I found an weird bug when mirroring big wings (maybe small ones to...) it appears that the joint strength between where the wing parts connects can get messed up, sometimes it's only a little so the plane lists to one side but sometimes it's so severe that the parts start to separate in flight.

To fix this I found I had to manually reconnect every part of the wing... not great but it works... if anyone knows a better way please let me know...