Narrative:

Fuel imbalance main tanks 1-4. Abnormal condition occurrence shortly prior top of decent point. Result; close flight plan fuel monitoring and QRH action reveals a sudden substantial loss of fuel related to main tank number 01 within a 30 minute period during the final phase of flight.research and inquiry of previous flights into maintenance logbook with inquiry amongst other crews also reveal an irregular intermittent trending history of fuel imbalance 1-4 events with moderate to heavy fuel over-burns for those flights also. I am unsure if these events of other crews have been reported. Suspect cause is a possible insidious fuel leak of main tank number 01 that may have increased in severity causing the rapid loss of a large amount of fuel within a 30 minute period prior to arrival; recommend serious mx investigation.planned flight time of 04:20 minutes; actual flight time of 04:50; additional 30 minutes of flight time due to ATC and some wind differences taken into account however is not a trigger for an imbalance event with an over-burn that does not match the additional time considered. Fuel monitoring from takeoff to prior the top of decent point over a distance of 2000 nautical miles constant at -0.1 kgs then within a 19 nautical mile distance block prior the top of decent point fuel trends to -0.3 kgs vs computer flight plan fuel. Shortly afterwards EICAS message 'fuel imbal 1-4' reveals main tank number 01 at 2;200 kgs vs main tank #04 at 4;100 kgs.initial covering of the QRH procedure was covered leading to meeting all the parameters of a suspected fuel leak. Further orh procedures directs an engine shutdown due to the suspected loss of fuel. Sensible judgment dictated that in this area due to the heavy pilot workload during decent and arrival phase within the heavy traffic terminal area and shortly before a landing with sufficient main tank 1 fuel; completion of a long complicated engine shutdown checklist would not be a viable option. An alternative attempt to troubleshoot within a short period was made leaving cross-feed valves open during descent and a constant main tank 1 monitoring program put into effect. This revealed a more rapid decrease in fuel in main tank #01 prior to landing and after approximately 10-12 minutes the cross-feed valves were subsequently closed to end the event.a non-eventful landing completed at destination however arrival occurred at approximately 5.7 tons below the recommended flight plan arrival fuel with a block in fuel of company minimum occurring at 9.0 tons. In the event an actual diversion to alternate had been required the estimated arrival would likely be an approximate total remaining fuel of less approximately 5.0 tons; this would be a fuel emergency event.additional factors 1. It was also noted during the preflight that the airplane drag/ff factor was incorrect at -0.3 however this relates to calculated vs totalizer FMS calculations and has no effect on the actual physical rate at which fuel decreases in a main tank.2. ATC arrival had changed the published arrival 3 times during the actual descent resulting in the aircraft finally being routed south of the airport then back with some speed control for the southern runway a benefit gaining a very short taxi time after landing; however most of the arrival changes occurred during high altitude resulting in more direct routing during the descent with little to idle power settings.3. Previous flight analysis example data from other crew;computer flight plan fuel 74.0; actual ramp fuel 77.1 (3.0 tons extra requested)computer flight plan recommended arrival fuel 14.6 actual arrival fuel 12.7This crew also experienced a brief fuel imbalance 1 -4 event during the departure phase with no further action required. The focus here needs to be on the time and rate at which main tank number 01 decreased in fuel prior to landing as the problem may be insidious in nature and difficult to fully detect and based on apparent past events and log book fuel history the focus and basis to trouble shoot should be on the EICAS message 'fuel imbal 1-4' and not the fuel burn itself.actual fuel analysis of the flight is as follows also with picture attachments;1. Computer flight plan total fuel (with 2.7 ton extra) 63.02. Captain decision fuel (1.0 ton extra) 64.03. Planned flight time/ actual flight time 04:20/04:50 (difference 30 minutes)4. Computer flight plan planned burn/actual burn 47.6/53.9 (difference 6.3 tons)5. Computer flight plan estimated burn 48.3 (P/M score -5.6 tons)6. Logbook fuel remain/ logbook burn 9.0/54.9

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Original NASA ASRS Text

Title: B747 Captain reported they had a rapid loss of a large amount of fuel within a 30 minute period.

Narrative: Fuel Imbalance Main Tanks 1-4. Abnormal condition occurrence shortly prior Top of Decent point. Result; close flight plan fuel monitoring and QRH action reveals a sudden substantial loss of fuel related to Main Tank Number 01 within a 30 minute period during the final phase of flight.Research and Inquiry of previous flights into Maintenance logbook with inquiry amongst other crews also reveal an irregular intermittent trending history of fuel Imbalance 1-4 events with moderate to heavy fuel over-burns for those flights also. I am unsure if these events of other crews have been reported. Suspect cause is a possible insidious fuel leak of Main Tank Number 01 that may have increased in severity causing the rapid loss of a large amount of fuel within a 30 minute period prior to arrival; recommend serious MX investigation.Planned flight time of 04:20 minutes; actual flight time of 04:50; additional 30 minutes of flight time due to ATC and some wind differences taken into account however is not a trigger for an imbalance event with an over-burn that does not match the additional time considered. Fuel monitoring from takeoff to prior the top of decent point over a distance of 2000 nautical miles constant at -0.1 KGS then within a 19 nautical mile distance block prior the top of decent point fuel trends to -0.3 KGS vs computer flight plan fuel. Shortly afterwards EICAS message 'FUEL IMBAL 1-4' reveals main tank Number 01 at 2;200 KGS vs main tank #04 at 4;100 KGS.Initial covering of the QRH procedure was covered leading to meeting all the parameters of a suspected fuel leak. Further ORH procedures directs an engine shutdown due to the suspected loss of fuel. Sensible judgment dictated that in this area due to the heavy pilot workload during decent and arrival phase within the heavy traffic terminal area and shortly before a landing with sufficient Main Tank 1 fuel; completion of a long complicated engine shutdown checklist would not be a viable option. An alternative attempt to troubleshoot within a short period was made leaving Cross-feed valves open during descent and a constant main tank 1 monitoring program put into effect. This revealed a more rapid decrease in fuel in Main Tank #01 prior to landing and after approximately 10-12 minutes the cross-feed valves were subsequently closed to end the event.A non-eventful landing completed at destination however arrival occurred at approximately 5.7 tons below the recommended flight plan arrival fuel with a block in fuel of company minimum occurring at 9.0 tons. In the event an actual diversion to alternate had been required the estimated arrival would likely be an approximate total remaining fuel of less approximately 5.0 tons; this would be a fuel emergency event.Additional factors 1. It was also noted during the preflight that the airplane DRAG/FF Factor was incorrect at -0.3 however this relates to calculated vs totalizer FMS calculations and has no effect on the actual physical rate at which fuel decreases in a main tank.2. ATC arrival had changed the published arrival 3 times during the actual descent resulting in the aircraft finally being routed south of the airport then back with some speed control for the southern runway a benefit gaining a very short taxi time after landing; however most of the arrival changes occurred during high altitude resulting in more direct routing during the descent with little to idle power settings.3. Previous Flight Analysis example data from other crew;Computer Flight Plan Fuel 74.0; Actual Ramp Fuel 77.1 (3.0 tons extra requested)Computer Flight Plan Recommended Arrival Fuel 14.6 Actual Arrival Fuel 12.7This crew also experienced a brief Fuel Imbalance 1 -4 event during the departure phase with no further action required. The focus here needs to be on the time and rate at which Main Tank Number 01 decreased in fuel prior to landing as the problem may be insidious in nature and difficult to fully detect and based on apparent past events and log book fuel history the focus and basis to trouble shoot should be on the EICAS message 'FUEL IMBAL 1-4' and not the fuel burn itself.Actual Fuel Analysis of the flight is as follows also with picture attachments;1. Computer Flight Plan total fuel (with 2.7 ton extra) 63.02. Captain decision fuel (1.0 ton extra) 64.03. Planned flight time/ Actual flight time 04:20/04:50 (Difference 30 minutes)4. Computer Flight Plan Planned Burn/Actual Burn 47.6/53.9 (Difference 6.3 tons)5. Computer Flight Plan estimated burn 48.3 (P/M Score -5.6 tons)6. Logbook Fuel Remain/ Logbook Burn 9.0/54.9

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.