Here is a plane made perpendicular to the Z axis at the origin. Notice the gaps between the edge of the STL and the plane. The plane has to move in the -Z direction toward the opposite side of the wing.
Here is a plane at exactly z = -30 mm and you can see there are gaps between the plane and the edge of the STL file.
So you can see the STL file cannot have parallel planes 30 mm apart without extending the surfaces to those planes. I suggest each plane be moved inward by 0.5 mm and cut the surfaces at that plane.
I don't use ICEM to mesh the air domain, I use Ansys Meshing, there is also Fluent Meshing. ICEM is probably not a good choice with an organic shape like this.
Turning off the visibility of edges does not change the surfaces at all, they are still there.