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redout
27th Jun 2008, 21:03
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.

FLX/MCT
27th Jun 2008, 21:05
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.

Todders
27th Jun 2008, 22:02
Also weight is what you wanna know when your calculating your performance etc.

wiggy
27th Jun 2008, 22:11
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.

Jumbo Driver
28th Jun 2008, 07:27
Simply because the weight (mass) doesn't change with temperature - but the volume does ...

Besides, what would be the benefit in knowing the volume of fuel in flight?


JD
:)

FE Hoppy
28th Jun 2008, 07:46
We need to know how heavy we are.

ChristiaanJ
28th Jun 2008, 22:49
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?

Denti
28th Jun 2008, 22:57
I would suspect your billed in litres as thats what i get on every fuel receipt. Fuel supplies dont think in weight (or mass) but in volume.

Jumbo Driver
28th Jun 2008, 23:10
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
:)

krujje
28th Jun 2008, 23:12
Not to be pedantic, but there IS a difference between weight and mass guys.... energy would be related to mass not weight...

Torstennnn
29th Jun 2008, 08:57
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?

ITCZ
29th Jun 2008, 11:27
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.

ChristiaanJ
29th Jun 2008, 12:58
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

Checkboard
5th Jul 2008, 10:56
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.

vapilot2004
5th Jul 2008, 17:41
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.

This I did not know Checkboard. :confused:



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
:)

Metering by volume is the more accurate method of dispensing fuel due, I imagine, to equipment limitations. It is easier to implement than the alternative - using scales. On a typical fuel bowser, a .00001% error in weight measurement is still a considerable amount. I would think even a moderately stiff wind on the sail area of the tank would affect accuracy during the weigh-in.

Checkboard
6th Jul 2008, 17:14
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.

Checkboard
6th Jul 2008, 17:17
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.

barit1
7th Jul 2008, 11:48
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 :ugh:

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.

SirToppamHat
27th Oct 2008, 19:57
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

Oilandgasman
27th Oct 2008, 20:47
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 (http://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.

SirToppamHat
28th Oct 2008, 11:45
Thanks Oilandgasman, that should do for starters.

STH

BelArgUSA
28th Oct 2008, 12:55
Let's continue to be Nerds and Geeks, and talk about liters, weight and mass.
xxx
I am surprised nobody talked much about jet engines...
After all, what these machines do for power is mass acceleration.
So, let us talk about the past century and jet engines.
Jet fuel is say, of a specific gravity of about .81 (call it 81% of weight of distilled water).
Of course, as mentioned, varies with temperature. Hot fuel is "lighter".
Now, let's talk about the old jet engines with water injection
There were the P&W JT3C-6 with water (called J-57 in the USAF/NAVY).
Some 707-120 and DC8-10 had these "wet engines".
Even the 747s... there were JT9D engines with water...
Last letter W in their designation, as JT9D-7AW, or -7FW
These 747 got some 2,000 lbs of thrust extra per engine using "water".
xxx
Now, most "aviation experts" (many here) said "water... to keep engine cold".
When you go full power, put water ON, keeps EGT low...
Can I smile...?
What water did really, was to increase accelerated mass flow.
Extra mass of water (specific gravity 1) to jet fuel (SG of .81)...
Result was more power... without increase in EGT, as water does not burn.
xxx
Imagine an inventor finding way to have "Hg (mercury) injectors" SG of some 13.
Would be a bit of vitamins for our engines...!
:}
Happy contrails

ChristiaanJ
28th Oct 2008, 17:01
I realized I wasn't fully clear about how water injection worked exactly, so I swiped this question/answer session from a discussion on airliners.net

Basic question if you please. Since I'm not a science type person, the injection of water to increase thrust has always seemed counter-intuitive to me since it would seem to have the effect of lowering the operating temperature of the engine (I've always understood the Law of Thermodynamics as indicating that to increase efficiency one must increase the temperature difference between the source & receiver. Do I have that right?). What causes the extra thrust? Superheated steam?

Without getting into a long mechanical engineering post here, the water is diffused (atomised) into the inlet and the combustion chamber (just forward of the burner cans).
As we all know, water does not burn. But what it does is drasticly increase the air flow density, and add additional oxygen as the water evaporates. This is basicly what gives all water injected engines the additional thrust, increasingly dense air flow and added oxygen.
Since the water is mixed with the air flow, and not contained in a pipe or boiler, it can never become superheated steam. The atom sized droplets boil off and vaporize as soon as they reach 212 degrees F (100 degrees C).

On airplanes like the KC-135A/Q and B-52F/G the J-57 produced around 11,600lbs of dry thrust for a fully cowled engine. The addition of water injection added about 800lbs of additional thrust for a total of about 12,400lbs.
In the KC-135A/Q we used 640 gallons of water during the 2 minutes water injection was used, the B-52 used 1280 gallons, as they had twice as many engines. In the KC-135 the water added 5,600lbs to the weight of the airplane. Water injection could be used, in the KC-135 only, down to 20 degrees F. This water was heated in the water tank, in the KC-135 through the use of 8 5KW heating elements. The water had to be heated to a minumum of 60 degrees F (15.6 degrees C) if the outside temp was 40 degrees F, or less to keep the water from turning into ice. The B-52 did not use water below 40 degrees F.

While less than exact (e.g., pressure at the compressor outlet / burner inlet is way above atmospheric, so the water 'boils' at more than 100°C) the quotes above at least give some idea of how it works.

What water did really, was to increase accelerated mass flow.
Extra mass of water (specific gravity 1) to jet fuel (SG of .81)...
Result was more power... without increase in EGT, as water does not burn.The SG is a red herring here; water injection would work pretty nearly just as well if water had the same SG as fuel.
Jet engine thrust is obtained by accelerating a large mass of air through the engine. The fuel provides the energy needed to accelerate the air, but the added mass flow of the burning fuel is negligeable relative to the mass flow of the air.

Unless I'm much mistaken, water injection works in two ways:
- the evaporating water cools down the air at the burner inlets, so more fuel can be injected without burning out the turbine, hence the mass flow of the air can be increased,
- the water also increases the total mass flow (in the example of the KC-135 above by over a ton/minute). But I'll have to get out my sliderule to figure out what percentage of total mass flow we're talking about.

CJ

BelArgUSA
28th Oct 2008, 17:24
Goeienavond Christiaan -
From an old jet engine book...
xxx
The increase in thrust is due to an increase in mass air flow, due to the inlet air temperature being reduced, plus the addition of the weight (mass) of the water-methanol itself
xxx
Again, I just repeat the "mis-information" of my past instructors...
Sorry, I am as educated as they were...
:8
Happy contrails

BelArgUSA
28th Oct 2008, 17:38
For info Christiaan - Some numbers from 747 engines...
Sorry , these are not metric units.
The dry JT9D-7F is a 48,000 lbs thrust engine, the -7FW (wet) is 50,000 lbs.
I do not have the mass airflow for the above engine.
xxx
For the JT9D-7Q, a 52,500 lbs thrust engine, it is 1,668 lbs/second.
TSFC is indicated as .378...
Fan thrust is 41,000 lbs, core thrust is 11,500 lbs.
Bypass ratio is 4.9 to 1...
And compression ratio is 24.4...
xxx
Hope it helps for your slide rule
:)
Happy contrails

bvcu
28th Oct 2008, 18:05
Dont forget the BAC 1-11 spey , and the Dart on the Andover [military avro 748] for those to young to remember. Cant remember all the theory but didnt the methanol have an effect as well as the water ? Have to dig the old rolls royce book out !!!

ChristiaanJ
28th Oct 2008, 18:32
This seems to turn into a "Water Injection" thread, but since we seem to have settled the original question.... why not !

BelArgUSA,
Goeienavond ook van hier.
Your book seems to match what I thought.
Thanks a lot for the figures, that will save me a lot of scratching around.

bvcu,
I think most of the ones you quote didn't smoke quite as spectacularly as the KC-135 or B-52..... :=
The methanol may have been there more as an anti-freeze? My guess, but I'd be pleased to find out more.

CJ

tocamak
28th Oct 2008, 18:44
Slightly off topic but similar in a way. Previous life on oil tankers and always wondered why not much was made of loading fuel without taking account of temperature for payment. Individually it probably doesn't make much odds but over a fleet per year must mount up. Oil business very careful about it as when loading 30 000t of gasoil in Venezuela at 30 deg.C and delivering to Boston in winter makes a huge change in volume. Ship calibrated in cubic metres and so dealt with metric tonnes with density at 15 deg. C but of course USA in barrels at 60F (not the same) with an API figure. Remarkably the figures worked out pretty closely.