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FROM: THE BOEING COMPANY

TO: Boeing Correspondence (MOM)

[MESSAGE NUMBER:MOM-MOM-15-0248-01B(R1)] Multi Operator Message

MESSAGE DATE: 20 Apr 2015 1233 US PACIFIC TIME / 20 Apr 2015 1933 GMT

CORRECTED COPY

This message supersedes Message Number MOM-MOM-15-0248-01B dated 19-Apr-2015 

04:03:24 PM US PACIFIC TIME. Please destroy all copies of Message Number MOM-MOM-15-

0248-01B.

Reason for Corrected Copy: Clarification was included to add ASG to the condition summary and 

to state that the Main and APU batteries do not need to be disconnected. Minor editorial 

corrections made.

FROM: THE BOEING COMPANY

TO: Boeing Correspondence (MOM)

[MESSAGE NUMBER:MOM-MOM-15-0248-01B] Multi Operator Message

MESSAGE DATE: 19 Apr 2015 1602 US PACIFIC TIME / 19 Apr 2015 2302 GMT

This message is sent to selected 787-8 and 787-9 Customers, Regional Directors, Regional 

Managers and Boeing Field Service Bases.

CATEGORY: Maintenance, Engineering, Flight Operations, and Management.

SERVICE REQUEST ID: 3-3222477079 

ACCOUNT: Boeing Correspondence (MOM) 

DUE DATE: 22-Apr-2015 23:59 

PRODUCT TYPE: Airplane 

PRODUCT LINE: 787 

PRODUCT: 787-8/-9 

ATA: 2422-02 

SUBJECT: Action - GCU Clock Failsafe Preventative Maintenance 

 

REFERENCES:

/A/ Power Deactivation AMM DMC-B787-A-24-22-00-00Q-510B-A

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SUMMARY:

This MOM is being sent to advise operators of an issue that could potentially interfere with 

normal operations of the 787. UTAS/Boeing recently confirmed a latent internal clock software 
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issue where, if a GCU operates continuously without being powered down for approximately 

248 days, it will enter a failsafe condition and the associated VFSG or ASG will go off-line. This 

message advises operators on methods to mitigate this issue until a GCU software update is 

available. 

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DESCRIPTION:

After a detailed software review and lab testing, UTAS and Boeing have determined that it is 

possible for an in-service 787 airplane which has not been powered down to a "Cold / Dark" 

state at some point during the last 8 months to experience a simultaneous failsafe of all six GCU 

positions. Please note that this condition has never occurred in-service. UTAS and Boeing 

believe that the probability is extremely low that a simultaneous failsafe of all six GCU positions 

will actually occur in the future because of relatively frequent airplane maintenance power 

down/power cycle activity. Also, please note that this condition is recoverable when the 

Generator Control Switches in the flight deck are cycled per existing QRH flight crew procedures. 

The FAA has indicated they intend to mandate action via an Immediately Adopted Rule 

Airworthiness Directive (AD). The following actions can be performed to prevent any potential 

operational impacts and are anticipated to be sufficient to show compliance with the pending 

AD.

DESIRED ACTION:

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1) Perform a Service Bulletin (SB) incorporation review on applicable 787s to determine if one or 

more of the following SB's has been completed with the last 120 days. These Service Bulletins 

require an airplane power cycle as part of incorporation.

787-24-0024

787-24-0039

787-42-0006

787-42-0009

787-42-0010

787-42-0013

787-42-0014

2) For all 787's in-service which have not had at least one of the above listed SB's incorporated 

in the last 120 days, deactivate/remove all electrical power from the airplane, to a "Cold/Dark" 

state for at least 15 seconds per ref /A/, as soon as convenient. (The Main and APU batteries do 

not need to be disconnected.)

3) Develop a scheduled maintenance task for each 787 in-service to perform step 2) every 120 

days until such a time that the planned GCU software update to resolve this issue is available 

and incorporated.

Boeing requests each 787 operator perform a preliminary SB incorporation / airplane power 

deactivation assessment for each airplane and provide the date that the power cycle activity 

was performed to Boeing by 22-Apr-2015. The date of the power cycle will be documented by 

Boeing and communicated to the FAA. 
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LONG TERM RESOLUTION PLAN:

UTAS and Boeing are developing a GCU software upgrade that is anticipated to be released in 

Q4 2015. Boeing is also creating an FTD to describe / track this project and it will be published by 

the end of April 2015. Boeing also anticipates that this new GCU software SB will be mandated 

by AD after it is published.

If attachments are referred to, and are not present, please access them by logging into 

MyBoeingFleet Service Requests application. If you have further questions, you may contact the 

appropriate Airline Support Manager.

https://myboeingfleet.boeing.com/servicerequests/index.html?entityId=3-

1H9PWSC&entityType=MultiOperatorMessage

John Murphy - 787 Fleet Chief

Courtney Makela

Director - 787 Services

Commercial Aviation Services

The Boeing Company

Corrected copy

If attachments are referred to, and are not present, please access them by logging into 

MyBoeingFleet Service Requests application using the link below. If you have further questions, 

you may contact the appropriate Airline Support Manager.

https://myboeingfleet.boeing.com/servicerequests/index.html?entityId=3-

1HC3RRU&entityType=MultiOperatorMessage