Fuel readout in Kilograms not litres
Thread Starter
Joined: Apr 2004
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From: UK
Fuel readout in Kilograms not litres
Had this question at a recent airline assessment.
Why is the cockpit fuel readout in Kilograms and not litres ?
Is it got to do with the specific densities. I was not to sure.
Why is the cockpit fuel readout in Kilograms and not litres ?
Is it got to do with the specific densities. I was not to sure.
Joined: Apr 2008
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From: Central Europe
The fuel temperature varies and so the density. For example +30°C on the ground an -30°C when airborne. So the volume varies and liter is a unit of volume. So only a weight unit can be accurate --> kilograms or pounds.
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From: The Winchester
Sticking my neck out and I'm certain someone will have more elegant explanation but it was once explained to me like this : Fuel mass is a direct measure of the number of fuel molecules you are carrying which in turn is a direct measure of the amount of chemical energy you are carrying...
Makes checking the loading /performance slightly easier as well.....
Right, off to get my tin hat.
Makes checking the loading /performance slightly easier as well.....
Right, off to get my tin hat.
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From: France
wiggy, you can put your tin hat back on the shelf.
You're right.
It's the weight of the fuel that determines whether you can carry enough 'energy' to take you from A to B plus diversion, not the volume.
So the weight is what you want to know, and what is displayed (derived from fuel tank level sensors + fuel density sensors).
Still, it's the volume that determines whether it actually fits in the tanks.....
Wasn't there a thread recently about whether you were billed for litres or kilograms?
You're right.
It's the weight of the fuel that determines whether you can carry enough 'energy' to take you from A to B plus diversion, not the volume.
So the weight is what you want to know, and what is displayed (derived from fuel tank level sensors + fuel density sensors).
Still, it's the volume that determines whether it actually fits in the tanks.....
Wasn't there a thread recently about whether you were billed for litres or kilograms?
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From: UK
Refuelling is metered by volume, which is why the S.G. and temperature are needed to convert your fuel onload into weight for loadsheet purposes.
However, wouldn't it be more sensible if it were billed by weight (mass?), as surely that is the truer measurement of the "energy" being supplied?
JD
However, wouldn't it be more sensible if it were billed by weight (mass?), as surely that is the truer measurement of the "energy" being supplied?
JD
Joined: Jun 2008
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From: heidelberg/germany
sorry - a novice not trying to be a smartallec, but...
is there really a difference between weight an mass?
the earth-accel. is nearly 10m/s^2 almost everywhere most of us would fly,
can't deviations be neglected?
is there really a difference between weight an mass?
the earth-accel. is nearly 10m/s^2 almost everywhere most of us would fly,
can't deviations be neglected?
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From: Australia
This has come up before on a discussion re: fuel quantity gauging using capacitance sensors.
I too, previously believed that fuel capacitance gauges measured conductivity of all the fuel in the tanks.
Apparently this is not so. A capacitance gauge is still a measure of level, not mass.
I'll search for the TechLog discussion. It was less than 6 months ago.
Kg/lbs are primarily used to have common units for uplift of fuel, pax, bags and aeroplane.
I too, previously believed that fuel capacitance gauges measured conductivity of all the fuel in the tanks.
Apparently this is not so. A capacitance gauge is still a measure of level, not mass.
I'll search for the TechLog discussion. It was less than 6 months ago.
Kg/lbs are primarily used to have common units for uplift of fuel, pax, bags and aeroplane.
Joined: Jan 2005
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From: France
krujje is right, of course, the energy is related to the mass, not the weight. Take a kilogram mass of fuel to the moon, and it will only weigh one-sixth, but have the same energy. Even on earth a kilogram mass does not weigh exactly a kilogram everywhere.
Torstennnn is right too, of course, in that the difference here on earth is so small that for our purposes we can neglect it. A kilogram on the scales is a kilogram mass, near as d@mn it
What's more, the energy per kilogram mass depends on the exact composition of the fuel, and the differences there are such, that the minute difference in weight becomes a second-order effect.
ITCZ,
Indeed, a capacitance gauge only measures fuel level (and by implication quantity). Specific gravity of the fuel is measured by a separate sensor (think of a float that sinks into the fuel more or less deeply, depending on the s.g.)
Fuel weight is then quantity x s.g.
"Kg/lbs are primarily used to have common units for uplift of fuel, pax, bags and aeroplane."
Not only for uplift as such, but also for calculating the C.G.!
CJ
Torstennnn is right too, of course, in that the difference here on earth is so small that for our purposes we can neglect it. A kilogram on the scales is a kilogram mass, near as d@mn it
What's more, the energy per kilogram mass depends on the exact composition of the fuel, and the differences there are such, that the minute difference in weight becomes a second-order effect.
ITCZ,
Indeed, a capacitance gauge only measures fuel level (and by implication quantity). Specific gravity of the fuel is measured by a separate sensor (think of a float that sinks into the fuel more or less deeply, depending on the s.g.)
Fuel weight is then quantity x s.g.
"Kg/lbs are primarily used to have common units for uplift of fuel, pax, bags and aeroplane."
Not only for uplift as such, but also for calculating the C.G.!
CJ
Joined: Aug 1998
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From: Ex-pat Aussie in the UK
capacitance guages are sticks of open layered "spongy" material. Each layer in the stick has a different capacitance if the sponge is full of fuel or not, so the stick just measures how much of the stick is sitting in fuel, compared to how much is sticking out of the top. The advantage is that there are no moving parts to fail, and only tiny voltages are required for operation (a good thing, when the thing is sitting in a fuel tank!)
It should also be mentioned that (on modern jets) there is a mass sensor in the fuel line leading to the engine, so the fuel flow is also in true kilos, and thus the Fuel Control Unit commands a set mass of fuel to the engine, not volume.
It should also be mentioned that (on modern jets) there is a mass sensor in the fuel line leading to the engine, so the fuel flow is also in true kilos, and thus the Fuel Control Unit commands a set mass of fuel to the engine, not volume.
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From: fairly close to the colonial capitol
It should also be mentioned that (on modern jets) there is a mass sensor in the fuel line leading to the engine, so the fuel flow is also in true kilos, and thus the Fuel Control Unit commands a set mass of fuel to the engine, not volume.
Refuelling is metered by volume, which is why the S.G. and temperature are needed to convert your fuel onload into weight for loadsheet purposes.
However, wouldn't it be more sensible if it were billed by weight (mass?), as surely that is the truer measurement of the "energy" being supplied?
JD
However, wouldn't it be more sensible if it were billed by weight (mass?), as surely that is the truer measurement of the "energy" being supplied?
JD
Joined: Aug 1998
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From: Ex-pat Aussie in the UK
Fuel flow can be measures as either a mass flow or a volume flow. Mass flow is preferred, as the calorific value of the fuel relates to its mass. Older engines, say as on a Handley Page Victor, used a primitve venturi fuel flow system. (Fuel run through a venturi, with a pressure of the fuel in the venturi sent to a pressure capsule, with pipe pressure in the outer chamber. The difference in pressure is related to the volume of flow.) Accuracy is about 2%. Other older types might use a variable orifice flow indicator.
Slightly more modern aircraft might use a free rotating turbine placed in the fuel flow, with a magnetic insert in the turbine. As the turbine rotates in the flow, the magnet spins about and an induction coil can measure the amount of rotation - however these don't cope well with high flow rates, as the error increases proportionately. They still only measure volume. All of the above can of course be fitted with a calibration device to convert the volume to a mass, if the density is known.
True mass flow indicators, like a Stator Torque Mass Flow Meter, swirl the fuel in the pipe, and then run the swirling fuel into a spring loaded turbine. The more the fuel can deflect the turbine, the more momentum, and thus true mass any given unit of fuel possesses. There are other similar systems, and these are used on modern turbines (as I said above) so that the Fuel Control Unit can demand an accurate mass (and hence acurate amount of energy) for the engine to use.
Slightly more modern aircraft might use a free rotating turbine placed in the fuel flow, with a magnetic insert in the turbine. As the turbine rotates in the flow, the magnet spins about and an induction coil can measure the amount of rotation - however these don't cope well with high flow rates, as the error increases proportionately. They still only measure volume. All of the above can of course be fitted with a calibration device to convert the volume to a mass, if the density is known.
True mass flow indicators, like a Stator Torque Mass Flow Meter, swirl the fuel in the pipe, and then run the swirling fuel into a spring loaded turbine. The more the fuel can deflect the turbine, the more momentum, and thus true mass any given unit of fuel possesses. There are other similar systems, and these are used on modern turbines (as I said above) so that the Fuel Control Unit can demand an accurate mass (and hence acurate amount of energy) for the engine to use.
Joined: Aug 1998
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From: Ex-pat Aussie in the UK
P.S. Most time when I order fuel around Europe, the fuel volume is converted to an equivalent volume at 15 degrees C on the fuel receipt. I believe we pay on ambient volume recieved, however.
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From: flyover country USA
Checkboard has given a great description of the basic flow measurement systems. My only comment is that the second method (volumetric turbine meter), under the right lab conditions, is probably the most accurate. However those conditions are stringent:
1) Flow-straightening pipe (10x or 20x the pipe diameter) upstream of the meter, and a shorter one (5x perhaps) downstream
2) Lab-type hydrometer, temperature compensated
3) Software corrections for SG, fuel viscosity and BTU (LHV) content
4) Maybe some more I'm forgetting after a decade away from this game
Only a high-quality test cell is likely to implement all this.
Needless to say, this is hardly practical for line operations, so the mass flowmeter is universally used today.
1) Flow-straightening pipe (10x or 20x the pipe diameter) upstream of the meter, and a shorter one (5x perhaps) downstream
2) Lab-type hydrometer, temperature compensated
3) Software corrections for SG, fuel viscosity and BTU (LHV) content
4) Maybe some more I'm forgetting after a decade away from this game
Only a high-quality test cell is likely to implement all this.
Needless to say, this is hardly practical for line operations, so the mass flowmeter is universally used today.
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From: The Road to Nowhere
I am not a techie, but have an interest in the specific case of refuelling a large Twin Turboprop I fly on quite regularly. The aircraft uses F34 (JP-8), which can have a range of acceptable SGs from 0.775 to 0.840 at 15°C(according to NATO docs anyway).
Of course fuel is rarely delivered at 15°C, and I would be interested to know how the SG varies with temperature. I believe it varies inversely to the temperature, but am not sure whether there is a constant factor (which I doubt), or whether the relationship is non-linear.
Could someone please point me in the direction of tables (or a graph perhaps) so that I can establish what the correct position is?
Thanks
STH
Of course fuel is rarely delivered at 15°C, and I would be interested to know how the SG varies with temperature. I believe it varies inversely to the temperature, but am not sure whether there is a constant factor (which I doubt), or whether the relationship is non-linear.
Could someone please point me in the direction of tables (or a graph perhaps) so that I can establish what the correct position is?
Thanks
STH
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From: Stonehaven
SG/Temperature conversion tables
There are books of tables published for these conversions covering all ranges of specific gravity and temperature. However this graph will solve your problem for most of the fluids you will come across in aviation.
www.fisherregulators.com/technical/tables/gravity.pdf
Measure the density in a flask using a hydrometer. ( for Jet A1 it should have a range of 0.75 to 0.85, this is stamped on the paper inside the glass.) At the same time read the temperature of the Jet A1 and apply both numbers to the graph to give you the corrected SG.
www.fisherregulators.com/technical/tables/gravity.pdf
Measure the density in a flask using a hydrometer. ( for Jet A1 it should have a range of 0.75 to 0.85, this is stamped on the paper inside the glass.) At the same time read the temperature of the Jet A1 and apply both numbers to the graph to give you the corrected SG.