How do you call these functions?
GI = GUI_INP(hObject, eventdata);
[sys,Inp,geom_aircraft] = Z_INP(GI);
This should work. At least partially, because sys and geom_aircraft are still undefined.
The code will fail, when the wanted figure is not found. Better use error() with a clear message, instead of just a disp(), because the following code will fail with confusing messages.
If you have a lot of open figures containing many objects, this
hfig_i_p = findobj('Tag','Initial_parameters');
will take some time, because it compares the Tags of all GUI-elements. Specify the object type instead:
hfig_i_p = findobj(allchild(groot), 'flat', 'Tag', 'Initial_parameters', 'Type', 'Figure');
Currently this is redundant, because the children of groot are figures only. But maybe this will change in the future.