Narrative:

Approximately 5 hours into our flight I was on a rest break and was notified to go to the cockpit. We had a fuel imbalance EICAS message appear. The right main fuel total was decreasing 100 pounds every 2 seconds until it reached an indication of 59.0 pounds. At the time; the engines were still feeding off the center tank and it had approximately 14.0 pounds indicated. Fuel checks prior to this had been accurate and a check of the calculated engine burn was in line with the flight plan. We turned the autopilot off to check the lateral balance of the aircraft and no discrepancy in balance what-so-ever was noticed. We then opened the maintenance fuel quantity display on the lower EICAS and noted that fuel probe 18 in the right tank showed a value of .9 while fuel probe 18 in the left tank had a value of 12.6. We also noted that fuel probe 19 in the right main tank; which we assumed was at a higher level physically in the tank; had a value of 11.1 and was in line with the value indicated on probe 19 in the left tank. We called dispatch through satcom and had them patch us to maintenance control to see if there was any history on this and have them query the aircraft to see what they could tell was going on with the aircraft. Tech reported that the aircraft had recently had a history of fuel indication problems with the right tank and 2 tank probes had recently been changed however; probe 18 was not one of the problem probes that had been changed. They also reported that a query of the aircraft indicated a right fuel probe error. We discussed that the location of the fuel probe 18 and the resulting total fuel indication of 59.0 for that tank seemed logical that the system wasn't reading anything above probe 17. We also discussed that as the fuel was used out of the right main tank that the system should return to normal once the level got below the probe in error. During this discussion the fuel quantity in the right main tank returned to an indication of 62.5 and after a short time again decreased to 59.0 at a rate of 100 pounds per 2 seconds. Confident that we only had a fuel indication problem; and had not lost any quantity of fuel; I made the decision to continue the flight as planned. I also made the decision to not run the fuel imbalance checklist that was called for as it was inappropriate for the problem we were encountering. Once the right main tank fuel quantity was reduced to a level below the probe in error; the fuel indication system returned to normal; our fuel burn and quantity was in line with the expectation of our flight plan progress sheet and no other discrepancies were noted for the remainder of the flight.

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Original NASA ASRS Text

Title: A B777's fuel quantity indication began decreasing unexpectedly during cruise. The Crew's troubleshooting and discussion with Maintenance Control determined that several fuel quantity probes in the effected tank were malfunctioning but as fuel burned quantity would become accurate.

Narrative: Approximately 5 hours into our flight I was on a rest break and was notified to go to the cockpit. We had a Fuel Imbalance EICAS message appear. The right main fuel total was decreasing 100 LBS every 2 seconds until it reached an indication of 59.0 LBS. At the time; the engines were still feeding off the center tank and it had approximately 14.0 LBS indicated. Fuel checks prior to this had been accurate and a check of the Calculated Engine burn was in line with the flight plan. We turned the autopilot off to check the lateral balance of the aircraft and no discrepancy in balance what-so-ever was noticed. We then opened the Maintenance Fuel Quantity display on the lower EICAS and noted that fuel probe 18 in the right tank showed a value of .9 while fuel probe 18 in the left tank had a value of 12.6. We also noted that fuel probe 19 in the right main tank; which we assumed was at a higher level physically in the tank; had a value of 11.1 and was in line with the value indicated on probe 19 in the left tank. We called Dispatch through SATCOM and had them patch us to Maintenance Control to see if there was any history on this and have them query the aircraft to see what they could tell was going on with the aircraft. Tech reported that the aircraft had recently had a history of fuel indication problems with the right tank and 2 tank probes had recently been changed however; probe 18 was not one of the problem probes that had been changed. They also reported that a query of the aircraft indicated a right fuel probe error. We discussed that the location of the fuel probe 18 and the resulting total fuel indication of 59.0 for that tank seemed logical that the system wasn't reading anything above probe 17. We also discussed that as the fuel was used out of the right main tank that the system should return to normal once the level got below the probe in error. During this discussion the fuel quantity in the right main tank returned to an indication of 62.5 and after a short time again decreased to 59.0 at a rate of 100 LBS per 2 seconds. Confident that we only had a fuel indication problem; and had not lost any quantity of fuel; I made the decision to continue the flight as planned. I also made the decision to not run the Fuel Imbalance checklist that was called for as it was inappropriate for the problem we were encountering. Once the right main tank fuel quantity was reduced to a level below the probe in error; the fuel indication system returned to normal; our fuel burn and quantity was in line with the expectation of our flight plan progress sheet and no other discrepancies were noted for the remainder of the flight.

Data retrieved from NASA's ASRS site as of April 2012 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.