Commercial Weight & Balance

[B767Driver]

[quote


So this leads back to my question "Applying the temperature now to convert a volume of fuel pumped 10 hours or 10 days ago will not necessarily result in the correct weight now, right?" It doesn't look like anyone has an experience where weight was mistaken because of a temperature change converting gallons to pounds. And it may be the 3% or so difference has been too small to be an issue.

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To be physically accurate, a mass of fuel is pumped into the fuel tank. The mass of this fuel will not change (negating the effects of evaporation). It is this mass of fuel that the engine needs to adequately perform the planned operation.

I stated previously that the volume of fuel will not change (and I stated that due to the fact that the change in volume of the mass of fuel with temp and density changes is miniscule). The change in weight of the mass of fuel can be somewhat more significant.

So, volume, density and weight of a certain mass of fuel will all vary with temperature. To obtain the proper quantity of fuel to complete the flight, be sure to correct the density of the fuel per its temperature.

A different mass of fuel will be required on a very cold day as opposed to a very hot day due to the density change. You are correct in saying that the quantity of fuel you have in your tank "10 days from now" may not be adequate for your planned operation. Let's say today it is 130* F outside and you topped off your fuel tanks. In ten days it is -50* . It is now likely that you need to add more mass of fuel to obtain the volume you desire (your full tank in this case).

Good topic.

 

[JHines]

[ QUOTE ]
[partial quote] So, volume, density and weight of a certain mass of fuel will all vary with temperature. To obtain the proper quantity of fuel to complete the flight, be sure to correct the density of the fuel per its temperature.

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F=MA. So the density and volume for a given mass change with temp, but not the weight - unless you plan on flying at some seriously high altitudes. Right?

 

[JHines]

Re: Here\'s the answer to the question you are really asking:

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[partial quote] All fuel delivery gizmos, be they updraft carburetors in a Cessna 172, fuel injectors in a Navajo, or fuel control units in a Rolls Royce RB211 high-bypass fanjet engine, deliver fuel by mass, not by volume, in order to achieve a given combustion ration, i.e., ratio of air to fuel by mass, not volume.

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I agree. But turbine equipment, in particular, operates in a closed loop. That is, it feeds the combustor just enough BTU's (or put another way, fuel mass) to maintain the set N1 (or T4 or whatever).

So back to my question above, given two hypothetical identical flight profiles on the same aircraft, each starting with 100% full by volume tanks (or "slam full" tanks, as we say in the South), but at different temperatures (ambient & fuel), do you or don't you see a difference in range, because at colder temps the same "full by volume" tanks have more fuel mass in them?

Or doesn't it matter because turbine pilots order fuel by weight instead of just having the tanks topped??

 

[B767Driver]

Re: Here\'s the answer to the question you are really asking:

[ QUOTE ]
do you or don't you see a difference in range, because at colder temps the same "full by volume" tanks have more fuel mass in them?[ QUOTE ]



This is a good question...one that I'll have to research. I know it's possible to "volumetrically top off" a fuel tank prior to reaching the planned fuel load, under extreme situations. (on a float type fuel gaging system, I don't think you could determine this.) (On a capacitance type gaging system, you would...since you are measuring the density of fuel.)

Here's my prelimary thoughts...(not verified)...that on a cold day your range/endurance would be greater because the fuel's mass per unit volume is higher...thereby permitting the fuel tank to effectively carry a "heavier" fuel load.

I'll see if i can verify this with engineering. This is beginning to get outside of my pay grade!

Anyone else?

 

[Ophir]

Re: Here\'s the answer to the question you are really asking:

I liked your explaination onthebeach, thanks.