Narrative:

Assigned by memphis center to descend via the BRBBQ2 RNAV arrival; except maintain 17000 feet. After passing brbbq on the arrival the aircraft was descending in profile mode; autopilot 2 engaged; 17;000 feet armed in the altitude alert window; aircraft descending at 1000 FPM. Memphis center called to advise us of traffic; aircraft Y at 16000 feet. We saw the aircraft on the TCAS display and as we were approaching the level off at 17;300 feet the aircraft abruptly pitched over and increased the descent rate from 1000 FPM to approximately 2000 FPM. The first officer; who was pilot flying; disconnected the autopilot and auto throttles immediately; as I was calling for the same; and corrected for the autopilot; which was obviously not going to honor the 17000 foot level off. It was too late; however; to avoid the TCAS RA that came right after receiving a TCAS TA; due to the increased rate of descent. We received a TCAS monitor vertical speed RA; followed quickly by a TCAS level off RA. The lowest altitude during the sequence was 16900; with a slight over control causing a climb to 17300; before receiving the TCAS clear of conflict annunciation and returning to 17000 feet assigned. The first officer reacted in a timely manner and an actual conflict with the traffic was avoided. I then reported the TCAS RA to memphis center; who acknowledged the call and gave us a frequency change to the next sector. A write up was entered to have the autopilot checked after the flight. This event was caused by autopilot 2 not honoring the 17000 foot armed minimum altitude in profile mode; suddenly increasing rate of descent at 17300 feet from 1000 fpm to 2000 fpm; causing a TCAS TA followed almost instantaneously by a TCAS RA.just a few minutes prior this event; the first officer and I had discussed the fact that profile mode is 'trust; but verify' leading both of us to be prepared for just such a case of autopilot non-compliance. The first officer; as pilot flying; reacted promptly and correctly avoiding an actual conflict with the traffic aircraft; while avoiding excessive altitude deviations.

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Original NASA ASRS Text

Title: Air carrier flight crew reported the aircraft pitched down requiring manual intervention and a TCAS RA with the traffic.

Narrative: Assigned by Memphis center to descend via the BRBBQ2 RNAV arrival; except maintain 17000 feet. After passing BRBBQ on the arrival the aircraft was descending in Profile mode; autopilot 2 engaged; 17;000 feet armed in the Altitude alert window; aircraft descending at 1000 FPM. Memphis Center called to advise us of traffic; Aircraft Y at 16000 feet. We saw the aircraft on the TCAS display and as we were approaching the level off at 17;300 feet the aircraft abruptly pitched over and increased the descent rate from 1000 FPM to approximately 2000 FPM. The First Officer; who was pilot flying; disconnected the autopilot and auto throttles immediately; as I was calling for the same; and corrected for the autopilot; which was obviously not going to honor the 17000 foot level off. It was too late; however; to avoid the TCAS RA that came right after receiving a TCAS TA; due to the increased rate of descent. We received a TCAS monitor vertical speed RA; followed quickly by a TCAS Level Off RA. The lowest altitude during the sequence was 16900; with a slight over control causing a climb to 17300; before receiving the TCAS clear of conflict annunciation and returning to 17000 feet assigned. The First Officer reacted in a timely manner and an actual conflict with the traffic was avoided. I then reported the TCAS RA to Memphis center; who acknowledged the call and gave us a frequency change to the next sector. A write up was entered to have the autopilot checked after the flight. This event was caused by autopilot 2 not honoring the 17000 foot armed minimum altitude in profile mode; suddenly increasing rate of descent at 17300 feet from 1000 fpm to 2000 fpm; causing a TCAS TA followed almost instantaneously by a TCAS RA.Just a few minutes prior this event; the First Officer and I had discussed the fact that Profile Mode is 'trust; but verify' leading both of us to be prepared for just such a case of autopilot non-compliance. The First Officer; as pilot flying; reacted promptly and correctly avoiding an actual conflict with the traffic aircraft; while avoiding excessive altitude deviations.

Data retrieved from NASA's ASRS site 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.