A340 FE part computed GW and CG

[Microburst2002]

FE WEIGHT COMPUTATION (BACKUP)
‐ When the aircraft is below 14 625 ft, and 255 kt :
GW = f(α, CAS, N1/EPR actual, CG from FE part, altitude)
‐ When the aircraft is above 14 625 ft, or 255 kt :
GW = TOGW – WFU
TOGW : Takeoff Gross Weight
WFU : Weight Fuel Used, acquired from the FADECs.
FE CENTER OF GRAVITY COMPUTATION (BACKUP/AFT CG WARNING)
The CG is computed from the position of the horizontal stabilizer, and is function of the N1/EPR,
Vc, ALT, MACH and GW from FE part.

I don't understand how each thing, GW and CG, use each other for computation. GW uses FE CG and CG uses FE GW. How can that be???

[HazelNuts39]

One of several ways to do it is by a process called 'iteration':

Step 1: Assume a CG position
Step 2: Compute Gross Weight
Step 3: Compute the horizontal stabilizer position for the computed GW and assumed CG
Step 4: Compare the computed to the actual position of the horizontal stabilizer.
- If the difference is less than a certain tolerance go to step 5, else -
- adjust assumed cg position and repeat steps 2, 3 and 4.
Step 5: Output GW and CG

Another method is interpolation:
Step 1: Compute GW and THS position for the CG on the forward limit
Step 2: Compute GW and THS position for the CG on the aft limit
Step 3: Interpolate GW and CG for the actual THS position

[Microburst2002]

Thank you. Sounds complicated, though

[DutchOne]

Its like the chicken and the egg. You need an egg to get the chicken, but no egg without a chicken. Hehehe

[Microburst2002]

I can answer the eggs and chicken question. But not this FE part thing...

[aidey_f]

Don't forget you're not starting from a null value, during the pre-flight one enters ZWF/ZFCG, the FCMC uses the fuel load and the input value to spit out an initial value for the FG/FM to work with.
Once you've got a starting value then the boxes can apply their magic to get a value out of what the aircraft is doing aerodynamically, as described above.

[Microburst2002]

It's like inserting coordinates in an IRS, then... If the insertion is not good, all subsequent computations are in error.