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Fuel density

I have just finished the AGK course for the new students in the aeroclub and I got a very interesting question.

Does the fuel density affect the performance of the “average” GA airplane?

I have never really thought about it but at work we always use kg when talking about fuel and while flying GA i always use liters.

Is it the difference just how the engine works? Piston vs Turbine? Or does the density of Avgas change less than Jet fuel?
And if density changes, can you lean more to get the same EGT for a lower Fuel Flow?

I would love to hear all your input on this.

EBZW, Belgium

Some related discussion here

Shoreham EGKA, United Kingdom

But that thread is more about the volumetric aspect.
I’m looking for more info about the energy aspect.

EBZW, Belgium

FlyingAppel wrote:

Or does the density of Avgas change less than Jet fuel

Change with respect to what? Temperature? or different Avgas types? Mogas?

The elephant is the circulation

Thermal expansion coefficients are similar for Jet-A1 and avgas 100LL, at 0.1% per K.

The energy content will be according to how many molecules of fuel can be combined with oxygen, so it will be related to fuel mass, not volume.

Shoreham EGKA, United Kingdom

@FlyingAppel of course it is mass what decides on fuel burn rate.

But contrary to “pro aviation” we seldom manage to repeat the engine settings on two flights. So the actual fuel burn rate depends much more on the pilot and also on how fast he wants to go. That’s why in the end it doesn’t matter whether we calculate as kg or l.

Also I assume that it’s easier for the fuel pump equipment to measure in litres and that’s what we translate into the numbers we have.


“pro aviation” we seldom manage to repeat the engine settings on two flights

+ “pro aviation” only uses Kgs (ok, some even still use lbs…), and would not bother with most of what has been discussed here. Assuming full capacity required + hot fuel = less kgs = less flight time, easy peasy

ain't the Destination, but the Journey
LSZF, Switzerland

A few experiences about fuel density:

When fueling for a self service pump fuel temperature seems to be disregarded. The meter is by the liter like for a car and you pay the per liter price. For JetA1 and AVGAS this seems to be the same.

When fueling from a truck I have seen fuel companies putting in the temperature and adjusting the number of liters accordingly. Also this seems to be similar for JetA1 and AVGAS. I have a feeling some companies only do this when temp is below 15°C. For example the fuel truck at Losinj does not seem to correct while I assume that fuel is often hot. This makes a few Euros difference on every fillup.

When actually fueling the truck drivers seem only to have meters for liters (or gallons). So whatever you calculate in lbs or kg will have to be translated into liters for those guys. We do it ourselves but maybe for the big planes the handling agent translates the crew fuel request into liters for the truck driver.

Nearly all European fuel companies quote JetA1 and AVGAS prices in liters. Only in Bosnia I have got prices in kg for JetA1.

Our plane (P46T) will display fuel quantity in lbs but at least from my research in the manual this seems to be not corrected for temperature as that computer does not even have a fuel temp sensor. The sensors in the wings detect the volume and then the display must have a fixed factor to display lbs.

Last Edited by Sebastian_G at 19 Nov 20:25

Yes it is a bit silly to display mass, when the flowmeter is measuring volume flow

Mass flow is actually nontrivial to measure. It is a well established technique in industry and lots of $$$ is made selling mass flowmeters, but none of them are simple, not anything like the 201B which most of us avgas burners use.

I think one could measure mass flow with the 201B (or whatever is used for your PT6) just by measuring the temperature at the transducer – because you just want it to be consistent, not accurate in absolute terms, and the expansion coefficient of the fuel will be known accurately.

You have sensors in the wings? Those are just level sensors and hardly accurate, but your flow totaliser has a fair chance of doing it properly.

Shoreham EGKA, United Kingdom

Peter wrote:

You have sensors in the wings? Those are just level sensors and hardly accurate

That is the amazing thing. In the Piper Meridian the actual fuel quantitiy measured is very accurate down to maybe 5-10 pounds. If you fly one hour at 250lbs/h fuel quantity will be down by 250lb +-5. This also seems to be the reason why it does not have a fuel totalizer, it is not required. Also such planes are often not fueled to the max so resetting a totalizer would not be as easy.

Peter wrote:

Yes it is a bit silly to display mass, when the flowmeter is measuring volume flow

This is probably a bit marketing. The big planes use kg/lbs and quite a number of smaller ones also use those units maybe to feel bigger ;-) The obvious advantage is on the w&b side. You do not have to do any math but then you have to do the math for the fuel truck driver.

But back to the original question. If the fuel mass changes due to temperature the weather is usually non standard and the performance changes due to the non standard air temperatures usually have a much bigger effect than the different fuel mass. For example in winter the fuel is more dense but then the air is thicker and fuel comsumption is a bit higher. This effect seems to be more important than the additional fuel mass available.
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