Statistically, over 80% of accidents that result directly from… — Page 67, Lesson 103

I want to walk you through one of the most dangerous conditions a pilot can face — disorientation. Statistically, over 80% of accidents that result directly from disorientation are fatal. That makes it the most dangerous of all orientation-related conditions. The most well-known example among pilots is called "The Leans." Let me define that precisely. The leans, also known as the somatogyral illusion, is a condition where your vestibular apparatus — that's your inner ear balance mechanism — is not sufficiently reliable to maintain an accurate model of your orientation. In plain terms, your inner ear tells you something that isn't true about your aircraft's attitude. This can happen in any phase of flight, and here's a critical point: the sensation can persist for up to an hour after the event that caused it. So even after you've corrected the aircraft, your brain may still feel like you're turning. I want to walk you through the two most common circumstances that cause the leans. First circumstance: you, the pilot, commence a very gentle slow turn. It's so gentle that the movement of the fluid inside your semicircular canals is not enough to cause a detectable bending of the hair cells — those are the sensory receptors that detect rotation. So, even though the aircraft is actually in a turn, your balance mechanism senses no change has been made — it thinks you're still straight and level. Then, when you make a normal return to straight and level flight, that roll-out movement is detected by the balance mechanism as a turn from straight and level. So now the aircraft is in reality straight and level, but you feel as though you are still turning. Your inner ear says "turning," your eyes say "level" — two conflicting signals. Second circumstance: you execute a prolonged turn — you hold the turn for so long that the hairs inside the semicircular canals have time to return to their upright, erect position while you are still in the turn. This gives you the erroneous feeling that you are straight and level, even though you're still turning. Then, as you roll out of the turn, the ends of those hairs move again, giving you a false impression of a turn — when in fact the aircraft is now level. Again, two conflicting signals. In both cases, your visual sense tells you that you are level, but your vestibular apparatus tells you that you are turning. So you have a sensory conflict. Now, the solution depends on your flight conditions. If you are in IMC — Instrument Meteorological Conditions, meaning you cannot see outside — the rule is simple and absolute: believe your instruments. Do not trust what your body tells you. If you are in VMC — Visual Meteorological Conditions, where you can see outside — then look out at the horizon. Your eyes, referencing the natural horizon, will override the false sensation from your inner ear. That figure shows the structure of the human ear, which is the organ responsible for these illusions. And there is a Figure 4.4 in the book that illustrates the prolonged-turn scenario I just described.