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bcradio

Fuel Level Goes Up on Takeoff?

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I was watching this video of 737-800 operations:

 

http://www.youtube.com/watch?v=uGxRJ6b7KL8&feature=c4-overview&list=UUH1aqQ298wuC6TexpafuSxA

 

Regarding the takeoff at 9:28 in the video, the fuel indicated when the takeoff roll starts is 9.9.  Further down it goes up to 10.0 and through rotation and liftoff it goes to 10.1 and 10.2 respectively.  Is that a quirk of the airplane, or has the video been edited with different clips spliced together?  I don't think they've used multiple clips as the departure on the ND looks the same.  Haven't seen it on the NGX videos.

 

I imagine it's the fuel moving within the tanks that give the fuel level measurement thingys a bit of a rough go?  Like parking my car on a hill?

 

Anyway, looking forward to the release of the 777!  I just hope my machine can run it.

 

 

 

Cheers,

Bert Collins

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Hi Bert,

Have you considered that speeding up at the same N2 more speed means more dinamic pressure and so more air entering into jet intake due to "ram effect" (air flow entering into the intake is proportional to dynamic pressure P1=Po*(1+0,2(Mach)^2)^3,5, so you need more fuel to mantain a constant air to fuel ratio (and then of course a constant N2%)?

About takeoff roll the strange thing is otherwise that, until 80 kts, fuel flow is corrected and realistic (i.e. 0,35 kg per kg of thrust) over that speed, if a well remembrer firstly it decreases then it increases more than realistic value but It's certainly a FSX limitation...

Ciao

Andrea

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Yes, but the number that changes is the total fuel on board number, not the fuel flow. 

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Just a stab in the dark, but is it something to do with the angle of the aircraft causing fuel to move around in the tanks?

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Our ACARS flight log regularly shows little discrepancies like that. Say you burn 400lbs during taxi - the Flt Log still says you blocked out with 23.0 and took off with 23.0. What happened to the 400lbs???

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Ok, I'm gonna go a bit physicsy on this:

 

As a analogy, consider you have a tub of water (or any other fluid) at rest. When you move (accelerate) this tub, the fluid will resist the change in motion, as per Newtons 1st law. So from this, the fluid will move up the side wall of the tank.

 

Now to how this affects the fuel level. When the aircraft accelerates down the runway for takeoff, the fuel in the tank will not be perfectly level in the tank, as explained by the analogy above. So now, depending on where the fuel quantity sensors are placed within the tank and how many of them there are, you may get a slightly erroneous  fuel level indication.This explains the increase during the T/O roll.

 

As for the rotation and liftoff, the angle of climb will effect the level in the tank, because one of the properties of liquids is that they have the ability to take on the shape of the container which they are in. Eg) If the tank is at 12 deg to the horizontal, the fuel will not be parallel to the bottom of the container, but it will still be roughly horizontal on the surface.

 

I would have though that Boeing would have implemented some kind of system to alleviate these kinds of situations, but I would hope the crew wouldn't be doing fuel quantity checks in the middle of a takeoff run either!

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That's kind of what I figured.  I wonder if the nose up pitch in cruise would indicate a different amount of fuel in the tanks compared to sitting on the ground. And if so, how much of a difference?  Not that a hundred pounds or so would make a whole lot of difference, but it is interesting to think about.

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