To inform crews that during climb the VNAV PATH mode can temporarily overshoot its programmed altitude.
Recent testing has shown that during VNAV climbs (using either flight director or autopilot), the VNAV PATH mode may temporarily overshoot the programmed altitude. This can occur during level offs at either waypoint altitude constraints or at the cruise altitude entered on the VNAV cruise page. Altitude overshoots of as much as 500 feet have been observed before the airplane returned to the programmed altitude.
These overshoots are caused by two factors:
If either of these events happens at or near the altitude where the FMC transitions to VNAV PATH for the level off, mode transition to VNAV PATH is delayed, and the airplane will overshoot the programmed altitude by an amount proportional to the climb rate.
Setting the MCP altitude to each intermediate VNAV waypoint altitude constraint will assure that level offs below the final cruise altitude will occur using the ALT CAP mode, and the overshoot will not occur. However, the final VNAV cruise altitude is always captured in VNAV PATH, and the overshoot will be possible, even if the MCP altitude is set to the final cruise value.
This anomaly only occurs with the following FMCs installed:
Avoid executing any FMC modifications when approaching either a waypoint altitude constraint or the final cruise altitude.
All intermediate waypoint altitude restrictions should be set on the MCP.
During all level offs in the VNAV PATH mode, be prepared to disconnect the autopilot/flight director and manually level the airplane as required.
This anomaly is corrected by Boeing Service Bulletin 757-34-0153 or Boeing Service Bulletin 757-34-0154 or Boeing Service Bulletin 757-34-0155. Refer to individual Service Bulletin for applicability.