In a two-seater plane (derived from Plane_Base_F) with a copilot turret, the copilot can always freelook, even if they took over the controls of the plane. This makes the copilot unusable on planes.

Note that the copilot CAN control the plane (joystick works fine), and if you do this on external view, it works as it should (taking controls locks the view behind the plane, just like the pilot). So the only real problem is the inside view.