I believe aircraft have the center of the model position in their center of gravity, while features have the center of the model at height 0. (z=0). Features could then be represented as lying on a z value of 0 for their center.
That would explain why using a vehicle as feature makes them partially buried.
One solution would be to export the lod, edit the lod file to move the model upwards until the center (the red circle) is the lowest part of the aircraft, then save the file, then import to DB as a new feature (dont overwrite the original parent record) . All this with LODEditor…. but Is a guess, I don’t know for sure.
Try at your own risk