Date: 201706
Local time of day: 0601-1200
Location: MEM.Airport, TN
Flight conditions: VMC
Light: Daylight
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent
Airspace: Class E ZME
ATC / advisory: Center ZME
Second aircraft: Small Transport
Aircraft component: Altitude Hold/Capture
Component problem: Design; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew FLC Overrode Automation; Flight Crew Took Evasive Action; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

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.


Second narrative: During descent on BRBBQ2 arrival into MEM; Memphis Center assigned an intermediate level off of 17;000 due to traffic. Aircraft was descending in Profile mode with 17;000 feet armed (and confirmed) in altitude window; autopilot 2 engaged; and at a rate of 1;000 feet per minute. Memphis Center then advised us of traffic at 16;000 which was also displayed on TCAS. As the aircraft was beginning to level off at 17;000 feet the autopilot began an abrupt transition from a 1;000 feet per minute descent to 2;000 feet per minute descent. As PF; I disengaged the autopilot and auto-throttles in order to level off at 17;000 feet. Unfortunately; this abrupt descent caused the TCAS TA to transition to a RA. Memphis Center was shortly notified by PM after aircraft was re-automated at 17;000 feet. During the maneuver it was estimated that the aircraft descended to 16;900 feet and climbed slightly to 17;300 feet. Additionally; during the course of the level off I was able to visually obtain the other aircraft in question which was safely below. Moments later we were assigned to the next sector and our descent continued into MEM uneventfully. Following aircraft shutdown procedures; the CA entered a write up describing the circumstances in order to have the autopilot checked prior to the next flight. This event was caused by AP 2 not honoring the selected altitude of 17;000 feet as well as unnecessarily transitioning from an 1;000 feet per minute descent to a 2;000 feet per minute descent when in fact it should have started a transition to level flight. This led to the TCAS RA. Monitoring of auto flight Profile Mode is essential during descent into airport terminal areas.


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