fuel consumption and CG movement

[IFLY_INDIGO]

In A320 as the fuel is consumed during the flight, the aircraft CG moves back towards the ZFWCG (but does not become ZFWCG unless we burn all the fuel).. I am wondering if there is any commercial transport cat airplane in which CG moves forward with fuel consumption ( I admit that would be very silly coz it would increase fuel consumption).. just out of curosity.. ????

[Denti]

Is it really that simple? Only one directional movement with fuel usage?

On the 737 the CG moves backwards as long as fuel from the center tank is used, and forwards when fuel from the main (wing) tanks is used.

[IFLY_INDIGO]

nice to know.. but in A320, CG always move backward with fuel consumption, irrespective of the tank..

[Wizofoz]

In the 747-400 it moves all over the place. The aircraft has a Tail tank, and the fuel sequence is automatically controlled to keep the CG in range.

Concorde used Fuel for trim, so it moved all over the place too.

[Mad (Flt) Scientist]

I am wondering if there is any commercial transport cat airplane in which CG moves forward with fuel consumption ( I admit that would be very silly coz it would increase fuel consumption).. just out of curosity.. ????

Yes there are. It all depends on the fuel layout. Global Express (which is Part 25 Transport Category) has it moving both forward and aft at different times, as the fuel burn is an attempt to maintain a reasonably constant - and hence reasonably aft - cg.

And why do you believe starting aft and moving forward is silly - the behaviour you describe, of starting forward and moving aft is actually worse, since it means that the start of every flight, when you are heaviest, is also furthest forward, and every flight starts heavy while not every flight ends empty, so the "good" part of the fuel burn curve may never happen.

What's important for fuel burn is trying to keep the cg as far aft as possible throughout the flight, consistent with a safe loading.

[Microburst2002]

CG in the 320 moves in both directions, depending on the circumstances.
For instance, when the transfer valves open the CG moves forward quite quickly.

[Checkboard]

Also, on the A320/319/318 series aircraft, the centre of gravity position has negligible affect on fuel burn, due to a complex interaction of aerodynamic drag factors.

[barit1]

Given that an aft CG is desirable (*) for min trim drag, fuel management should generally move it that direction.

However when there are many tanks a la 747, the initial fuel burn would logically be from fwd (center section) tanks, then whatever is necessary to keep within aft CG limit.

* Granted that reduced-natural-stability a/c don't have this issue...

[Swedish Steve]

However when there are many tanks a la 747, the initial fuel burn would logically be from fwd (center section) tanks, then whatever is necessary to keep within aft CG limit

But it isn't. The B744 stab tank starts to empty in the climb, as soon as there is room in the centre tank. The reason is that if the stab tank does not empty for any reason, the aircraft will go out of CG limits later.

[Iamneon]

Here below the story of fuel economy of the a320 family;
from getting the grips with fuel economy;

The A320 family does not show the same SR variation with CG as the other aircraft. The aft CG produces worst SR at FL290, crossing over to show an improvement at higher flight levels. The SAR variation is much smaller also. This is due to a complex interaction of several aerodynamic effects. The SAR can be considered effectively constant with CG position. Loading is therefore not critical for fuel economy for the A320 family.

Hope this helps

Ciao

[barit1]

Swedish Steve:
But it isn't. The B744 stab tank starts to empty in the climb, as soon as there is room in the centre tank. The reason is that if the stab tank does not empty for any reason, the aircraft will go out of CG limits later.

Fair enough, but I'm sure this is done for reasons of practical simplicity. The most fuel-efficient method would be to drive the CG to near the aft limit, then maintain it there as long as possible via split feed.

I recognize this may not provide required contingency operations, though; and the practical difference in specific range may not be significant.

[IFLY_INDIGO]

the behaviour you describe, of starting forward and moving aft is actually worse, since it means that the start of every flight, when you are heaviest, is also furthest forward, and every flight starts heavy while not every flight ends empty, so the "good" part of the fuel burn curve may never happen.

Thats is very enlightening indeed...:)

[IFLY_INDIGO]

FMS calculates aircraft CG using the ZFWCG and Fuel consumption data, very mechanically. if in flight, you change the ZFWCG by any amount, FMS updates the aircraft CG instantly. What could be the use of the value of aircraft CG for FMS?

[Microburst2002]

Does any other system interface with the FMGS for %CG? maybe flight controls? FAC?

Anyway, however negligible the effect of CG on fuel comsumption may be, the FMGS still uses it to compute EFOB.

[Denti]

Dunno about the FMGS, the FMC certainly does in a boeing. We had an incident where some very curious captain (PNF) surprised his FO during cruise in a 738 by entering an MAC of 36% into the FMC when it was in really around 20%. It resulted in overlapping yellow bands and immediate stickshaker activation, flown stall recovery and lateron some very embarassed explanation at a tea-appointment without biscuits.

[Basil]

One problem with leaving the B744 stab transfer till later is that, if there is a transfer malfunction, you are very soon out of trim for landing.
As it is you will have, IIRC, about 5hrs to sort it out or land.

[IFLY_INDIGO]

Here below the story of fuel economy of the a320 family;
from getting the grips with fuel economy;

The A320 family does not show the same SR variation with CG as the other aircraft. The aft CG produces worst SR at FL290, crossing over to show an improvement at higher flight levels. The SAR variation is much smaller also. This is due to a complex interaction of several aerodynamic effects. The SAR can be considered effectively constant with CG position. Loading is therefore not critical for fuel economy for the A320 family.

Hope this helps

Ciao

great help indeed... :)

[Bus Driver Man]

FMS calculates aircraft CG using the ZFWCG and Fuel consumption data, very mechanically. if in flight, you change the ZFWCG by any amount, FMS updates the aircraft CG instantly. What could be the use of the value of aircraft CG for FMS?

For GC <25% add 2 kts to Vls

Probably one of the reasons the FMGS calculates the GC.
Also, for info for the pilots in case of landing distance calculation. Forward CG (<25%) increases landing distance by 2% on a dry rwy and 3% on wet or contaminated runways.

[IFLY_INDIGO]

For GC <25% add 2 kts to Vls

Probably one of the reasons the FMGS calculates the GC.
Also, for info for the pilots in case of landing distance calculation. Forward CG (<25%) increases landing distance by 2% on a dry rwy and 3% on wet or contaminated runways.

yeah.. that's one great purpose indeed...:ok:

[president]

Why would overlapping of the yellow bands cause stick shaker? Stick shaker activates at the top of the red/black bar

[john_tullamarine]

Not an Airbus man, but two comments of a general nature.

(a) the other consideration for fuel usage programming is that we prefer to keep fuel in the outer wing panels for bending relief - another input into the walnut moving.

(b) For GC <25% add 2 kts to Vls ... Forward CG (<25%) increases landing distance by 2% on a dry rwy and 3% on wet or contaminated runways.

Characteristic stall. Forward CG (at higher weights) produces the maximum stall speed which then goes into general speed and distance calculations. It appears that the Airbus has adopted two paddocks to get a general advantage with the forward penalty being a higher approach/landing speed and, naturally enough, a modest distance penalty.