The tip of the fin and rudder. It is torn off. At first
wild speculations were offered in explanation: "the plane
was inverted ...", "a propeller or other object hit it ...",
but the most logical explanation is that the force of the
cartwheel upon impact simply exploded the tip. There is a
rudder counterweight in there which would amplify the
forces. No sign of dirt or other impact was found, so that
is offered as the most likely explanation.