Narrative:

ZAU directed us to fly heading 080 degrees then further right to heading 100 degrees. Using heading select mode of autoplt, I steered the aircraft to that heading. While in the turn, TCASII issued a TA. The navigation display showed the conflicting traffic at 12 O'clock, less than 10 NM, and 300 ft above us climbing. We acquired the aircraft visually. The TCASII then issued a descend advisory. I disconnected the autoplt, finished the turn to 100 degrees and established a TCASII directed descent of 800 FPM from FL370 to FL366. TCASII then issued a clear of conflict message and I smoothly returned the aircraft to FL370. At the start of the TCASII maneuver, the first officer advised the ATC controller we had a TCASII alert and were descending. The controller told us he had cleared the conflicting traffic to climb from FL350 to FL390. The aircraft climbed at a much slower rate than he (the ATC controller) had anticipated. I think this incident could have been avoided if: 1) the medium large transport autoplt system were programmed to use greater bank angles for turns at cruise (we were at 10 degrees bank for commanded turns to 080 degrees and 100 degrees). This would have perhaps increased the diverging tracks of the 2 aircraft soon enough to avoid setting off TCASII. 2) make more pilots aware of their responsibility to report to ATC any time their aircraft rate of climb drops below 500 FPM. The controller then may have issued heading changes early enough to avoid TCASII alerts.

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Original NASA ASRS Text

Title: AN ACR MLG DSNDED IN RESPONSE TO A TCASII COMMAND.

Narrative: ZAU DIRECTED US TO FLY HDG 080 DEGS THEN FURTHER R TO HDG 100 DEGS. USING HDG SELECT MODE OF AUTOPLT, I STEERED THE ACFT TO THAT HDG. WHILE IN THE TURN, TCASII ISSUED A TA. THE NAV DISPLAY SHOWED THE CONFLICTING TFC AT 12 O'CLOCK, LESS THAN 10 NM, AND 300 FT ABOVE US CLBING. WE ACQUIRED THE ACFT VISUALLY. THE TCASII THEN ISSUED A DSND ADVISORY. I DISCONNECTED THE AUTOPLT, FINISHED THE TURN TO 100 DEGS AND ESTABLISHED A TCASII DIRECTED DSCNT OF 800 FPM FROM FL370 TO FL366. TCASII THEN ISSUED A CLR OF CONFLICT MESSAGE AND I SMOOTHLY RETURNED THE ACFT TO FL370. AT THE START OF THE TCASII MANEUVER, THE FO ADVISED THE ATC CTLR WE HAD A TCASII ALERT AND WERE DSNDING. THE CTLR TOLD US HE HAD CLRED THE CONFLICTING TFC TO CLB FROM FL350 TO FL390. THE ACFT CLBED AT A MUCH SLOWER RATE THAN HE (THE ATC CTLR) HAD ANTICIPATED. I THINK THIS INCIDENT COULD HAVE BEEN AVOIDED IF: 1) THE MLG AUTOPLT SYS WERE PROGRAMMED TO USE GREATER BANK ANGLES FOR TURNS AT CRUISE (WE WERE AT 10 DEGS BANK FOR COMMANDED TURNS TO 080 DEGS AND 100 DEGS). THIS WOULD HAVE PERHAPS INCREASED THE DIVERGING TRACKS OF THE 2 ACFT SOON ENOUGH TO AVOID SETTING OFF TCASII. 2) MAKE MORE PLTS AWARE OF THEIR RESPONSIBILITY TO RPT TO ATC ANY TIME THEIR ACFT RATE OF CLB DROPS BELOW 500 FPM. THE CTLR THEN MAY HAVE ISSUED HDG CHANGES EARLY ENOUGH TO AVOID TCASII ALERTS.

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.