Hey,
I flew my elevon aeroplane yesterday in manual perfectly fine, but when I set it to autotune, it nosedived forward out of the sky, straight into the ground. Why would this be? Any help is greatly appreciated! 
Parameters and flight log in the link below:
Its almost always because you have the flight control servo reversing wrong. Then you also have the transmitter channel or rc input wrong to make up for the flight controller reversing problem so it all works ok in manual but as soon as you ask the flight controller to fly it by itself it crashes because when it tries to make ot go up it goes down so it tries even further up which makes it go further down
This is the solution…
Before you ever try to fly it in any mode, hold the craft in your hand, choose fbwa and make sure control surfaces respond correct direction for stick input then ALSO make sure control surfaces respond correct direction to oppose any movement of the airframe by your hand ie try to rolling the airframe left and make sure the left elevon or aileron goes down then check pitch and yaw also.
Thanks Scott, appreciate your help.