But it would still require action from a pilot, who is known to be disorientated, and probably quite upset about their imminent collision with the ocean.
Perhaps an alternative would be to use a belt that vibrates differently depending on the angle of the aircraft. Humans are good at integrating data without conscious thought. This has been used with compass direction on the ground to dramatically improve spatial awareness without any conscious effort http://www.wired.com/wired/archive/15.04/esp.html.
Even little planes have those. What they need is a switch to tell the computer that they expect no descent whatsoever below xy000 feet, to stop it before it starts.
I was going to suggest this in my post. A "dead-man"'s limit. A plane would lock you out and right itself it detected you dropped below some pre-defined altitude (e.g. within an acceptable angle without landing gear deployed or a greater than acceptable angle with or without landing gear deployed). Whatever altitude is necessary for the plane to right itself from the steepest descent. Something like that would make it impossible for someone to storm a cockpit and nose-dive a plane. It would need appropriate remote electronic and mechanical overrides, though to prevent possibly losing a plane if it was triggered incorrectly. But at least a plane flying level on lockout autopilot would buy some time opposed to it definitely being destroyed by the nose-dive.
