Narrative:

Following captain and first officer's last break; turnover was accomplished and aircraft autopilot was on in LNAV/VNAV modes. Approximately 20 minutes after crew changeover; as aircraft was approximately 80 NM miles north of tzh; captain noticed that the aircraft had reverted to heading select mode. Aircraft was still on the magenta track line and LNAV was reengaged. Captain took control of aircraft while first officer briefed approach and landing. First officer finished his approach briefing and control of aircraft was turned over to first officer. We had been given a descent to 11;600 meters and we were assigned the STAR KM 7B vice the KM 7A which control had told us to expect. Approx 3 NM prior to tzh; captain again determined that the aircraft had reverted to heading select mode. Aircraft was still on the magenta track line and LNAV was reengaged. Aircraft began turn to KM as programmed; but rate of turn was somewhat slow. First officer reengaged heading select mode and increased aob to 25 degrees. ATC instructed us to fly heading 070 and then subsequently told us to fly direct to KM. Approach and landing were normal without any further instance of aircraft reverting to heading select mode without command. Following landing; crew discussed possible causes. I had been watching all FMA changes while I was backing up correct meters to feet calculations and did not see anyone engage heading select. We wrote a frm report on a possible faulty heading select switch and briefed maintenance personnel in zbaa of the possible issue.

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Original NASA ASRS Text

Title: During the last portion of a flight to ZBAA B777 flight crew notices that the aircraft has come out of LNAV and is in heading select although still on the magenta line. LNAV is reengaged but 80 NM later it is noted in heading select again and again LNAV is reselected and seems to operate normally for the remainder of the descent and approach.

Narrative: Following Captain and First Officer's last break; turnover was accomplished and aircraft autopilot was on in LNAV/VNAV modes. Approximately 20 minutes after crew changeover; as aircraft was approximately 80 NM miles north of TZH; Captain noticed that the aircraft had reverted to Heading Select mode. Aircraft was still on the magenta track line and LNAV was reengaged. Captain took control of aircraft while First Officer briefed approach and landing. First Officer finished his approach briefing and control of aircraft was turned over to First Officer. We had been given a descent to 11;600 meters and we were assigned the STAR KM 7B vice the KM 7A which Control had told us to expect. Approx 3 NM prior to TZH; Captain again determined that the aircraft had reverted to Heading Select Mode. Aircraft was still on the magenta track line and LNAV was reengaged. Aircraft began turn to KM as programmed; but rate of turn was somewhat slow. First Officer reengaged Heading Select mode and increased AOB to 25 degrees. ATC instructed us to fly heading 070 and then subsequently told us to fly direct to KM. Approach and landing were normal without any further instance of aircraft reverting to Heading Select mode without command. Following landing; crew discussed possible causes. I had been watching all FMA changes while I was backing up correct meters to feet calculations and did not see anyone engage Heading Select. We wrote a FRM report on a possible faulty Heading Select Switch and briefed Maintenance personnel in ZBAA of the possible issue.

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.