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Guest yoyomike

Fuel flow prediction in FMC

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Guest yoyomike

Hey guys, I was just wondering where the FMC gets the data for predicting the fuel estimate on arrival. Does it read the fuel burnoff "base" from the aircraft.cfg file under "fuel flow scalar" of the general engine data? Or is it hard coded into the gauge itself?I just want to test a few things. I'd appreciate any help you can give me on this.thanks, guys!mike

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Vangelis can tell you more but it's using a performance library built into the FMC that's very similar to how the real deal operates. Takes all sorts of things into account besides momentary fuel usage - step climbs further downrange, adjustment for direct to's, climb setting/derates etc...


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Guest yoyomike

Thanks for the reply, sir Tabs.Yup, I've flown the B744 on the longer range routes and I've impressed with the accuracy of the fuel prediction. Given that accuracy, I'm sure there's a built-in "calculator" accounting for the step climbs, takeoffs, landings, and approach.The reason why I am curious about editing the fuel flow is because I have access to some real life dispatch material, and after comparing, I noticed that the PMDG 744 has a slightly higher fuel burnout that the ones I see on this dispatch material-about 200 kilograms of fuel more. (I match everything - wind condition, temperature, gross weight, cost index, speed restrictions etc with what was really planned). I also consulted a friend of mine who is very experienced with programming FDE's and according to him, the panel should be able to read the aircraft.cfg file for a "base" factor or coefficient or something that the FMC will base it's fuel prediction on - the calculations/corrections for T/O, CLB, DES and LDG will come after that too.Thanks again for the reply Sir Tabs. Sir Vangelis? :)

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Guest yoyomike

Oh yeah, these dispatch forms also use the same engine as PMDG was programmed. The GE-CF680C2B1F.

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I think such minor deviations are not uncommon. The real FMC uses the fuel flow and drag correction factors on the perf init page to account for such issues. remember also that no 2 planes will perform exactly the same. Send two airplanes out from any fleet, and u'll get slightly different numbers.Paul

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Guest yoyomike

Hi Paul, I'm sorry I wasn't thinking again when I last posted. By 200 kilos more, I meant 200-300 kilograms more per hour, and much more on takeoff and landing. It's such that if my form would indicate 15 tons of fuel on landing, my FMC would already predict something at around 11-12 tons. It's a big difference, Captain.cheers,mike

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200-300 per hour is certainly significant. I think that that same parameter corrects this also. Remember that whe u look at ur charts tho, that airlines can derate the engines so that the catual full rated thrust is never availabe to the pulots. Another post recently showed that this is the case with Virgin.Paul

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Guest yoyomike

yup. What's the limit to Virgin's 744's? So you think it's the fuel flow scalar, sir?:)cheers,mike buenaventura

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