AF330:
Ok so the center of gravity is the point at which the entire weight of the plane may be considered as concentrated so that if supported at this point the plane would remain in equilibrium in any position.
1) So does this point keep changing during the flight? Where is that point generally located?
Yes. Fuel is kept in the outer tanks as long as possible during the flight to provide wing bending relief. That means fuel is normally burnt from the centre tank first (if the aircraft has one), then from the wing inner tanks and finally from the wing outer tanks. On a swept wing aircraft the centre tank is obviously forward of the wing inner & outer tanks. Consequently, the CG moves aft during the flight as fuel is burnt by the engines.
To complicate matters, the long range Airbii have a trim tank in the tail. Fuel is transferred from the centre/inner tanks to the trim tank after take-off to move the CG aft. The fuel system aims to keep the CG close to an aft CG target, approximately 2% forward of the the aft CG limit, to reduce drag.
Where is the CG located? It depends how the aircraft is loaded and what stage of flight you're talking about, so it could be anywhere between the forward limit (typically about 15% MAC) and the aft limit (typically about 40% MAC). The fuel system of the long range Airbii aims to keep the CG at about 38% MAC (roughly) during the cruise.
2) Ok, as I don't know all of these computers, I just would like to compare with the A320:
a) I have heard about the FMGS/FMGC about the A320, but never about the FMGEC. Has the A320 a FE?? If not, who controls GW and C.G?
b) Can we say that FCMC = FQI + FLSCU?
c) Does an A320 have a FWC? So each time a computer detects a problem, it sends it to the FWC (and the FMGEC maybe, no?)
d) FCPC...hmm... can we say that FCPC is kind of ELAC on Airbus? ELAC + SEC? ELAC + FAC? FAC + SEC?
I'm not familiar with the A320, so I'm not going there!
3) Ok, so the FCMC is controlling the gross weight and the center of gravity. But is it calculating the fuel's GW? Is it calculating the plane's C.G?
The FCMC isn't 'controlling' the aircraft's gross weight. The aircraft is loaded to a certain GW while it is on the ground. The GW then reduces during flight as fuel is used.
The FCMC calculates the weight of fuel on board by measuring the fuel volume and density, and displays that calculated figure on the ECAM. It also calculates the fuel on board by subtracting the fuel used from the fuel on board at engine start and generates a caution if there is a discrepancy between the two calculated figures.
The FCMC also calculates the aircraft's CG using the ZFW and ZFWCG input by the pilots during the preflight, and the sensed fuel quantity & distribution - as discussed previously!
4) So you have a AFT CG Monitor inside the FE.
The FWD CG is calculated by getting the CG calculated by FCMC?
CG calculated by FCMC - AFT CG calculated by FE = FWD CG?
I'm not entirely sure what you're asking, but the aircraft only has one CG at any one time. That CG moves during flight (as discussed above) and can be anywhere between the forward CG limit and the aft CG limit. Those limits are defined when the aircraft is certified.
The FE does a GW/CG computation independently of the FCMC, using various aerodynamic inputs (speed, thrust, altitude, THS position, elevator position). The GW/CG values computed by the FE are used as a backup in case of dual FCMC failure, and are also used to generate an 'Excess Aft CG' warning if the CG is found to be too far aft.