Narrative:

We were descending to 11000 ft with detroit approach. Aircraft (fokker 70) was in altitude capture phase at approximately 11300 ft. Received TCASII TA. TCASII target indicated intruder was 1900 ft below our altitude, climbing. Within 1-2 seconds received 'climb' RA. Intruder was not visually idented. Since autoplt vertical speed mode was disabled due to altitude capture phase, I disconnected autoplt and climbed manually according to TCASII presentation. At approximately 11400 ft received TCASII 'monitor vertical speed' message. Approximately 1-2 seconds later received TCASII 'crl of conflict' message. I re-engaged autoplt and selected vertical speed descent to return to assigned altitude of 11000 ft. For reasons unknown, autoplt did not capture altitude at 11000 ft. At approximately 10600 ft I disconnected autoplt and climbed manually back to 11000 ft. There was some cockpit confusion during this sequence of events due to following reasons. We received an airspeed assignment, altitude change, and RA all at about the same time. Could not understand reason for RA when TCASII indicated intruder was 1900 ft below our altitude. Necessity for disconnecting autoplt due to vertical speed mode inhibit. Autoplt did not capture altitude when descending back to assigned altitude. Controller questioning reason for RA when there appeared to be adequate vertical separation between aircraft. Controller did state he 'had no problem' with our maneuver. Note: TCASII software version 6.04A is installed in our aircraft.

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Original NASA ASRS Text

Title: ACR RESPONDS TO A TCAS ALERT NEAR DTW.

Narrative: WE WERE DSNDING TO 11000 FT WITH DETROIT APCH. ACFT (FOKKER 70) WAS IN ALT CAPTURE PHASE AT APPROX 11300 FT. RECEIVED TCASII TA. TCASII TARGET INDICATED INTRUDER WAS 1900 FT BELOW OUR ALT, CLBING. WITHIN 1-2 SECONDS RECEIVED 'CLB' RA. INTRUDER WAS NOT VISUALLY IDENTED. SINCE AUTOPLT VERT SPD MODE WAS DISABLED DUE TO ALT CAPTURE PHASE, I DISCONNECTED AUTOPLT AND CLBED MANUALLY ACCORDING TO TCASII PRESENTATION. AT APPROX 11400 FT RECEIVED TCASII 'MONITOR VERT SPD' MESSAGE. APPROX 1-2 SECONDS LATER RECEIVED TCASII 'CRL OF CONFLICT' MESSAGE. I RE-ENGAGED AUTOPLT AND SELECTED VERT SPD DSCNT TO RETURN TO ASSIGNED ALT OF 11000 FT. FOR REASONS UNKNOWN, AUTOPLT DID NOT CAPTURE ALT AT 11000 FT. AT APPROX 10600 FT I DISCONNECTED AUTOPLT AND CLBED MANUALLY BACK TO 11000 FT. THERE WAS SOME COCKPIT CONFUSION DURING THIS SEQUENCE OF EVENTS DUE TO FOLLOWING REASONS. WE RECEIVED AN AIRSPD ASSIGNMENT, ALT CHANGE, AND RA ALL AT ABOUT THE SAME TIME. COULD NOT UNDERSTAND REASON FOR RA WHEN TCASII INDICATED INTRUDER WAS 1900 FT BELOW OUR ALT. NECESSITY FOR DISCONNECTING AUTOPLT DUE TO VERT SPD MODE INHIBIT. AUTOPLT DID NOT CAPTURE ALT WHEN DSNDING BACK TO ASSIGNED ALT. CTLR QUESTIONING REASON FOR RA WHEN THERE APPEARED TO BE ADEQUATE VERT SEPARATION BTWN ACFT. CTLR DID STATE HE 'HAD NO PROB' WITH OUR MANEUVER. NOTE: TCASII SOFTWARE VERSION 6.04A IS INSTALLED IN OUR ACFT.

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.