unaware that we were going to pass so close. In his attempt to accommodate our request for a lower altitude, he set up a proximity constraint which worked out just a bit too close. If we had been in a non TCASII aircraft and had not ourselves become aware of the traffic, the 2 aircraft could have passed much, much closer given the controller's distraction. The TCASII alerted us to the convergence with enough time to query the controller which allowed him to react.

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Original NASA ASRS Text

Title: ACR X TCASII TA DSCNT TO OCCUPIED ALT HAD LTSS FROM ACR Y. SYS ERROR.

Narrative: ON J80 WBOUND AT FL310, THE RIDE BECAME BUMPY AT THIS ALT SO THE PF ASKED ME TO REQUEST LOWER ALT. UPON OUR REQUEST, ZOB ASKED US TO STAND BY A MOMENT, THEN HE ISSUED US DSCNT CLRNC TO FL280. JUST PRIOR TO LEVELING AT FL280, THE TCASII ISSUED A TA. WE OBSERVED A TARGET ON THE TCASII SCREEN AT OUR 2 O'CLOCK POS, CLBING FROM 600 FT BELOW US AND CONVERGING. WE QUESTIONED ATC AS TO THE TFC PRESENCE AND HE IMMEDIATELY ISSUED AN IMMEDIATE 90 DEG L TURN. AT NO TIME DID WE ALTER OUR COURSE TO AVOID THE CONFLICT EITHER FROM A RADAR VECTOR OR AN RA FROM THE TCASII. THE CLOSEST THE 2 ACFT GOT APPEARED ON THE TCASII TO BE 4 MI HORIZ AND CO-ALT CONVERGING AT 90 DEGS. THE CAPT CALLED TO ASK ATC HOW SUCH A SIT HAD OCCURRED. THE CTLR INDICATED THAT HE HAD SET UP OUR DSCNT SO AS TO CROSS IN FRONT OF THE CLBING TFC WITH THE PROPER VERT SEPARATION BASED ON HIS ESTIMATE OF WINDS AND EACH ACFT'S SPD. HE ALSO INDICATED THAT HE BECAME UNAWARE THAT WE WERE GOING TO PASS SO CLOSE. IN HIS ATTEMPT TO ACCOMMODATE OUR REQUEST FOR A LOWER ALT, HE SET UP A PROX CONSTRAINT WHICH WORKED OUT JUST A BIT TOO CLOSE. IF WE HAD BEEN IN A NON TCASII ACFT AND HAD NOT OURSELVES BECOME AWARE OF THE TFC, THE 2 ACFT COULD HAVE PASSED MUCH, MUCH CLOSER GIVEN THE CTLR'S DISTR. THE TCASII ALERTED US TO THE CONVERGENCE WITH ENOUGH TIME TO QUERY THE CTLR WHICH ALLOWED HIM TO REACT.

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.