When engine is on it would seem like that just having a character or a tall enough object below the main rotor is enough to break it, even though no visual collision is happening.

Edit: Here's two screenshots off of a situation where the mrot breaks immediately when engine is turned on: http://i.imgur.com/Rr8kXwI.jpg http://i.imgur.com/NjWDtyW.jpg

Please notice how the blade looks to be positioned in the world in different views, I'm not sure if that has something to do with the collision problem or if it's a independent case of its own or if that's intended to look like that (for optimization reasons).

As I'm new to this medium, I accidentally attached one of the pics to this issue as I thought I only we're attaching them to this note. Sorry for any inconvenience!