Fuel Density and Metering
 

So on a recent long haul trip from a cold climate to a warm climate I noticed a big difference in the specific gravity of the fuel uplifted to the aircraft in the cold country vs the hot country. This is of course expected but got me thinking.

When we are flying out of a hot country, with temperatures in the high 30s/low 40s (Celsius) and with hot fuel being pumped into the tanks what is the effect on the aircraft once we are airborne and the fuel cools to near or below freezing? As we climb the fuel Density will increase and hence the volume will decrease. Question, is this accounted for during fuel planning? And is fuel metered to the engines by weight or volume?

This then brings me to the next question. Why do we do fuel planning by weight? Sure we need to know the weight of the fuel in the tanks. But when deciding burn should we not be more concerned with litres and then work out the weight afterward? Thanks.

A kilogram of fuel will have the same amount of specific energy as another kilogram. A litre on the other hand will have a different specific energy depending on the temperature change. Jet aircraft will calculate fuel flows based on weight to take the temperature variable out of the equation. 2000kg/hr will produce the same amount of energy whether the fuel is at 30C or 10C

:ok:

Yep, it's not the volume of fuel that's important in terms of the energy a given volume contains, it's the number of "fuel molecules"..which is related to density.

Putting it another very simplistic way, it might need the energy contained in 10 billion billion molecules of fuel to fly from XXX to YYY....those 10 billion billion molecules will have a total mass - so as long as I can get that mass into the tanks I'll have the right number of molecules for the flight and from a range POV it doesn't matter if the fuel occupies a volume of e.g.1000 litres or 10,000 litres.

Of course in some circumstances the fuel is so warm you can't get your 10 billion billion molecules in the tanks - then you've got a problem...

(Edit to add: Before the Physicists and planners pile in :Yes, yes it's first approximation answer given in the context of the question)

wiggy: a quick qustion for you if I may - years ago I used to do a lot of transpacific flying.
The uplift in the US was always JetA versus the more usual Jet A1 elsewhere. The rumours were that the calorific output of the Jet A was less than the A1 - do you know if that really was the case? ie. the howgozit was always worse ex-US than going.

Meikleour

TBH no idea but it's an interesting question and I'd be interested in the answer.

Exxon says the "Net Heat of Combustion, MJ/kg" has the same minimum value, 42.8 if that helps.

Volume vs Weight
 

If I were selling fuel, I would want to sell it by weight rather than by volume, i.e. I would be selling energy rather than gallons of liquid. In cold country, I wouldn't want to give away energy when the fuel was very dense.

I don't know how modern fuel delivery equipment works but, in the old days, fuel from the bowser was delivered by volume. When it got in the plane, the fuel gauges measured it in pounds. The crew monitored it on the gauges and when the required number of pounds were on board, they stopped fueling. It is possible that when fueling with hot fuel you might not be able to accommodate the volume of fuel needed in order to have enough energy in the tanks.

My understanding is that modern aircraft measure the fuel volume and temperature and then calculate the weight. Is this right?

The issue is that one can fuel up most jet aircraft with different fuels. And no where on our refueling panel can we select which type of fuel is being uplifted. Jet A, A1, B, JP4 etc. How does the aircraft know which one is in the tank?

Anyway, I'm seriously confused about this matter. I just want clarification so as not to get caught out.

My ops manual currently says the final uplift must be +/- 625 litres of planned uplift for the aircraft to be dispatched.

I believe this is to cater for the change in SG.

Yep, and as a pilot or operator, I'd want to buy it by weight since I have to lift the darn stuff into the air. ;)

Nice that there is a "coincidence of motives".

If you cast your mind back to Rutan's Voyager round-the world flight you'll have seen an example of just this.

The flight was due to depart at around dawn, and the aeroplane was positioned on the runway the previous evening where it was checked out and fully fuelled - brim-filled in fact to avoid moisture condensation in the tanks. Overnight temps in the desert fell very low, and so by morning it was pretty cool and the fuel on the aeroplane shrank.

Someone actually checked the fuel by dip-stick, and found the tanks weren't completely full. They assumed this was due to fuel system air pockets having worked their way into the main tanks (curiously by an almost identical amount on each side), so that person had the fuel tanks topped-off again. This meant that the aeroplane was actually carrying (IIRC) a couple of hundred pounds of extra fuel - putting it above MTOW. The extra fuel in the wings, coupled to the negative AoA ground attitude and the bumps, caused the wing tips to scrape on the ground shortly after the start of the take-off roll. Before it left the ground the wing tips were so badly abraded that the winglets fell off a few minutes into the flight...

PDR

Short answer is yes - the aircraft measures the volume of the fuel and the fuel density and uses that to calculate the mass of fuel.
BTW, while the energy per unit mass of jet fuel is close, it's not a constant - there is some variation around the mean, although it's pretty much in the mud compared to other variables like engine-to-engine efficiency and airframe drag differences.
Back in the old hydro-mechanical fuel control days, there were adjustments on the fuel controls (adjustable on-wing) for fuel density. My understanding is the adjustment was seldom used, but it was available.
Today, with FADEC, that density adjustment has gone away. The FADEC controls closed loop to rotor speed or EPR, and accels/decels are to rate of change of rotor speed (normally N2dot (or N3dot for Rolls), although N1dot does come into play sometimes). There are maximum fuel flow rates based on fuel metering valve position during transients (weight fuel flow/burner pressure - often called "Woof/PB") but those rarely if ever come into play.
About the only time fuel density might have a noticeable affect on FADEC is during initial lightoff during starting. FADEC uses a fixed fuel metering valve position - which will result in a fixed volume of fuel - until the engine lights and starts to accel. That's one of the reasons it can be hard to start when it's really, really cold.

Capacitance type fuel indications systems (which are in 90% of the turbine powered aircraft out there) are not effected by the density changes due to temperature.

There usually isn't a separate computation being performed regarding volume/temperature on the airplane side of the system.

If gauge shows 2500 lbs, there's 2500 lbs of fuel on board. Give or take a bit for calibration errors etc etc.

Temp of the fuel is only one factor, density of the feedstock that was cracked makes a difference, too. There's no controlling that, either.

GF

plhought, are you sure about that? There appear to be plenty of capacitance based fuel gauging systems that have a densitometer, which should be unnecessary if you are correct.

In the types I've been involved with I've never seen a method of measuring density within the aircraft. Tiny planes to big ones. I could understand tank farms and trucks using a capacitive indication system in conjunction with a density measurement to establish accurate volume or specific gravity for distribution for use.

Upon further reading, I guess some fancy aircraft do use fuel temp probes to correct for density. I didn't think capacitance probes would be effected by it. Guess they can be affected.

The capacitive fuel probes, coupled with the indication system, give a reading in pounds, and as such doesn't care how hot or deep the fuel is. Refuelers should carry a conversion chart that give the pounds of fuel delivered, knowing the volume and fuel temp.

I must say I'm confused here, and open to being corrected. But my understanding was that capacitative probes work by having a capacitor comprised of coaxial plates - a tubular outer plate and a solid inner plate with an air gap. The capacitance of the probe is proportional to the co-incident area of the plates and the dielectric constant of the air between them.

When you put this probe into a fluid it essentially becomes two capacitors in parallel - one with an air dielectric and the other with a fluid dielectric. So the capacitance changes such that it is the coincident area of the plates which are immersed in fluid times the fluid dielectric constant plus the coincident area of the plates above the fluid times the air dielectric constant.

As such the probes themselves only ever measure fuel *level* in the tanks, and any conversion to gallons/litres/lbs/kgs/pickle-bottles or WHY is achieved with calibration curves in the metering system. This would suggest that if the tanks were filled with warm fuel such that the gauges showed half-full, at the end of a cold night the gauges would indicate a lesser figure for the same amount of fuel UNLESS the fuel metering system included a temperature/density correction system of some sort.

Is my understanding wrong?

PDR

PDR1, as I recall it the B747 & B744 have a capacitive probe mounted sideways close, but not too close - water - to the bottom of each tank to provide a density calibration. This has led to problems, big ones, when refueling with fuel which tends to stratify because the fuel is then non homogeneous so your calibration will not be the average. The early B747-SP flights SYD-SFO & SYD-LAX caused us a lot of grief because the USA West Coast fuel tended to stratify and these A/C were dedicated to that run. Since we were usually at MTOW errors could not be tolerated and determining the actual fuel load became very difficult. Often we just took a deep breath and buggered off into the wild blue a bit unsure of actual weights. Undesirable of course, but no other option really.