Narrative:

I was seated in the [relief pilot] 2 right hand observer seat; which provides limited visibility of aircraft instrumentation. As [relief pilot] 2 on the short flight; I did not operate as a flying pilot and only acted to relay information during troubleshooting.upon reaching 10;000 ft. I began to calculate the flight plan waypoint times. We began to encounter light; borderline moderate; turbulence and I looked at the radar; which was only depicting green returns within about a 20 mile radius at wx+t -20M setting. The captain reduced the speed via the MCP (mode control panel) and verbally stated his rationale due to the turbulence. I returned to concentrating on trying to write the times in turbulence when I felt what seemed to be a higher than normal deck angle and heard the captain remarking something along the lines of 'what is this doing.' I looked at the captain's pfd (primary flight display) from my seat saw the airspeed significantly below the bugged speed; I looked at the standby indicator and saw what appeared to be an airspeed of 216 kts.; similar to that on the pfd. At this point the turbulence had mostly subsided and we were in visual conditions. The pli (pitch limit indicator) displayed on the pfd and there was a slight vibration/buffet felt. The captain disengaged the autopilot and advanced the thrust levers and advised he had control of the airplane. The first officer (first officer) who was receiving a pm (pilot monitoring) line check grabbed the controls and the captain restated he had control of the aircraft. The captain rectified the decaying airspeed; however the aircraft would not maintain the desired speed as set in the MCP and/or FMS (flight management system). The captain then elected to use flch (flight level change) and the aircraft slowly regained speed and continued the climb and leveled off with the autopilot engaged. The four person crew discussed the situation but could not understand the reason for decaying airspeed with VNAV engaged. A call was placed to consult with the company regarding the situation. At this time I completed the flight plan times.as the flight progressed; a noticeable buffet appeared in the aircraft and another call was placed to maintenance regarding vibrations in the right engine. It is unknown if this vibration was a contributing factor related to the previous situation; but remained during all future phases of flight. In the descent into ZZZZ; an increase in vibration was noticed by all crew members out of approximately FL300 and again at approximately FL190. The aircraft also had trouble leveling off in the descent; continually overshooting altitude level offs by at least 20 - 40 ft. At one point; a lower altitude was set and flch was selected to descend; and the aircraft began to climb; requiring disengagement of the autopilot and manually selecting thrust to force the aircraft not to climb. On the initial approach segment; the aircraft again was not responding to MCP/FMS inputs and the captain was required to hand fly the approach to landing.I personally completed the pre and post flight walk arounds and did not note any defects. Maintenance in ZZZZ pulled engine data and stated there was a significant engine vibration detected. The captain made several log book notations as to the erratic behavior of the aircraft automation and the vibration.

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Original NASA ASRS Text

Title: Boeing 777 Relief Pilot reported autopilot anomalies and engine vibrations inflight.

Narrative: I was seated in the [Relief Pilot] 2 right hand observer seat; which provides limited visibility of aircraft instrumentation. As [Relief Pilot] 2 on the short flight; I did not operate as a flying pilot and only acted to relay information during troubleshooting.Upon reaching 10;000 ft. I began to calculate the flight plan waypoint times. We began to encounter light; borderline moderate; turbulence and I looked at the radar; which was only depicting green returns within about a 20 mile radius at wx+t -20M setting. The Captain reduced the speed via the MCP (Mode Control Panel) and verbally stated his rationale due to the turbulence. I returned to concentrating on trying to write the times in turbulence when I felt what seemed to be a higher than normal deck angle and heard the Captain remarking something along the lines of 'what is this doing.' I looked at the Captain's PFD (Primary Flight Display) from my seat saw the airspeed significantly below the bugged speed; I looked at the standby indicator and saw what appeared to be an airspeed of 216 kts.; similar to that on the PFD. At this point the turbulence had mostly subsided and we were in visual conditions. The PLI (Pitch Limit Indicator) displayed on the PFD and there was a slight vibration/buffet felt. The Captain disengaged the autopilot and advanced the thrust levers and advised he had control of the airplane. The FO (First Officer) who was receiving a PM (Pilot Monitoring) line check grabbed the controls and the Captain restated he had control of the aircraft. The Captain rectified the decaying airspeed; however the aircraft would not maintain the desired speed as set in the MCP and/or FMS (Flight Management System). The Captain then elected to use FLCH (Flight Level Change) and the aircraft slowly regained speed and continued the climb and leveled off with the autopilot engaged. The four person crew discussed the situation but could not understand the reason for decaying airspeed with VNAV engaged. A call was placed to consult with the company regarding the situation. At this time I completed the flight plan times.As the flight progressed; a noticeable buffet appeared in the aircraft and another call was placed to maintenance regarding vibrations in the right engine. It is unknown if this vibration was a contributing factor related to the previous situation; but remained during all future phases of flight. In the descent into ZZZZ; an increase in vibration was noticed by all crew members out of approximately FL300 and again at approximately FL190. The aircraft also had trouble leveling off in the descent; continually overshooting altitude level offs by at least 20 - 40 ft. At one point; a lower altitude was set and FLCH was selected to descend; and the aircraft began to climb; requiring disengagement of the autopilot and manually selecting thrust to force the aircraft not to climb. On the initial approach segment; the aircraft again was not responding to MCP/FMS inputs and the Captain was required to hand fly the approach to landing.I personally completed the pre and post flight walk arounds and did not note any defects. Maintenance in ZZZZ pulled engine data and stated there was a significant engine vibration detected. The Captain made several log book notations as to the erratic behavior of the aircraft automation and the vibration.

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.