The Nervous System, Ear, Hearing and Balance — Page 67, Lesson 101

I want to walk you through the otoliths and how they work, because this is where a very dangerous illusion for pilots begins. The word "otolith" literally means "stones in the ears." They are small grains of chalk embedded in a fleshy medium that contains hair cells. They are located at the base of the semicircular canals. Their job is to detect two things: tilting of the head and linear acceleration. The otoliths are contained within chambers called the utricles and the saccules. Here is how they work. When you experience an acceleration greater than 0.1 metres per second squared, that force causes the hair cells inside the otoliths to bend. Your brain interprets that bending to determine the new position of your head. That is the normal, useful function. But here is the problem for a pilot. When you experience linear acceleration — for example, during the take-off roll — the otoliths are moved backwards by the inertia. That bending of the hair cells sends a signal to your brain that your head is tilting backwards. So your brain interprets that as "I am climbing." The reverse happens during deceleration: the otoliths move forward, and your brain gets the false impression that you are pitching down, as if you are descending. This false sensation is called the somatogravic effect, or the somatogravic illusion. This illusion can be reinforced by information from the nerve cells in your muscles, from pressures due to gravity. Those two separate effects combine to create an almost overpowering illusion of climb or descent. The book states that this has led to catastrophic results. There is an important note here about the aircraft instruments. If the aircraft is fitted with an air-driven artificial horizon, this false feeling of pitch-up can be reinforced by the instrument itself. When the aircraft accelerates linearly, the artificial horizon has an acceleration error that makes it show a climbing turn to the right. So the instrument agrees with your false sensation, and you are even more convinced you are climbing. During deceleration, the reverse applies: the instrument reinforces the feeling of an apparent descent. The semicircular canals and the otoliths together make up the vestibular apparatus. This apparatus helps you maintain spatial orientation and controls other functions, such as eye movement to keep a stable picture of the world on your retina when you move your head. Now, let me give you the sobering statistics. Spatial disorientation has been a contributory factor in 37% of accidents in general aviation and 12% in commercial transport operations. It is described as the most dangerous of conditions, and over 80% of accidents resulting directly from disorientation are fatal. That is why understanding the somatogravic illusion is not just academic — it is survival information.