Specific Gravity
04-01-2007, 06:12 PM
#1
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Joined APC: Mar 2006
Position: BE300 and DA50
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Specific Gravity
If your aircraft holds 50,000 lbs of fuel, and you get it filled up after a flight and, after adding all the fuel gauges together, it comes out to 51,000 lbs. How much fuel do you have? I'm wondering about specific gravity of fuel. I'm a bit confused on this subject and would like more info.
04-01-2007, 06:48 PM
#2
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Joined APC: Mar 2007
Posts: 40
Specific gravity is just a ratio of the density of a certain amount of material (jet fuel in this case) to the weight of an equal amount of distilled water at a set temperature. For example, 1 gallon of Jet A at 60 deg. F may weigh 6.78 lbs and a gallon of water weighs 8.35 lbs. When you take 6.78/8.35, you end up with .81. That is your specific gravity of the Jet A sample you took. If you were to add an additive like Prist, which is heavier than Jet A, to the fuel, and still took the identical size sample, the fuel would be slightly heavier per gallon. This would make the SG higher. The reason some people debate the weight of Jet A is because it actually changes. Altitude, temperature, and other atmospheric changes affect the specific weight of the fuel which will affect the SG.
So in your example, your gauges may be calibrated for (or you are multiplying by) a different SG (weight/gal) of fuel than you are being filled with. Often when you pull fuel from an underground tank (cooler, thus more dense and heavier per gallon) and fill your aircraft, the outside air plus the sun will increase the temperature of the fuel causing it to expand, therefore increasing the amount (gallons) of fuel in your tanks. (Understand the weight will stay the same - unless you initially fill full and then when the fuel expands, the extra flows out the vents.) When you get to a higher altitude and the fuel cools and contracts, you will actually have less fuel in your tanks (gallons) but will weight proportionately more.
PS, since the specific gravity will be higher (more dense) at higher altitudes, the fuel is more potent per gallon than at lower, warmer temps. This helps to increase your power while burning less fuel.
Confusing? Let me know
So in your example, your gauges may be calibrated for (or you are multiplying by) a different SG (weight/gal) of fuel than you are being filled with. Often when you pull fuel from an underground tank (cooler, thus more dense and heavier per gallon) and fill your aircraft, the outside air plus the sun will increase the temperature of the fuel causing it to expand, therefore increasing the amount (gallons) of fuel in your tanks. (Understand the weight will stay the same - unless you initially fill full and then when the fuel expands, the extra flows out the vents.) When you get to a higher altitude and the fuel cools and contracts, you will actually have less fuel in your tanks (gallons) but will weight proportionately more.
PS, since the specific gravity will be higher (more dense) at higher altitudes, the fuel is more potent per gallon than at lower, warmer temps. This helps to increase your power while burning less fuel.
Confusing? Let me know
04-01-2007, 06:51 PM
#3
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Joined APC: Jan 2006
Position: Engines Turn Or People Swim
Posts: 39,201
Specific gravity is a ratio of the density of some material (usually a liquid) compared to the density of water.
The SG of water = 1/1 = 1.0
The SG of Jet A (or gasoline) would be less than 1.0, since these fuels are lighter than water.
Density is a little more relevant for the pilot in routine flight operations. Fuels such as gasoline, AVGAS, and Jet A experience density changes with temperature (lower temp = higher density). This means that a given volume (say a gallon) of fuel will be denser and actually contain MORE fuel molecules at colder temps.
In GA and automotive use, density is ignored and we buy fuel and plan fuel consumption based on volume (ie gallons) and ignore the temperature affects.
In light piston airplanes the affects of temperature on fuel capacity are slight and can usually be ignored, although full fuel tanks on a hot day will provide somewhat less range. This is why fuel leaks out of parked airplanes on hot days...it expands and can't fit in the tanks. The tanks are still full, but there are fewer fuel molecules in that volume (since some of them are now in the puddle on the pavement).
Auto gas is sold to consumers by volume (gallons)...have you ever noticed that it is usually pretty warm at the pump? The gas station owners like to sell warm gas because they are actually giving you LESS fuel molecules in each gallon. Naturally THEY buy gas by weight, so they pay the same regardless of temperature.
In larger and turbine airplanes we measure fuel by weight because this gives us an exact measure of the amount of fuel molecules that we buy and plan for. The airplanes fuel system does care about temperature and density because it measures volume and then applies density to determine the fuel weight.
In the case where you physically filled the airplane up, it might be possible to have more fuel (by weight) than the airplane is supposed to hold if the the fuel temp was abnormally low. In this case I suspect that technically the airplane should not be flown with significantly more fuel by weight than what it's rated for. This situation should have been avoided because the fueler should deliver fuel by weight, not volume, so he should have stopped at 50,000# even if the tanks weren't full.
Also note that while volume may change in flight as fuel cools at high altitudes, the amount of fuel molecules is still the same, so you get the same amount of energy (and range) out of the fuel regardless of it's density.
The SG of water = 1/1 = 1.0
The SG of Jet A (or gasoline) would be less than 1.0, since these fuels are lighter than water.
Density is a little more relevant for the pilot in routine flight operations. Fuels such as gasoline, AVGAS, and Jet A experience density changes with temperature (lower temp = higher density). This means that a given volume (say a gallon) of fuel will be denser and actually contain MORE fuel molecules at colder temps.
In GA and automotive use, density is ignored and we buy fuel and plan fuel consumption based on volume (ie gallons) and ignore the temperature affects.
In light piston airplanes the affects of temperature on fuel capacity are slight and can usually be ignored, although full fuel tanks on a hot day will provide somewhat less range. This is why fuel leaks out of parked airplanes on hot days...it expands and can't fit in the tanks. The tanks are still full, but there are fewer fuel molecules in that volume (since some of them are now in the puddle on the pavement).
Auto gas is sold to consumers by volume (gallons)...have you ever noticed that it is usually pretty warm at the pump? The gas station owners like to sell warm gas because they are actually giving you LESS fuel molecules in each gallon. Naturally THEY buy gas by weight, so they pay the same regardless of temperature.
In larger and turbine airplanes we measure fuel by weight because this gives us an exact measure of the amount of fuel molecules that we buy and plan for. The airplanes fuel system does care about temperature and density because it measures volume and then applies density to determine the fuel weight.
In the case where you physically filled the airplane up, it might be possible to have more fuel (by weight) than the airplane is supposed to hold if the the fuel temp was abnormally low. In this case I suspect that technically the airplane should not be flown with significantly more fuel by weight than what it's rated for. This situation should have been avoided because the fueler should deliver fuel by weight, not volume, so he should have stopped at 50,000# even if the tanks weren't full.
Also note that while volume may change in flight as fuel cools at high altitudes, the amount of fuel molecules is still the same, so you get the same amount of energy (and range) out of the fuel regardless of it's density.
04-02-2007, 03:39 PM
#4
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Thread Starter
Joined APC: Mar 2006
Position: BE300 and DA50
Posts: 5
Thank you for your in depth explanations. Let me give you a better example of what I'm not quite sure of. Our airplane holds 15500# of fuel. We landed after a 4.5 hr flight and asked for the airplane to be filled up. When I got back into the plane to start it up and program the FMSs the fuel gauges (one for each of the three tanks) added up to 15800#. My question is, should I enter 15800, or 15500. How much gas do we really have, and if the gauges say 15800, will we be able to go further on that, or do we still have the same amount by volume.
Thanks for all the info. You guys are too smart for me so you may have to dumb it down a bit if you can. Thanks again.
Thanks for all the info. You guys are too smart for me so you may have to dumb it down a bit if you can. Thanks again.
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