The graveyard spiral is an illusion that graphically illustrates the hazards of trusting your everyday sense of up and down while flying an airplane. Imagine yourself flying in thick clouds in a large jet. You are at high altitude preparing to let down to your final destination, a very busy time during any flight. You become distracted, and before you know it you realize that you have entered a descending turn. Relying only on your "sense of up," you apply controls to counteract the turn. The big plane doesn't ever respond very quickly to the controls, so it takes quite some time before you start to roll out of the turn. It is during this delay that the seeds of the graveyard spiral are planted.
Your inner ear is very good at
detecting reasonably rapid changes in your body's position, but it can
be fooled when it experiences sensations not normally felt on the ground.
During the delay between your realization that you were in a turn and the
time the aircraft started to roll out, your inner ear became comfortable
with the turn. Since you were turning you were undergoing circular motion,
and you experienced an acceleration of a little more than one G acting
directly through the seat of your pants. Your inner ear, which initially
detected the roll part of the turn becomes fooled when the roll
stops and the turn stabilizes. During that delay it decides that the acceleration
it feels through the seat of your pants must be down, even though
you are still turning. You now feel as if you are flying in a straight
line with your wings level.
Once your control input finally
begins to make the jet come out of the turn, your inner ear correctly detects
a roll in the opposite direction to the original turn. But remember,
you already think you are flying level. You interpret this roll as your
plane going into a turn opposite to the direction of the original turn.
You now counteract the roll to
get back to what you think is level, unwittingly allowing your inner ear
to cause you to roll back to your original angle of bank. Trusting your
ears, you cheerfully fly this descending turn for the rest of your life.
