Flight Crew Last 90 Days 100; Flight Crew Total 13250; Flight Crew Type 7100
Troubleshooting; Confusion
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types
Air Traffic Control Issued New Clearance; Aircraft Equipment Problem Dissipated; Flight Crew Returned To Clearance; Flight Crew Became Reoriented; General Maintenance Action
Aircraft
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.
I was the pilot flying First Officer on this flight to ZBAA. The Captain and I were both back from our last break and had received a complete crew change briefing. Both the Captain and I had verified the briefed information and aircraft status to include altitude clearance and settings using meters conversion and the FMA modes engaged of LNAV and VNAV. The autopilot was on and speed was 280 knots. Roughly 80 NM north of TZH on B458; the Captain noticed the aircraft was now in heading select mode. Neither I nor the Captain had selected this mode. LNAV was reselected. The approach briefing was completed and all FMC and mode control panel settings verified. At approximately 3 NM prior to TZH with the ensuing point KM requiring a turn to the east; we noticed the airplane was again in heading select instead of LNAV. I did not see a lead turn magenta line painted on the nav display and immediately initiated a turn to the east toward KM. As we were completing that turn we were given a heading of 070 degrees and then a direct KM clearance. We continued a normal descent to a right downwind east of the airport and landed on Runway 01. All four pilots discussed the issue after the flight. None recall any evidence of inadvertent pushing of the heading select knob button. We all agreed it is a common practice to slew the heading bug to current heading. It was speculated that the heading select switch may have malfunctioned due to wear or improper adjustment or loosening and commanded heading select mode change (push function) instead of the pilot input heading slew (rotate function). It is also possible that programming error in the FMC may have been a factor (i.e. flyover vs. lead turn or active waypoint issue causing heading select reversion; etc.). We wrote up the switch and discussed the issue with ZBAA Maintenance to facilitate troubleshooting.
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.