Boeing Bulletins

FMC-22 : VNAV Descent Through MCP Altitude

15 November 1996

Effectivity

Please refer to source document.

Reason

This bulletin provides information informing flight crews of temporary recommended procedures for VNAV descents.

Information in this bulletin is recommended by The Boeing Company, but may not be FAA approved at the time of writing. In the event of conflict with the FAA approved Airplane Flight Manual (AFM), the AFM shall supersede. The Boeing Company regards the information or procedures described herein as having a direct or indirect bearing on the safe operation of this model airplane.

Background Information

Operators have reported a condition in which the airplane may descend below the MCP ALTITUDE WINDOW value during VNAV descent.

Boeing has determined this anomaly can occur on all 757 airplanes except for airplanes that both:

Have "VERSION 2.0" displayed on the FMC CDU IDENT page after the operating program number, and
Incorporate the VNAV ALT mode.

VNAV descents may continue below the MCP Altitude Window setting when both of the following conditions have been met.

The HOLD AT line on the RTE LEGS page contains a B (at or below) altitude constraint, and
The MCP ALTITUDE WINDOW is set for the same value as the B altitude constraint.

The anomaly will not occur when a B altitude constraint is entered on the waypoint line above the HOLD AT waypoint line.

Operating Instructions

For VNAV descent, flight crews should not use B (at or below) altitude constraints on the HOLD AT line of the RTE LEGS page for holding patterns in the descent profile.

Administrative Information (Extract)

This Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes in the operators fleet have been modified by Boeing Service Bulletin 757-34-0118 or Boeing Service Bulletin 757-34-0119 or Boeing Service Bulletin 757-34-0120 or Boeing Service Bulletin 757-34-0123.