B747-400 Tail Fuel Density

[avrflr]

Why does the amount of fuel required in the tail change with SG? I asked a line maintenance bloke and he didn't have a clue.

[Mad (Flt) Scientist]

Either it's a volume restriction, being converted to mass, or a mass restriction being converted to volumetric. Which way is it?

[spannersatKL]

As Mad Scentist says Density = Mass / Volume hence Volume = Mass/Density ....applies to all tanks! Also on -400 as the tail tank is on a long moment arm from the CG if the density were high then the trim would be affected.

[galaxy flyer]

MfS:

While we are on this subject, why can't I fill up our products with the number of pounds I would like? Yes, I know SG is the issue, but frustrating.

GF

[john_tullamarine]

GF,

don't have the specifics on the product .. but, if you measure quantity added in volume (litres, gallons, whatever - as is obviously the case from your post), one has to do the conversion from required mass at the relevant SG to come up with the number of volume units for the bowser chap ... pain in the neck but only a minor nuisance apart from the philosophical problems associated with risk of error.

.. then again, you might talk one of the commercial operators into requiring a change in the system (at huge expense) so that the gauges all read mass units ... ?

trust that you have recovered from the rigours of the Avalon airshow ...

[BelArgUSA]

Aircraft weight limitations (call it "mass" if you use nerds and geeks vocabulary)...
They are based on kilos or pounds, depending on what units you use.
There are no aircraft limitations which use "liters" or "gallons" for certification.
xxx
Aviation gasoline specific gravity is .69...
Meaning a liter of gasoline weighs 690 grams per liter.
Air Force "wide cut" JP-4 specific gravity is .78...
Which is 780 grams per liter. That is the fuel generally used by USAF.
Excellent for low temperature concerns... if you are in Alaska in winter.
Jet-A fuel has .812 gravity.
So 812 grams per liter.
And Jet-A1 has .813 gravity. US Navy calls it JP-5 -
Bla bla bla - 813 grams per liter. Most airlines have that fuel in tanks.
xxx
Specific gravities above are based on 15ºC temperature.
On a hot day, in Jeddah (Holy be thy name) Jet-A1 might be only .79 SG...
And on a cold winter day in Khabarovsk, Jet-A might be .835 SG.
xxx
Your tanks are volume, meaning - if you "fill-up" volumewise...
Full tanks in a given 747, you might be 5 tons over limitations on cold days.
And on hot days, your tank "weight capacity" might be only 5 tons less.
xxx
Definitely brings a CG question about the tail tanks in a 747-400...
You might end with a CG outside of the "aft limit" on a cold day.
xxx
Be aware that your tanks gage readings are compensated for temperature.
They measure weight... NOT volume.
xxx
:)
Happy contrails

P.S.
In this day and age, of expensive car gasoline (petrol for my UK friends) -
Your pump delivers gallons/liters... you pay by volume.
Remember: your best buy is cold gasoline/petrol.
Because your engine uses weight for energy, not volume.
Best time to buy - early morning, or in... winter.

[mustafagander]

The "amount" must refer to volume - we plan the fuel load in Kg. At times the volume is insufficient, even on the ER. Bugger, tech stop and real long night. OTOH think of the overtime!! :}

[avrflr]

OK, I should have asked "why does the mass of fuel required in the tail change?"

To give a specific example, the fuel load requested was 135,000 kgs with 3900 kgs in the tail. SG was 0.795. The engineer had written down a few other tail figures from the manual, e.g. at 0.790 3800kgs was required. I would have guessed that as the SG goes down, more fuel would have to go into the CWT (bringing the CG forward) and therefore more fuel would be required in the tail. The opposite appeared to be true, according to the engineer's notes.

[Mad (Flt) Scientist]

3900 kgs in the tail. SG was 0.795. The engineer had written down a few other tail figures from the manual, e.g. at 0.790 3800kgs

You can put less mass of fuel in at lower SG because the tank VOLUME is fixed. Basically, if you fill the tank to the brim, the volume constrains what can go in. Thus the mass of fuel in the tank is determined from a fixed volume times the (variable) SG.

3900/0.795 and 3800/0.79 are similar numbers indicating a (near constant) volume. I suspect the masses have been rounded to the nearest 100kg.

(Incidentally, that's what galaxy flyer was alluding to; some aircraft end up being volume-limited, rather than mass limited, especially if using low SG fuels. Since mass of fuel is what propels the plane, being volume limited is quite annoying if you're trying to get a long range....)

[avrflr]

I believe the capacity of the tail fuel tank is ~10,000 kgs. Certainly there was plenty of spare volume. The fuel in the tail is used to keep the center of gravity within safe limits, if you filled it up to capacity I suspect the aircraft would not fly far.

[UKpaxman]

As said above, the only thing that can change the SG of a liquid is the temperature. Tank volumes will be based on 15 deg C (standard volume), the fuel will have an SG at it's measured temp (ambient'ish) but the SG will need to be converted to an SG at 15 deg C to calculate the mass in the tank.

[Mad (Flt) Scientist]

If it's not the volume of the tail tank, then it must be the volume of some other tank - the way it's working (decreasing mass with decreasing SC) means it must be a volume constraint somewhere. How about this?:

The tail tank mass is balancing the mass in some other tank forward. That forward tank is full. Therefore the mass in the forward tank falls as SG drops, therefore the mass you (need/can have) in the aft (tail) tank also falls.

[ballpoint]

Being a 744 engineer myself, I think it works this way:

The (full) wing can contain more Kgs of fuel when the SG goes up.
And it´s basic practice first to let the wing fill to the max capacity of +/- 110000 Kgs. using volumetric shutoff before filling the CW tank.
So, with more Kgs of fuel in the wing, you can put less in the CW tank and therefore less in the tail.
If in use, the tail will contain 1/10th of the amount that is in the CW.
So, 30000 in the CW means 3000 in the tail.

With regards,

Ballpoint

[galaxy flyer]

Here is the problem many corporate jet operators have, being volume limited. We advertise and customers want to make max range, pilots fill up the plane to the shutoffs and do NOT get the advertised mass of fuel, on which, the advertised range is based. While the Jet A1 standard density is about .81 at 15C, you almost never exceed .795 or so, and that difference is not insignificant on a max range sector.

However, it seems low density fuel actually has greater energy content volumetrically, so compensates a bit (not fully) for the lower fuel weight. Or so I hope.

GF

[john_tullamarine]

the only thing that can change the SG of a liquid is the temperature

not quite .. for fuels, the source crude also is relevant.

For GF's concern, it is necessary to arrange for high SG, high energy go-gas. So, for instance, QF flew a 744 nonstop London (?) to Sydney by doing just this ... albeit that they didn't have all that much at the far end, according to the tales ...

[galaxy flyer]

John:

My research got so detailed that I realized that I needed to have the bowser filled from a tank at the farm that had been filled to the brim 36-48 hours earlier so that the high density (high SG) fuel settled to the bottom. I suppose refueling in Whitehorse in the winter would have also helped. Speaking of Whitehorse, that was the one place I just about got every promised pound of fuel in a GLEX. 44,900 out of 45,100 design load.

A engineer friend of mine in the C5 right after our changeover to JP8, told the fueler to "fill it up" after the pilots asked for something just short of a full load. When it was done, he was 15,000 pounds over the "flight manual full weight of fuel".

GF

I'll be at Singapore, if you are at the show.