AF330:
1) So does the plane tend to pitch-up as you burn fuel? But how do you reduce drag? If you pitch up, you reduce drag!
If nothing else changes, then yes, the aircraft would tend to pitch up as fuel is used and the CG moves aft. In a conventional aircraft the CG is forward of the centre of lift, creating a pitch moment that tends to pitch the aircraft nose down. To counteract that moment, the horizontal stabiliser must produce a down force to keep the aircraft balanced. That downforce adds to the aircraft's weight, which also acts downwards, and the wings must therefore produce more lift (i.e. upwards force) to keep the forces in balance so that the aircraft can maintain level flight. If the wings produce more lift they also produce more induced drag.
As the CG moves aft, the amount of downforce the stab must produce reduces and so does the drag. The Airbus fuel system tries to keep the aircraft's CG as far aft as possible during the cruise to reduce drag and improve performance.
The effect of CG on induced drag is discussed in the following link:
http://www.pprune.org/tech-log/35785...uced-drag.html
2) Sorry, what does MAC mean? Could you please explain?
MAC = Mean Aerodynamic Chord
The location of an aircraft's CG is usually expressed as a certain percentage of the MAC, e.g. 35% MAC means the CG is located 35% of the distance along the MAC.
The following link has a simple explanation:
Mean Aerodynamic Chord (MAC) - SKYbrary Aviation Safety
Ok, the FCMC needs the fuel's volume and density to get GW.
But I didn't understand how is FOB calculated.
Fuel weight (i.e. FOB) = Fuel volume x Fuel density
The FCMC measures the volume of fuel in the tanks and its density and calculates the FOB. The result is displayed on the ECAM and fuel system page.
3) You know the weight of fuel. Why does it take fuel used during engine start??? What are the 2 calculated figures? I had XXX lbs at departure. Now, I know my fuel quantity by knowing the fuel density, no? Confused!
The FCMC performs two calculations of the fuel weight. The first is described above. The second calculation is performed using a different method - it subtracts the fuel used (by measuring fuel flow) from the FOB at engine start. The two figures are then compared and if there is a discrepancy the FCMC generates an ECAM caution. A discrepancy could be an indication of a fuel leak.
4) The ZFWCG is given by the pilot. But does the plane calculate it again (and changes the value)? Like for fuel: when you enter "block", the FMGS can automatically change it If it wrong.
No. The ZFWCG is determined by the way the aircraft is loaded with passengers, baggage & cargo, etc. The ZFWCG doesn't change after the aircraft is loaded on the ground (if it does, you're in trouble!).
5) The FWD CG is also controlled by FE?
No, the FE provides an 'EXCESS AFT CG' warning if the CG is too far aft, but there is no such warning if the CG is too far forward, simply because it's not necessary. If the aircraft is loaded correctly on the ground, then the CG can't move too far forward - it can only move backward as fuel is used or transferred to the trim tank in the tail. The reason an 'EXCESS AFT CG' warning is provided is because the fuel system tries to keep the CG just forward of the rear limit during the cruise. If there is a problem with the system (too much fuel pumped aft, or not subsequently pumped forward as the CG moves aft) then the CG could move too far aft.
Do you mran that it calcualtes FOB by subtracting weight of fuel before engine start - weight of fuel now?
It will then give the fuel's weight which has been burned..
No. See above.