To inform flight crews of a Pegasus-FMC anomaly resulting in the lock up of one or both FMCs and to provide temporary operating instructions for lock-up resolution.
Boeing has confirmed operator reports of in-service single and dual Pegasus-FMC lock-up events resulting from a data-bus communications anomaly. When the anomaly occurs, one or both FMCs may lock up preventing normal CDU access and control of the FMC(s). This lock-up anomaly and the information and recommended operating instructions contained in this Operations Manual Bulletin are only applicable to Pegasus-FMC software versions.
Operator reports indicate the most common lock-up event involves a single FMC; however, dual FMC lock-up events have also been reported. An FMC lock up is indicated by the continuous display of the SINGLE FMC OPERATION scratchpad message with no response to CDU function or line-select keys and may be accompanied by display of the L,R FMC FAIL advisory-level EICAS alert message(s), illumination of the amber FAIL light on the CDU(s), and the amber MAP flag on the associated HSI display(s). In some cases, both FMCs may continue to operate normally, but no data communication or data comparison occurs between the master FMC and the spare FMC.
Honeywell is aware of this Pegasus-FMC anomaly. The planned fix to the problem is under investigation.
If the SINGLE FMC OPERATION scratch-pad message displays on the ground on either CDU, or both FMCs lock up as described above on the ground or inflight, a single attempt at cycling both FMC circuit breakers can be accomplished, flight conditions permitting, as follows:
Note: If in-flight, Boeing recommends the following procedure be accomplished with an autopilot and the autothrottle engaged due to the requirement of a flight crew member leaving his/her station to achieve access to the overhead circuit breaker panel.
Begin route entry by re-entering ORIGIN airport identifier to ensure previous route is initially deleted.
If normal FMC operation is restored, LNAV and/or VNAV may be engaged, as needed.
The temporary procedural steps provided above should restore normal FMC operation; however, some FMC faults may preclude normal operation. If the above procedure does not restore normal FMC operation, DO NOT accomplish a second attempt as this may result in further systems’ degradation.
If the SINGLE FMC OPERATION scratch-pad message displays during flight, accomplish the FMC FAIL checklist as published in the Boeing Quick Reference Handbook (QRH), or operator equivalent.
Accomplishing the checklist steps contained in the FMC FAIL checklist will configure the airplane systems for single FMC operation. For Pegasus-FMC equipped airplanes which interface with CDUs equipped with the MENU mode-select key, continued ETOPS operation should not be compromized due to the alternate navigation functionality.
This Operations Manual Bulletin will be revised to include service bulletin information when available. This Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes Operations Manual in the operator’s fleet have been modified.
Boeing Maintenance Tip 757 MT 34-044 is related to this Operations Manual Bulletin.