Jump to content

Sign in to follow this  
Guest Boeing747-430

fuel and fuel density question

Recommended Posts

Guest Boeing747-430

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

Share this post


Link to post
Guest Vagabond78

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

Share this post


Link to post

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.

Share this post


Link to post
Guest D17S

Ahh. . . . Fuel quantity. It

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    2%
    $540.00 of $25,000.00 Donate Now
×
×
  • Create New...