as I'm an engineer too I've been on the same search like you for some years and found nothing appropriate.
Esspecially if you want to transfere 3D-models from a professional CAD-system to something linke DirectX-geo-files (the .x-files FMS uses) you have the big problem, that nearly any CAD-system uses NURBS/BREP representation for geometry. But that cannot be used for high performance display like a flightsim.
Here we use polygon-representation (a simple face-by-line-by-point representation)...
The main problem now is that there exist some good translators between NURBS/BREP and polygon but all of them just convert what is defined in the BREP-model - and that is far too much infomation.
What you in fact need is a combination of translator and poly-count-reducer to decrease the number of polygonal surfaces in the 3D-model.
Those poly-count-reducer exist - but for really high prices!
Often their are shipped with professional animation software (like Lightwave 3D, 3D Studio Max, Maya, etc.), because in that field they have the same problems.
In the past I have done some commercial 3D modellings and from that experience I can say that the best way to create a low-poly-count-3D-model from a CAD-model is to remodell it manually( !!! ).
Tools like Metasequoia (freeware) are very good for that. You can load the CAD-original as DXF into (3D-) background and start remodelling your low-poly-model right at the same position.
If you want to see whats possible with Metasequoia just have a look onto my FMS-page (see signature) e.g. the B-17.
Hope this answer brings some light into darkness!
