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Fuel calculations

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  • ESzczesniak
    Fuel planning is actually quite a bit more complicated in reality for these modern jetliners. The approach taken is typically a work backwards approach. You start with how much fuel you are required to have when you land. Then you add on how much fuel you needed to get to the alternate if you needed. Then you add your fuel to taxi in to the gate. Then you add your hold/contingency fuel you might have needed for delays on approach. Then you add your flight plan fuel. Then you add your taxi out fuel. And that's the simplified version that boils down to this:

    FAR reserve
    Alternate fuel
    Taxi to gate fuel
    Hold/contingency fuel
    Flight plan fuel
    Taxi from gate fuel.

    All of the above except alternate and flight plan fuel are usually fixed numbers you use based on experience. Those two however are usually calculated for a fuel burn rate. You'll get some data that you're aircraft burns say 10,000 lbs/hour (that's more like a 747) and your cruise speed is 450 kts TAS. So if you have a 900 NM journey, you'll need 2 hours to get there so 20,000 lbs of fuel. However, you need to correct that distance for wind, adding to the distance for a headwind and subtracting for a tailwind. Normally, this is a kind of backwards/circular calculation where you determine that you 900 NM journey would take 2 hours to complete without wind. In this time a 100 kts headwind would have blown you 200 NM backwards off course (wind is NM/hour) so your new trip distance is equivelant to 1100 NM. So you now do 1100/TAS and find a new time of about 2.25 hours and plan your fuel with this.

    Fuel calculations actually get even more complex when you realize your fuel burn rate varies depending on cost-index, cruise altitude, speed and aircraft weight. On top of that you can add-on varied fuel burns to top of climb based on FLEX/derated takeoffs and departure density altitudes.

    So bottom line, you're planning get's you in the ball park. But to do it like the pros is even more involved.

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  • bigdave
    whereabouts on avsim is it; i've had a good look but i haven't found it.

    i am full of questions, but i'll keep looking and trying to find out anyway.

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  • RingwaySam
    Which 737? Default, PMDG or other?

    Theres a nice fuel calculator on Avsim which you can use - You can pick your reserve fuel and alternative destination fuel etc, it's pretty good.

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  • bigdave
    started a topic Fuel calculations

    Fuel calculations

    i mainly fly the 737 on FS so i'll use this

    it has a total fuel capacity of 20112 Litres. I don't know how to calculate an EFB but i am attempting one below

    10% is therefore 2011.2 which is rounded up automatically to 2012.
    1% = 201.12 L

    lets say i'm doing a 1 hr + 30 mins flight (airspeed @ 330 mph x 1.5= 495 miles)
    495/2370 (maximum range) x 100 = 20.88 = 21%

    i believe the estimated fuel burn is 4223.52L (201.12 L x 21%)

    EFB / 90 mins = 46.92 = 47 Litres/p/min

    47x 90 = 4230 (much better round answer to work with)
    you are provided by an Estimated Fuel Burn if you create a flight plan of any kind. The EFB alone will most definitely not get you to your destination. HOW MUCH MORE FUEL IS REQUIRED?
    Reserve fuel is only to be used if the fuel for the main part of the flight has been used up. Do i add an extra hour's worth of fuel?

    how much reserve fuel (in terms of time) do i put in?
    1/2 hour, 1 hour, 1.5 hours, 2 hours?