787 „disappearing” fuel

[Sidestick_n_Rudder]

After converting to the 787, I have noticed the fuel gauges as well as FMC fuel predictions tend to be quite unreliable, namely:

- during flight there is usually a marked difference between “totalizer fuel” and “calculated fuel” in the FMC. Usually on order of 500kg, with the totalizer fuel being less than calculated fuel. This seems to be associated with switching off the CTR tank pumps, as if this 500kg was disappearing somewhere in the system

- FMC fuel predictions seem to be “optimistic”, ie they initially show higher amount of fuel on landing, which is decreasing as the flight is getting closer to destination. We usually land at up to ~1 tonne less than what was initially predicted by the FMC

Anyone noticed similar thing?

How about other Boeings? I’ve only flown Airbus before and the gauges, as well as FMGC predictions seemed to be much more reliable.

No big deal in day to day ops, but can be treacherous when operating close to minimum fuel and / or in case of diversions…

[sycamore]

I would have thought that `upload/download of fuel`according to engineering should be able to show up any significant discrepancies....or have you not reported it...?

[Commander Taco]

The 767 and 777 have the same characteristic. I’ve seen the 767 and 777 totalizer fuel versus FMC calculated fuel drift apart by as much as 800 to 900 kilos by mid-flight, at which point the two values start to drift back together so that by TOD they become very close. If memory serves, wing-bending is the culprit but perhaps TD can weigh in on this.

As to your FOD dropping, on the Boeings (at least when I last flew the 767/777/787), the SOP was to not load the arrival/approach until sometime prior to top of descent, whereas on the Airbus, we’d load the whole shebang from takeoff runway to approach at destination. Obviously the Boeing method will result in, depending on the airport, a substantial decrease in fuel over destination once the arrival is loaded. At airports like PEK or PVG I’d typically see as much as 1,500kilo decrease in FOD.

[tubby linton]

Airbus publish the tolerances of the fuel system and over the years have been aiming to make them more accurate. The A300-600 used to have 1T that would just disappear out of the fuel total and then would never come back. The most accurate measurement is fuel used.
The accuracy they quote is 1% of current fuel on board plus 1% of maximum tank capacity.

[eckhard]

In my experience, the 787-9 was worse than the 787-8. If I was rostered on a -9, I would always add an extra tonne automatically, to take care of this phenomenon. Some folk reckoned the lost fuel was hiding in the pipework somewhere.....

[FlightDetent]

Was it burned over the expected amount or just indicated missing on the gauges ?

[STBYRUD]

Sounds like the 787 does the same as what I've seen on 737s and the 777 - the center tank, especially when you're drawing the last half hour from it before it becomes empty, overreads by a bit - resulting in a 'sudden' loss of fuel on the OFP once the center tank is empty.

[FullWings]

On the 777 it seems to be a mixture of two things: occasional inaccuracy in the FoB measurement mid-flight when partially loaded, and rather optimistic fuel predictions from the FMC.

The good news is that it seems to get more accurate when you’re down in single figures. I’m not sure what level of detail the FMC goes into on the approach, but you often see 30-40 track miles for 100kg burn which obviously isn’t right, especially as you dirty up on finals.