The import of the IGES file in FEMAP results in a THICK solid part, you should understand that this geometry is not adequate to perform a midsurface extraction using command GEOMETRY > MIDSURFACE > AUTOMATIC, this command is useful only for generating surfaces from thin-walled solid geometry. The midsurfaces can then be used as the basis of plate meshes.
In any case, if the exterior surface of your 3-D solid wing is really the skin of the real wing, then you can always create a surface geometry using command GEOMETRY > MIDSURFACE > OFFSET TANGENT SURFACES, applying an offset value of half of the thickness of skin.
I was thinking about this problem and came up with the following, a way to create a mid-surface shell mesh representing a solid wing -
I ran a free-free modal run of this shell mesh -
And compared with a solid tetra mesh of the orignal geometry -
First mode bending is pretty close, so it appears the stiffness is satisfactory.