A340 FE part computed GW and CG
 

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???

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

Thank you. Sounds complicated, though

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

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

Omlette, anyone?
 

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.

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