APU fuel burn on the B777

[Analyser]

Why is it that the APU fuel burn changes with weight at a fixed altitude.I can understand that it changes with altitude due to the drag on the APU door but cant seem to figure out with different weights and at the same altitude.

weight 300t. 260t. 220t
FL 310 230kg/hr. 220. 195.
FL100. 240. 240. 230.

[First IFE]

Probably has to do with a sub optimized APU inlet design and therefore different burns as a result of AoA changes with weight changes.

[Dave Therhino]

18% burn rate change seems way too large for the AOA difference to be the cause, and the boundary layer is pretty thick at the door anyway. I suspect it may be electrical load difference related somehow. Are the numbers for engine out operation?

[Vessbot]

Does the electrical load change with weight?

[Dave Therhino]

It can, but not in a specific way. If the center tank is empty those pumps are off, but the weight at which that occurs varies. If you have a lower payload maybe fewer ovens are used to heat meals, but I think galley loads would be shed if you were engine out. It would help to know the condition the burn numbers are for. Engine failure? IDG out dispatch?

I don't know the answer to the OP's question - I'm speculating.

[First IFE]

My experience with APU design (I understand, can be different use case) is it is brute force, I.e. the entire design is not optimized other than to support power or cooling needs. Installation is a fallout. I understand as pointed out it is way back in the thick boundary layer, but given that there is nothing else detailed in the manual other than weight, seems it has to do with AoA. I am also speculating...

[Analyser]

Sorry for the late response.Numbers are from the FCOM.
Normal figures for APU fuel burn
No non-normal associated with the numbers

[Vessbot]

That's the great thing about web forums where you don't pay by the word, you can enjoy the exploration of questions you find interesting, that aren't "the greatest concern," or even any practical concern.

[TURIN]

It doesn't matter.
The fuel tank gauges aren't that accurate anyway.

[birdspeed]

My thoughts are that’s speed related. The higher the weight the greater the scheduled speed. So more aerodynamic drag.

[FullWings]

I would be wary of dismissing things like this as irrelevant, as they sometimes expose significant errors on investigation. Some time ago I was looking at the QRH, in the cruise, at some of the tables, including stopping distances and pitch/power for UAS. There were bits that just seemed odd and I found difficult to understand, so I queried them with our tech guys. Some time later they said that they’d got back to Boeing, who’d gone *whoops* and published new (correct) tables...

[Vessbot]

My thoughts are that’s speed related. The higher the weight the greater the scheduled speed. So more aerodynamic drag.
These numbers are burn over time, not over distance.

[turbidus]

While for the Bus...this has some good information....(page 21 for APU)

ECS rules?

https://ansperformance.eu/library/airbus-fuel-economy.pdf

[Machdiamond]

birdspeed you may be on to something: greater weight, scheduled speed means higher ram pressure recovery at the APU inlet and thus higher power capacity and fuel flow.

[Analyser]

Thanks birdseed. Sorry for the late reply but sounds accurate.