AF330:
Yes, if I understand you correctly then you seem to be more or less correct, with the following comments:
Direct Law
So it can be dangerous during take-off to move the SS, one of your wings can touch the RWY...
Yes, theoretically, if you used a large aileron input with the sidestick. However, remember that Direct Law is a 'reconfiguration' flight law - you would not take-off in Direct Law intentionally.
You would only encounter Direct Law after multiple in-flight systems failures. On the A330, the system reconfigures from Normal Law to Alternate 1 Law to Alternate 2 Law and finally to Direct Law, depending on the level of failure. The failures that would lead to Direct Law include:
- Triple IR failure,
- Triple PRIM (Flight Control Primary Computer) failure,
- Dual elevator fault, or
- All engines out with PRIM 1 failed
FCeng84:
From the reference it appears that for the single aisle models the CG and GW estimates used by the control laws are completely independent of pilot input of ZFW and ZFCG. For the long range models the picture does not look quite as clear with the stated impact on control laws characterized as "slightly affected".
I realize that the aerodynamics based estimates will only be available in air and then only after a certain period of time to allow the associated estimation algorithms to converge. If the control laws are dependent on CG and GW during takeoff roll and/or initial climbout I suspect that either default values coded with the control laws or the data input by the pilots will have to be used.
WRT the long range Airbii, my understanding is that the control law gains are normally modified using the GW/CG values calculated by the FCMC from the ZFW/ZFWCG data input by the pilots. The GW/CG values calculated independently by the FE are used as a backup in the event of a dual FCMC failure, and are also used to trigger the 'Excess Aft CG' warning. I don't know if there is some kind of comparison going on behind the scenes in-flight - there is nothing mentioned in the books.
During the take-off roll, the flight control system is in Normal Law Ground Mode. There is a direct relationship between the sidestick deflection and elevator deflection, with full authority. As far as I'm aware the control law is not modified for GW/CG during take-off. The stab position is set manually (automatically on later aircraft) after engine start, according to the GWCG value (or loadsheet GWCG) and there is no auto-trim. Consequently, during take-off the pitch control operates exactly the same as it would in a conventional aircraft. It transitions to Flight Mode shortly after lift-off and it's only then that the FCMC derived CG value starts to affect the control laws.