Collision Models - Non flyable aircraft dropping through static model?

[Leg2ion]

Hi All - reaching out for some help/advice if possible?

I have recently created an airfield (using Blender) , and set up collision shells and lines. When I export it to EDM - then add it into a mission and position any other models on it - they show as sunken in Mission Editor.

When I go to 'fly' - my aircraft is sat on top. If I position a static helo next to mine it is sunken. If I position a dynamic helo (set to start on ground) - it sits on the tarmac up until the point it starts up - at which point it drops into the tarmac.

I have found a few threads asking the same but no resolution that I could see?