This bulletin provides information for flight crews regarding an FMC anomaly.
An operator has reported incidents of VNAV incorrectly allowing descent through the selected MCP altitude. This may occur while descending enroute to a holding pattern. When a VNAV descent is in progress and an altitude constraint is entered in 1R on the HOLD page, the altitude constraint will be displayed in large font on the respective HOLD AT line on the LEGS page. This condition results in the altitude constraint being placed on the HOLD, not necessarily on the waypoint. In some cases, VNAV may allow the airplane to descend below the constraint / HOLD altitude, even if the MCP window is set at the HOLD altitude.
A corresponding anomaly does not exist while the airplane is in a climb. While performing a VNAV climb to a holding pattern, the airplane will not climb through the MCP altitude.
This anomaly has been verified by Boeing and exists in the 200K, 700K, 1-Meg Non PIP FMCs. The anomaly will not occur on those FMCs which incorporate the VNAV ALT mode.
The pilot can identify the applicable FMC by reference to Line 5 on the IDENT page. The applicable FMC will show DRAG FACTOR on Line 5L and F-F Factor (fuel flow factor) on Line 5R.
For information purposes, the non-applicable FMCs will show DRAG/FF (drag and fuel flow factor) on Line 5L and CO DATA on Line 5R (PIP FMC) or OPC on Line 5L and DRAG/FF on Line 5R (PEGASUS). The installed CDU does not provide a reliable indication of the installed FMC because some airplanes have been retrofitted with PIP or PEGASUS FMCs while retaining the basic CDU.
When directed to descend and hold at a specified altitude at a waypoint in the route, enter the altitude constraint on the LEGS page. This can be done prior to or after creating the HOLD but must be done on the LEGS page.
This anomaly will be corrected by Boeing Service Bulletin 757-34-0192 or 757-34-0210 or 757-34-0211. Refer to individual Service Bulletin for applicability.