While the most common issue with normals is having them backwards, you can also have them pointing in any other direction. Not sure about the "faces being visible only from the inside" part... I think Orbiter figures that out with the order of the vertices in each triangle, and the normals are only use for lighting. The D3D9 folks should know this.Nah. As mentioned, that mesh has no backfaces. If the normals were facing the wrong way, you'd only see them from the inside. Also, I did try shipedit just to be on the save side, didn't change anything.
EDIT: yes, you need to save in Shipedit.
1) File >> Load mesh...
2) Transform >> Generate Normals ... >> Apply and Close
3) File >> Save mesh ...
Shipedit can also do a sanity check with Calc >> Check mesh.