The Breakdown:
Something disturbs the plane in straight and level flight at cruising speed. The right wing drops slightly. The plane enters a gentle sideslip to the right. In a slip a crossflow of air initiates a dihedral response to pick up the right wing.
The vertical tail fin responds, simultaneously, by yawing the plane slightly to the right. If the plane is spirally unstable the rudder (tail fin) forces the plane around before the dihedral had time to pick the right wing up.
The rudder produces a yaw which introduces an over-banking tendency while the plane turns. In a turn to the right the left wing is moving, at the moment, faster through the air and generates more lift. At the same time the right wing is slowed a bit and generates less lift. The net result is the left wing goes up slightly and the right wing drops. The over-banking tendency cancels out the attempt of the dihedral to the right wing to return the plane to level flight. Net result the right wing stays down.
The slight sideslip to the right continues and the dihedral tries again to right the right wing to stop the sideslip. The vertical tail again over-rides the dihedral and the plane enters a new over-banking effect. This process will repeat itself unless stopped.
If this isn't stopped the plane enters a steeper bank and a tighter turn. This continues if you don't interfere.
The g-load increases due to the centrifugal force build-up. The increase in load forces the plane to drop its nose and pick up speed. The plane has a built-in tendency to keep itself at a constant Angle of Attack. The extra load, created by the centrifugal force build-up, creates a situation where the plane can only maintain a constant Angle of Attack by picking up additional speed. The dive combines with the spiral turn to create a spiral dive.
When an increase in the bank reaches a certain point another effect forces the nose of the plane downward toward the earth. In this deep bank the earth is on the right side and the sky is on the left side. The rudder (vertical fin) continues to push the plane around to the right. It now is pushing the plane into a dive toward the earth.
In summary, this is what a plane "wants to do" once it is in a turn. The plane wants to do this even when the pilot is on the controls.
This, in previous posts, is the anatomy of the "death spiral" that causes many lost lives where instrument flight skills are lacking and/or the pilot tries to fly an airplane so it makes him "feel good or more 'natural'."
Flight Controllers have testified about their inability to talk an untrained pilot out of a "death spiral." They try to put the aircraft into a flight attitude where their sensations are "more natural." In effect, they don't believe what the plane's instruments are telling the pilot about the actual attitude of the plane (a deep spiral dive).
This is a safety lesson on proper instrument pilot training. Knowing the normal tendency of a plane, in a turn, can set you up for a vestibular illusion that may end your and your passengers lives in a needless spiral dive accident.
