Put a constant force upwards on the Tail-wings.
And the same force in the opposing direction.

When turning off the opposing force, the airplane should tumble,
and lean downwards.

(or simply only turn on the upward force when the airplane is
damaged..)

You could put som additional random forces on the wings
and body
turning on an off randomly during falling, wich makes the airplane
tumble even more.

But as Henning said, you cant simulate aerodynamic behavior and turbulances by
using simple ridgid-physics.

look at Mr. Galileo and what he said about a falling feather
in vacuum.