View Full Version : SG of fuel and fuel distribution

22nd Apr 2008, 00:55
The aircraft i fly (airbus) has a fuel distribution chart corresponding to a sg of 0.785

Now obviously when the sg is higher, the litres of fuel required to make up the weight of fuel is less.
We give our fuel order in tonnes.

So if we order 20T @ sg 0.785, we get about 25480L
And if we order 20T @ sg 0.805 we get about 24800L

Hence a difference of about 700L

How would a higher sg affect the fuel distribution?

thanks in advance

Old Fella
22nd Apr 2008, 02:15
Your flight plan is based on a fuel burn in kilos/hr I assume. It doen't matter what the SG of the fuel is, you just want a certain fuel load weight and so the distribution will be unaffected. Your fuel load is displayed in Kgs, your burn rate is measured in Kgs and your fuel burnt is shown in Kgs. You plan to arrive at destination with a certain weight of fuel remaining. Don't see where there should be any problem with the fuel being of a SG higher than .785 Kgs/litre, just means the volume required for a given weight will be less.

22nd Apr 2008, 13:11
the effect of SG is different from airplane to airplane. It is more pronounced on aircraft with a trim tank.

The fuel loading logic on the A300-600 is that fuel is only loaded in the trim tank when the inner / outter / center are all full. So depending on SG, for a specific KG load, you may or may not have fuel in your trim tank. This has a big effect of TOW CG.

On the A330, the fuel loading sequence is automated, such that fuel is dristributed to all tanks as per a set algorithm, which limits the CG shift. However around the 36 to 38 ton fuel load, the last ton or more of fuel solely goes to the trim tank. So again, for a Block Fuel of 37 tons of fuel, SG does play a significant part.

Yet, the FCOM is clear. Enter ZFW & ZFCG, and go.

The SG effect on CG is only important in determining if you are inside the envelope. It is not important in terms of Stab Setting.

22nd Apr 2008, 16:04
Old Fella,

Your fuel guage may read in lbs, but the probes in the tank are measuring volume. They're usually temperature compensated to take care of thermodynamic expansion, but the basic SG of the fuel is just assumed to be standard.

I've done a few fuel cals where we put known mass quantities in an airplane and check them against the readout and you might be surprised to know how little accuracy there is. About +/- 5% is the best you can hope for. Remember, according to the FAA, your fuel guage only needs to be accurate when the ship is empty.

22nd Apr 2008, 16:09
Modern (even as early as the 1960s) capacitance-type fuel probes compensate for density and give a true reading in pounds or Kg.

22nd Apr 2008, 18:04
Capacitance type probes are what I was talking about. They compensate for density only via temperature. Some aircraft may have additional sensors to measure density, but not any of the ones I've worked on.

Here's a good Av Week article about the subject:


22nd Apr 2008, 18:28
Haven't we been there some time ago?

Or this may have been on a Concorde forum, where we went through exactly this question, and looked at the sensors. Remember Concorde? Fuel quantity was in kgs, and a damn sight better than +/- 5 %.

22nd Apr 2008, 19:05
I can't say I know much about Concorde. Did it use fuel for Mach trim? Obviously, there are ways to get more precise, but unless you're using fuel for something more critical than just running the engines, most folks don't go to the trouble.

That said, I haven't worked on an airliner. Perhaps the customers demand more than the minimum since they're so obsessed with carrying just enough fuel to get there. But, on the military and GA side of the house, no one seems to care much if your weight calculations are a few percent off. That's why we weigh the actual aircraft before and after each flight when doing performance tests. The answers are highly weight dependent and the fuel system can't be trusted.

22nd Apr 2008, 19:43
I can't say I know much about Concorde. Did it use fuel for Mach trim?
Yes it did.
In the case of Concorde there was a significant shift aft of the centre of lift when going supersonic. Rather than trim using the control surfaces (with the resulting added drag), clever management of fuel burn, combined with fuel transfer aft (and forward during deceleration), was used to move the centre of gravity to the optimum position for most of the flight.

Concorde had a total of 13 separate tanks (carefully numbered 1 to 11, plus 5A and 7A... I bet somebody was superstitious...).

Tanks 9 and 10 (fully forward) and tank 11 (in the tail) were designated "trim tanks" and those were the ones used primarily to shift the CofG when needed.


23rd Apr 2008, 01:04
Since the early 80's airliners have been equipped with capacitance fuel quantity measuring systems that not only use 1960's technology compensated volumetric measurement, but the digital computer receives data from a densitometer that measures actual fuel density. The densitometers employ radio-active isotopes and require special handling procedures.

Old Fella
23rd Apr 2008, 01:33
As well as knowing flight planned fuel I have always determined two things before uplift. (a) no contamination in the fuel to be uplifted and, (b) density of the fuel to be uplifted. Knowing density of fuel to be uplifted and fuel remaining before uplift it is simple to calculate the amount of fuel to be uplifted. I have had to uplift fuel in US Gallons, Imp Gallons and litres worldwide, the one common value always determined before uplift being uplift fuel density. Always, fuel loading was in either lbs or Kgs depending on aircraft type. Fuel flight plan always used fuel required in either lbs or Kgs. At least two individual methods of determining fuel on board had to be used and a maximum difference allowed between fuel by uplift vs fuel by gauge was 3% with the lesser amount being considered fuel on board. Outside the 3% differential the reason had to be determined. If known fuel on board before uplift and amount uplifted equals desired fuel load at the completion of refuelling, no problems. As I previously said, fuel distribution for any aircraft I have operated as a F/E has been determined by weight of fuel, not volume.

23rd Apr 2008, 09:36
The statement that military fuel mass is not accurate is total bull. The fuel systems in modern fighters or bombers feed centre of gravity into the fly by wire system, so an accurate knowledge of the weight of fuel in each tank is needed.

For this reason, the fuel system on EFA (for example) is categorised as a level 1 system (failure likely to lead to total loss), while those on civil aircraft are generally level 2 (failure likely to cause divert and stress in cockpit).

On the civil side, the accuracy specified for the B777 was +/- 2% through the range of loads in the tank, with (from memory) +/- 0.5% close to empty.