Date: 202009
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B787 Dreamliner Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Troubleshooting; Time Pressure; Situational Awareness; Workload
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While descending on the RNAV ARRIVAL within a mile or two of waypoint ZZZZZ the autopilot; for unknown reasons; went into a degraded mode. While in the degraded mode the aircraft continued straight ahead at ZZZZZ not staying on the LNAV course. By the time we recognized the situation and got the autopilot out of the degraded mode the controller informed us we had missed the turn and he gave us a heading. Flight continued normally from there.


Second narrative: I was the flying pilot. We were arriving on the RNAV STAR into ZZZ. At the time of the incident we were between the ZZZZZ and ZZZZZ1 waypoints; very close to the ZZZZZ waypoint. ATC instructed us to descend. The autopilot was engaged. I pushed the MCP FLCH button to start the decent. Immediately afterward I expected the aircraft to pull the power back and start a descent via the FLCH mode. Immediately the aircraft showed an 'autopilot mode' caution message on the ECAIS. Both the captain and I then realized that the aircraft was not descending as per the selected mode. We did not get a horn or a red autopilot warning message. Also the PFD showed that the autopilot was still engaged via the A/P FMA but the aircraft was not descending. We weren't sure if the aircraft was connected to the autopilot or not as per the conflicting information the aircraft was indicating. We then noticed that the aircraft airspeed was slowing and wasn't descending. I disconnected the autopilot and pushed the throttles forward to correct the low airspeed and pushed the yoke forward to start the aircraft descent. We also disconnected and reengaged the flight directors. We then noticed that the yellow 'autopilot mode' caution message had disappeared. Since we believed that the problem had been resolved I reengaged the autopilot. At that time the aircraft continued to descend and fly at the correct assigned airspeed. We didn't however notice that LNAV had not reengaged since we had just reengaged the autopilot and believed that all the modes had been reengaged correctly via the autopilot. At this time; only about 10 seconds later; we noticed that the aircraft was not tracking the arrival course correctly and immediately turned to the left towards the FMC arrival course to intercept. About 5 seconds later ATC said 'it appears that you missed your turn'; and the gave us a 090 heading. The captain told ATC that we were having autopilot issues and were already correcting back to the STAR arrival course. We were then given two more heading changes. We followed ATC directions for the remainder of the approach and landing to a safe landing with no further incident. I'm not certain; but I may have accidentally pushed the autopilot disengage bar while I was reaching for the FLCH button to start the descent.


Synopsis: B787 flight crew reported an autopilot malfunction that caused a track deviation.