I am a trifle confused at the data in the 737-800 (etc) fuel load estimations. According to the table a 2000-mile journey of 4h 40 mins at FL 370 should require 22,300 lb of fuel. However, when the journey is set up in the FMC, the fuel required is about 28,000lb. Both figures exclude diversion and contingency fuel.The discrepancy seems to be considerable.

The difference sounds about right considering: taxi;holding;min landing fuel. ZFW and wind will also play a part. Are you sure you are comparing like with like ?

The fuel load estimation chart shows the basic for the journey (climb, cruise, descend) as requiring 22,300 lb of fuel. If you go into the aircraft and programme a journey of that length (and at the same height, etc) into the FMC and adjust the fuel until you have exactly the amount needed, there seems to be quite a difference.

The FMC has entries for your cruise winds and temperatures aloft. Temperatures aloft affect engine performance. Your IAS and altitude also play a role. Another entry that can affect things is your cost index which controls you best optimized altitude.Did you enter any reserve into the FMC? What do the PROG and forecast pages show for fuel left at destination. Have you tried the PMDG fuel planner if you have it?

I have just completed a test, the basis for which is Heathrow to Tel Aviv at fl350 winds zero and no reserve or diversion allowances. These are the figures I get via the various sources.FS flight plan : 15.855 lbs of fuelFlight Load Estimation chart : 22,200 lbs of fuelFMC : 27,800 lbs of fuel (So far as I can tell this does not take winds or diversions into account since wind speed/direction is not entered)5,600 lbs is a fairly significant discrepancy, is it not?

what is your Zero Fuel Weight ?Actually , if i remember correctly, you wont get a realistic "fuel remaining " figure from the FMC until you are into the climbor cruise.22,700 is what my 737 planner tells me is the trip fuel for that flight (with 111,600 zfw)... so add alternate/holding/min landing/taxi (together approx 9,300), and make the flight. See what your estimate is in the cruise.Peter

Edited January 26, 201214 yr by pschlute

Yep that is correct. Once in the cruise estimate is 9,200 remaining.

what is your Zero Fuel Weight ?Actually , if i remember correctly, you wont get a realistic "fuel remaining " figure from the FMC until you are into the climbor cruise.22,700 is what my 737 planner tells me is the trip fuel for that flight (with 111,600 zfw)... so add alternate/holding/min landing/taxi (together approx 9,300), and make the flight. See what your estimate is in the cruise.Peter

But surely the FMC figure is the basic route until alternate (etc) figures are added.

All the FMC figure will only tell (on the PROG page) is how much fuel you will have left when you land. If you have calculated your fuel correctly as I have detailed above, this should be 9,200 lbs.This agrees with the tables in the PMDG documentation and agrees with my flight planner app: 22,700 trip fuel + 9200 reserves/min landing fuel/taxi fuel. Load your flight with 31,900 lbs and test it.As I said, the estimate from the FMC is only reliable once you are in the cruise.Peter

I am grateful for that but, for the sake of exercise, planning a route with 0 wind and no alternate surely the FMC and Load Estimation chart should agree, shouldn't they? Or does the FMC automatically add in an amount for wind and diversion - that, at least, is what it seems to be doing.

Ok i am confused now. Where in the FMC are you seeing a fuel estimate ?What is your ZFWWhat is your Cost Index

Edited January 27, 201214 yr by pschlute

Ok i am confused now. Where in the FMC are you seeing a fuel estimate ?What is your ZFWWhat is your Cost Index

I enter the details of the flight in the FMC in the normal way and then get the fuel from the Progress Option. This tells you how much fuel you will have left on reaching your destination and, for obvious reasons, should be close to that shown in the chart.However, if you want to test the system by assuming 0 wind and no diversion airport (in other words origin to destination only) then since wind and diversions are not entered into the FMC, if you load the fuel indicated by the chart, it should be very close to that shown by the FMC/Prog. In fact, the latter tells you you have not enough fuel on board by quite a considerable margin.Or, is it the case that the FMC automatically allows for a diversion?I usually have a cost index of 100 and a ZFW of 118.8

The Estimated fuel remaining on the PROG page will NOT give you a correct number until you are airborne and in the cruise. It does not take into account a diversion automatically. If you load up your flight with 31,900 fuel (22,700 trip and 9,200 reserves/taxi), and take off and climb to cruise alt you will see the PROG page telling you to expect 9,200 fuel on arrival.Peter

And if you do an N1 climb for a considerable time you may get an insufficient fuel warning from the FMC until established in the cruise.

I use this site to plan fuel and load:http://fuel.aerotexas.com/LOVE the sheets it gives me and it's been very accurate thus far. :)