# fuel and fuel density question

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i recently posted a topic about my trip to the airport and into a real 744. me and the mx guy where talking and then he showed me the mx page on the lower EICAS for fuel, which listed fuel density, quantity, ect. and the reading for density showed about 6.7, but he said that its wrong and its more around 6.8. and the fuel load for the flight was i believe 312,000 lbs. and he did some calculation on his calculator and said that that difference in shown density and actual density is around 800(i think thats what he said). but i was focusing on the lower EICAS display and didnt know what he said before the calculation. and i was thinkin to myself yesterday. how can it make a difference? isnt 312,000 lbs of fuel 312,000 pound of fuel regardless of density? i understand that the 6.8 density would have a lower volume. but what does the airplane use to determine how much fuel is in it? does it have a device that meaures mass or does it measure volume? and then i was thinking about it at night yesterday, and i thought up a theory. maybe the plane detects how much fuel it has by volume, then uses the density and the density formula, D=m/v, to determine the mass, thus an incorrect density reading would make a difference because the plane would think it has X amount of fuel but really it would have Y. is this correct? so to sum it up. why would density matter when measuring fuel? how does the plane determine how much fuel it has onboard? and also, if the computer commands, for instance, 1000 lbs of fuel per minute to the engine, how does the plane know how much is 1000 lbs? thanks for any help.adam

Hi Adam,This is a really good question. I can't say for sure how the 747 measures its fuel load, but to me it would seem much easier to measure the volume of fuel in the tanks and then convert it a mass using the density than to have scales under each tank to determine the mass of the tank. That way doesn't make any sense esp. since the wings are tanks. So I think what he was saying when he said it was a difference of 800 was the difference in mass due to incorrect density calculations.As far as the mass flowrate of fuel is concerned, (this is speculation on my part based on my engineering classes and Navy experience) I would imagine the fuel system measures the pressure drop in the fuel lines to the engine and then uses the denstiy information to calculate the mass flowrate to the engine. Check out this link for some theortical calculations of flow rate based on the venturi effect.http://www.efunda.com/formulae/fluids/venturi_flowmeter.cfmThen venturi flowmeter would use the pressure drop, fluid density and crossectional areas of the fuel pipe to determine the fluid velocity, volumetric flowrate mass flowrate.I hope this helps and I hope someone who knows the 747 better than myself can give us a definitive answer.Cheers,Erichttp://www.precisionmanuals.com/images/forum/747400.jpg

Fuel computer in an airplane like that is really a complicated stuff. The fuel is measured by two methods - totalizer and 'sensed'. Totalizer simply subtracts the amount of fuel burn from the original fuel (so it must precisely know the fule flow). The sensed fuel is the one that it measures directly in the tanks. No doubt temperature and other things must be carefully accounted for since jet fuel has high coefficient of thermal expansion.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

Michael J.

Ahh. . . . Fuel quantity. It

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