B777 flight crew has a fuel system malfunction while en route.

Attributes
ACN	718982
Time
Date	200611
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : zan.artcc
State Reference	AK
Altitude	msl single value : 34000
Aircraft 1
Controlling Facilities	artcc : zan.artcc tower : clt.tower
Operator	common carrier : air carrier
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : relief pilot
Experience	flight time last 90 days : 200 flight time total : 11600 flight time type : 4400
ASRS Report	718982
Person 2
Affiliation	company : air carrier
Function	flight crew : relief pilot
Experience	flight time last 90 days : 120 flight time total : 12000 flight time type : 5570
ASRS Report	718983
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	aircraft equipment other aircraft equipment : fuel eicas message other flight crewa other flight crewb
Resolutory Action	flight crew : diverted to another airport flight crew : landed as precaution
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

While acting in the capacity of relief pilot (PNF) and occupying the first officer seat in cruise flight at FL340 east of yev; the fuel imbal EICAS illuminated and alerted us of an imbal between fuel tank #1 and fuel tank #2. The fuel quantity in fuel tank #1 was about 4000 pounds lower than the fuel tank #2. The other relief pilot (PF) and I ran the corresponding fuel imbal checklist; checking all parameters per the checklist. The fuel continued to decrease in the left tank at an abnormal rate and we called the captain to the cockpit and initiated the fuel leak checklist. Communication was established with maintenance dispatch and the B777 assistant chief pilot. We also attempted to contact boeing B777 technical support; which was unsuccessful. During the troubleshooting stage of the flight; we disengaged the autoplt and determined that the aircraft was in trim; with no adverse roll. The fuel tank #1 gauge indicated a fuel imbal of more than 40000 pounds from the fuel tank #2 gauge and was continuing to decrease. We also pulled up the fuel system maintenance page on the FMC to check the fuel probe reading and found that one of the probes did not read close to any reading in the opposite #2 tank or its own tank. This helped the captain to decide that it was a fuel gauge malfunction and not a fuel leak. After assessing the situation with all the participants; the decision was made by the captain to divert to anc. In the descent the #1 tank fuel gauge started to increase fuel quantity; which helped to reinforce our decision. We made a smooth overweight landing in anc.

Original NASA ASRS Text

Title: B777 FLT CREW HAS A FUEL SYS MALFUNCTION WHILE ENRTE.

Narrative: WHILE ACTING IN THE CAPACITY OF RELIEF PLT (PNF) AND OCCUPYING THE FO SEAT IN CRUISE FLT AT FL340 E OF YEV; THE FUEL IMBAL EICAS ILLUMINATED AND ALERTED US OF AN IMBAL BTWN FUEL TANK #1 AND FUEL TANK #2. THE FUEL QUANTITY IN FUEL TANK #1 WAS ABOUT 4000 LBS LOWER THAN THE FUEL TANK #2. THE OTHER RELIEF PLT (PF) AND I RAN THE CORRESPONDING FUEL IMBAL CHKLIST; CHKING ALL PARAMETERS PER THE CHKLIST. THE FUEL CONTINUED TO DECREASE IN THE L TANK AT AN ABNORMAL RATE AND WE CALLED THE CAPT TO THE COCKPIT AND INITIATED THE FUEL LEAK CHKLIST. COM WAS ESTABLISHED WITH MAINT DISPATCH AND THE B777 ASSISTANT CHIEF PLT. WE ALSO ATTEMPTED TO CONTACT BOEING B777 TECHNICAL SUPPORT; WHICH WAS UNSUCCESSFUL. DURING THE TROUBLESHOOTING STAGE OF THE FLT; WE DISENGAGED THE AUTOPLT AND DETERMINED THAT THE ACFT WAS IN TRIM; WITH NO ADVERSE ROLL. THE FUEL TANK #1 GAUGE INDICATED A FUEL IMBAL OF MORE THAN 40000 LBS FROM THE FUEL TANK #2 GAUGE AND WAS CONTINUING TO DECREASE. WE ALSO PULLED UP THE FUEL SYS MAINT PAGE ON THE FMC TO CHK THE FUEL PROBE READING AND FOUND THAT ONE OF THE PROBES DID NOT READ CLOSE TO ANY READING IN THE OPPOSITE #2 TANK OR ITS OWN TANK. THIS HELPED THE CAPT TO DECIDE THAT IT WAS A FUEL GAUGE MALFUNCTION AND NOT A FUEL LEAK. AFTER ASSESSING THE SITUATION WITH ALL THE PARTICIPANTS; THE DECISION WAS MADE BY THE CAPT TO DIVERT TO ANC. IN THE DSCNT THE #1 TANK FUEL GAUGE STARTED TO INCREASE FUEL QUANTITY; WHICH HELPED TO REINFORCE OUR DECISION. WE MADE A SMOOTH OVERWT LNDG IN ANC.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.