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APU fuel burn on the B777

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APU fuel burn on the B777

Old 23rd Mar 2020, 12:37
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APU fuel burn on the B777

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.
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Old 23rd Mar 2020, 20:34
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Probably has to do with a sub optimized APU inlet design and therefore different burns as a result of AoA changes with weight changes.
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Old 24th Mar 2020, 04:20
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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?
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Old 24th Mar 2020, 04:24
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Does the electrical load change with weight?
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Old 24th Mar 2020, 06:51
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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.
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Old 24th Mar 2020, 19:16
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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...

Last edited by First IFE; 25th Mar 2020 at 02:19.
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Old 17th May 2020, 13:21
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Sorry for the late response.Numbers are from the FCOM.
Normal figures for APU fuel burn
No non-normal associated with the numbers

Last edited by Analyser; 17th May 2020 at 18:22.
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Old 17th May 2020, 16:31
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The question seems no more than academic to me, and in reality, irrelevant. I'm certain that the answer is no more than rounded figures recorded under various test conditions. Between 300T and 210T in an aircraft burning 7500 KG (roughly) per hour, regardless of percentages placed on APU burn, the median difference is quoted as 14 kg/hr.

Off the top of my head, I'd go with intake entry angle at various AOA, APU Bleed/Electric demand under load, airplane configuration/speed blah ... blah ... blah. Nothing important really.

If you're flying a B777 and this is your greatest concern, you're doing pretty damned good.

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Old 17th May 2020, 17:19
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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.
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Old 17th May 2020, 20:01
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It doesn't matter.
The fuel tank gauges aren't that accurate anyway.
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Old 25th May 2020, 08:51
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My thoughts are that’s speed related. The higher the weight the greater the scheduled speed. So more aerodynamic drag.
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Old 25th May 2020, 10:41
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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...
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Old 25th May 2020, 13:08
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Originally Posted by birdspeed View Post
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.
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Old 25th May 2020, 15:49
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While for the Bus...this has some good information....(page 21 for APU)

ECS rules?

https://ansperformance.eu/library/ai...el-economy.pdf
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