Narrative:

I flew aircraft X [on a transpacific flight]. The flight plan drag/fuel flow (ff) number was -3.6. I have been tracking fuel burn predictions in the FMC with aircraft that have ff numbers in the -3.x to -4.x range for some time. I find that on these aircraft; FMC fuel over destination (FOD) predictions are not reliable. In this example at top of climb (TOC) the predicted fuel over destination was 21.5 with a sabre flight plan remaining fuel (remf) of 15.0. Our cost index was 20. We flew the flight plan speed; cost index (ci) and altitude profile. The tow chg was -1154 pounds under flight plan weight. I would expect that being below flight plan weight; we would see improved performance throughout the flight. But not on these aircraft. At each waypoint the FMC FOD would decrease significantly. At 2:00 hours into the flight the FMC FOD was 20.5. At 4:00 into the flight FMC FOD was 19.1. At 6:20 FMC FOD was 17.6. At 8:14; FOD was 16.6. At 10:49; about 30 minutes before top of descent (TOD); it was showing 15.3. The [paperwork] is also of interest. I got the first [fuel update] at TOC and it predicted 15.3 FOD. At W141 where the FMC predicted 19.1 the [paperwork] showed 14.2. At tobby it showed 13.3 and the FMC indicated 15.4. Actual winds aloft and temperature were very close to flight plan. The fuel log showed a steady decrease as well; although not as rapid. At TOC we were up .3. At 4:07 hours we were down .8. At 7:00 hours it was down 1.2. And from 9:00 hours to the TOD point it was down 1.6. Critical point fuel was never a factor. Actual fuel at touchdown was 15.1 with block in about 14.7; but we got some big shortcuts during the arrival phase. I find the aircraft that have a ff number in the -2.x range are much more predictable. Anything in the -3.x to 4.x range; I don't trust the FMC. We no longer track drag. But could this be attributed to airframe drag or rigging issue that is being addressed by the sabre flight plan; but not the FMC? The beauty of the 787 is the speed range and the ability to make up time if running late. One of the factors to evaluate the feasibility of bumping up the speed is the FMC ETA and fuel predictions. An overly optimistic FOD prediction by the FMC could easily lead a crew to increase the ci significantly to make up a late departure; only to run short of fuel at the destination or have implications if a diversion were needed.

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Original NASA ASRS Text

Title: B787 Captain reported he observed overly optimistic fuel projections in the FMC on aircraft using Drag/Fuel Flow numbers in a specific numerical range.

Narrative: I flew Aircraft X [on a transpacific flight]. The flight plan Drag/Fuel Flow (FF) number was -3.6. I have been tracking fuel burn predictions in the FMC with aircraft that have FF numbers in the -3.x to -4.x range for some time. I find that on these aircraft; FMC Fuel Over Destination (FOD) predictions are not reliable. In this example at Top Of Climb (TOC) the predicted Fuel Over Destination was 21.5 with a Sabre flight plan Remaining Fuel (REMF) of 15.0. Our Cost Index was 20. We flew the flight plan speed; Cost Index (CI) and altitude profile. The TOW CHG was -1154 LBS under flight plan weight. I would expect that being below flight plan weight; we would see improved performance throughout the flight. But not on these aircraft. At each waypoint the FMC FOD would decrease significantly. At 2:00 hours into the flight the FMC FOD was 20.5. At 4:00 into the flight FMC FOD was 19.1. At 6:20 FMC FOD was 17.6. At 8:14; FOD was 16.6. At 10:49; about 30 minutes before Top Of Descent (TOD); it was showing 15.3. The [paperwork] is also of interest. I got the first [fuel update] at TOC and it predicted 15.3 FOD. At W141 where the FMC predicted 19.1 the [paperwork] showed 14.2. At TOBBY it showed 13.3 and the FMC indicated 15.4. Actual winds aloft and temperature were very close to flight plan. The fuel log showed a steady decrease as well; although not as rapid. At TOC we were up .3. At 4:07 hours we were down .8. At 7:00 hours it was down 1.2. And from 9:00 hours to the TOD point it was down 1.6. Critical Point fuel was never a factor. Actual fuel at touchdown was 15.1 with block in about 14.7; but we got some big shortcuts during the arrival phase. I find the aircraft that have a FF number in the -2.x range are much more predictable. Anything in the -3.x to 4.x range; I don't trust the FMC. We no longer track DRAG. But could this be attributed to airframe drag or rigging issue that is being addressed by the Sabre flight plan; but not the FMC? The beauty of the 787 is the speed range and the ability to make up time if running late. One of the factors to evaluate the feasibility of bumping up the speed is the FMC ETA and Fuel predictions. An overly optimistic FOD prediction by the FMC could easily lead a crew to increase the CI significantly to make up a late departure; only to run short of fuel at the destination or have implications if a diversion were needed.

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.