B777 flight crew reported diverting to an alternate after experiencing an apparent fuel leak.

Narrative:

Enroute to [an international destination] an overweight landing and diversion to [a suitable alternate] became necessary due to a sudden loss of fuel. We had been ahead of flight plan fuel estimates by approximately 700 lbs at previous waypoints and fuel balance and quantity had been checked approximately 5 minutes prior to getting the fuel imbalance EICAS warning due to right wing fuel quantity suddenly being approximately 5000 lbs below left tank fuel. The fuel imbalance checklist was accomplished and we were now going to be 4900 lbs below flight plan fuel estimate at next waypoint and it appeared we must have developed a fuel leak. Because we still had lots of fuel and were not yet fuel critical I first coordinated with dispatch regarding a divert because of this fuel disparity. The relief pilot was awakened early to inspect the wings and engines for evidence of a fuel leak; and we then notified ATC of our intentions and needs. We then requested and received an inspection and escort to the gate by fire/arff after an uneventful overweight landing. The performance of all of my crew; including flight attendants was great and I have no suggestions at this time to anyone involved in our handling of this event to have improved our situation. The diversion and landing although busy and task saturated; went smoothly.

Original NASA ASRS Text

Title: B777 flight crew reported diverting to an alternate after experiencing an apparent fuel leak.

Narrative: Enroute to [an international destination] an overweight landing and diversion to [a suitable alternate] became necessary due to a sudden loss of fuel. We had been ahead of flight plan fuel estimates by approximately 700 lbs at previous waypoints and fuel balance and quantity had been checked approximately 5 minutes prior to getting the Fuel Imbalance EICAS warning due to right wing fuel quantity suddenly being approximately 5000 lbs below left tank fuel. The Fuel Imbalance checklist was accomplished and we were now going to be 4900 lbs below flight plan fuel estimate at next waypoint and it appeared we must have developed a fuel leak. Because we still had lots of fuel and were not yet fuel critical I first coordinated with dispatch regarding a divert because of this fuel disparity. The Relief Pilot was awakened early to inspect the wings and engines for evidence of a fuel leak; and we then notified ATC of our intentions and needs. We then requested and received an inspection and escort to the gate by Fire/ARFF after an uneventful overweight landing. The performance of all of my crew; including Flight Attendants was great and I have no suggestions at this time to anyone involved in our handling of this event to have improved our situation. The diversion and landing although busy and task saturated; went smoothly.

Data retrieved from NASA's ASRS site 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.

Attributes
ACN	1362779
Time
Date	201606
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Daylight
Aircraft 1
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Fuel Quantity-Pressure Indication
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Inflight Event / Encounter Fuel Issue