PGBosak

Fuel consumption unrealistic in lower altitudes?

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On my yesterdays overnight flight i forgot set altitude bug for initial climb level before going to sleep.

So the aircraft leveled at FL200 for exact 8 hours. When i came back, i saw the aicraft still cruising at FL200

so i checked fuel prediction. It showed the same 12 tons at arriving in klax as calculated before t/o.

 

I wouldn't say that's normal behaviour. :Shame On You:

 

 

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This isn't a bug, and you should avoid finger wagging until you actually understand the systems and functions of the aircraft...

 

The FMC's predictions are based on your following its guidance. If you put a higher altitude in there, the FMC is assuming you are going to actually fly that. If there is a STEP SIZE entered, then the FMC is going to assume you will fly that, too. If you look at the cruise page, I'm betting you'd find the proper cruise altitude in there. If you look at the ND, I'm betting you'd find your proper T/C just hanging in front of you that whole time. If you look at the LEGS page, I'm betting you'd find that the LEGS page is assuming you'd by flying the altitude that you told it you'd be at (and not simply where you are right now).

 

Moral of the story: the FMC planning is only as good as you following its guidance. The PROG page predictions wouldn't be accurate if you took off and flew on a completely different route than you told it you would be flying, too.

 

So...next time: research first. Finger wag only after you confirm you're right, otherwise...well...there's a famous Abraham Lincoln quote that might fit here...

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(...) well...there's a famous Abraham Lincoln quote that might fit here...

 

Found it ( :P):

 

"The things I want to know are in books; my best friend is the man who'll get me a book I ain't read."

(Abraham Lincoln)

 

So:

  • your best friend = Kyle Rodgers
  • book you ain't read = FCOM

:dance:

 

(And you all thought it's the *OTHER* quote, didn't you?)

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Finger wag only after you confirm you're right

 

i think you didn't get my point, from the logical view.
 
When the prediction says from the start following all the SC the aircraft will land with 12 Tons left and i actually land with 12 Tons knowing i flew 8 of 14 hours at FL200
we don't talk about prediction anymore, but actual consumption and that lead us back to my initial topic.
 
 
here's a famous Abraham Lincoln quote that might fit here.

this one?

 

http://www.bilder-upload.eu/show.php?file=40b0e6-1487253416.jpg

 

Btw i just saw the meaning (here) behind that smiley, thats not what i intended to say. Sorry! That Finger gesture has normally a different meaning. (in the offline world)

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I'm not sure I follow the finger wagging having a different context. To me, that's what a Catholic school teacher does to someone when a child has done something naughty.

 

As far as the issue goes, I think you're more in the realm on coincidence than repeatable/provable/actionable information. I'm assuming you were flying with weather, too, yes? In order to actually have something to chase, I'd need FF values for that time to compare to perf data charts. Still, I'd avoid the finger wag until I had actual, hard data to present to someone.

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repeatable/provable/actionable information.

 

No problem. Here is all the information to repeat this flight yourself :

 

aircraft 77W

fuel: 127.84 tons

zfw 222.5 tons

t/o weight 350.0 tons

CI 85

(t/o 00:15z, weather 16.02.2017 ASN)

intital Level FL200 after 8 h climb to 330 then SC as computed.

 

YSSY N0491F290 KADOM1 KADOM A576 TESAT B450 NOBAR A579 VIRAR/N0501F310 A579 NN B581 WACOS/N0494F330 B581 CAMOS/N0488F350 B581 WEDES/N0480F370 B581 FICKY C1177 ROSIN BUFIE4 KLAX

 

weather forecast uploaded for FL310 FL330 FL350 FL370.

 

As customer my job ends with informing you. With or without finger smiley.  :wink:

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If you entered FL200 in the FMC instead of FL3xx or whatever it was expecting, then your fuel predictions would have been correct. (Or, more accurate) It spent those 8 hours thinking you were 'about' to start a climb to your planned FL3xx.

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What PGBosak (is that your full real name, BTW?) is talking about is: 

 

(...)
we don't talk about prediction anymore, but actual consumption and that lead us back to my initial topic.s one? (...)

 

His plane was flying lower than predicted for many hours, at a less fuel-efficient FL, and still the fuel used matched the prediction. Actual fuel consumption is his point here.

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is that your full real name, BTW

It's in his signature as a graphic, like mine.

 

Though FL200 is less efficient, ground speed might well have been higher, so fuel used could balance out. He doesn't say what speed the aircraft flew at while at FL200 so it's hard to be sure. The planned flight level is initially 290, so not massively different. Also the climb to FL330 came 8 hours later than planned, therefore at a lighter weight, so burning less fuel.

 

A lot of factors come into play so the answer isn't always the intuitive one.

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It's in his signature as a graphic, like mine.

 

(...)

 

I'm sorry for that. Right now sitting in front of a crappy PC with onboard graphics only, most pics are not displayed here.

 

Yes, I was wondering about winds at different altitudes, affecting flight time, too.

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His plane was flying lower than predicted for many hours, at a less fuel-efficient FL, and still the fuel used matched the prediction. Actual fuel consumption is his point here.

 

exactly!  :drinks:

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exactly!

 

The PMDG modeling is correct.  The driving force is the Cost Index. 

 

blaustern out

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To the OP,

 

you are correct. It should be more considering your statements, but so so many things may have effected the ultimate reading. The best way is to go back to FL200 and read off your fuel flow. Compare that to the PFPX tables for example if you have it or any other source of fuel data for the 777. PFPX even has the fuel flow validation feature which is great. I'm sure you'll find that the PMDG model is spot on FCOM values, and that it was just a freak sequence of data that lead to your result.

If you don't have any source of fuel flow data, you can post your FL, weight, speed, OAT, N1 and fuel flow and I'm sure or anybody else on here can check it out for you.

 

Cheers,

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