This bulletin provides information informing flight crews of temporary recommended procedures for VOR remote tuning.
While remote tuning a VOR with the FMC during a recent test flight, a dual FMC restart occurred with a loss of performance data and deactivation of the active route. Investigation has revealed that this anomaly is due to a conflict between the FMC tuning master, which is normally the left FMC, and the FMC guidance master, which is determined by which autopilot is engaged. Therefore, this anomaly only occurs when remote tuning a VOR while the right autopilot is engaged, but does not occur every time this situation is encountered.
The indication to the flight crew that this anomaly is about to occur is that when a VOR frequency or identifier is line selected to the VOR display line on Progress page 1/2, the data remains in the scratch pad and does not transfer to the line. If no further action is attempted, an FMC generated short term interrupt will occur, the scratch pad will clear and normal operation will be restored. However, a second attempt to remote tune will result in a long term restart and a loss of performance data and deactivation of the route. A third attempt will result in dual FMC failure (single failure with -952 and -953 software).
Until this anomaly is corrected, do not remote tune VOR stations using the FMC when the right autopilot is engaged.
This anomaly is corrected by Boeing Service Bulletin 757-34-0143.