The Nervous System, Ear, Hearing and Balance — Page 72, Lesson 104

I want to walk you through two of the most important illusions that affect a pilot’s sense of motion in flight: the somatogyral illusion and the somatogravic illusion. These come from the vestibular system—the part of your inner ear that gives you your sense of balance and movement. Let’s start with the somatogyral illusion. This illusion happens because the semicircular canals in your inner ear cannot accurately register a prolonged rotation—what we call a sustained angular velocity. Here’s how it works. Inside each semicircular canal, there are tiny hairs surrounded by a fluid called endolymph. When you start turning, the fluid moves and bends those hairs, and that bending tells your brain you are rotating. But if the turn continues at a steady rate for a long time, the fluid settles down and the hairs gradually straighten back to their neutral position. Your brain then loses the sensation of turning, even though you are still rotating. Now, the critical moment comes when you either reduce the turn rate or level the aircraft. At that point, the fluid moves again, bending the hairs in the opposite direction. Your brain interprets that as a sensation of turning in the opposite direction—even though you are actually slowing down or stopping. So the formal definition is this: a somatogyral illusion is the sensation of turning in the opposite direction that occurs whenever the body undergoes angular deceleration from a condition of sustained angular velocity. There is a well-known dangerous outcome of this illusion called “The Graveyard Spin.” Imagine you are in a prolonged spin. The endolymph in your semicircular canals has settled, and the sensing hairs have straightened, so you no longer feel the spin. When you initiate a recovery—by applying opposite rudder and moving the controls to stop the rotation—the hairs are displaced again. This gives you a strong, false sensation that you are now entering a spin in the opposite direction. If you succumb to that false sensation and react to it, you will re-enter the spin in the original direction. That can be disastrous. Now let’s move to the somatogravic illusion. This one involves the otoliths, which are the structures in your inner ear that sense linear acceleration—forward, backward, up, and down motion, not rotation. The somatogravic illusion is the illusion of pitching up or down as a result of the movement of the otoliths due to linear acceleration. On take-off, this illusion is exacerbated—made worse—by the resultant vector of the g-forces acting on you and the aircraft. In other words, the combination of forward acceleration and gravity can trick your otoliths into making you feel like the nose is pitching up more than it really is, or pitching down, depending on the situation. There is another condition associated with the vestibular apparatus that I want to mention: vertigo. Vertigo is a loss of spatial awareness in which you experience a rotating, tumbling, or turning sensation. It can be caused by disease in the inner ear, but it can also occur in a healthy individual under certain circumstances. Those circumstances include blocked eustachian tubes, sudden pressure changes in the inner ear from sneezing or strong blowing of the nose, accelerations or high g-loadings, and drug-induced causes. Alcohol intoxication is specifically noted as provoking vertigo. So to summarise: the somatogyral illusion is about false sensations of rotation after prolonged turns, leading to the graveyard spin. The somatogravic illusion is about false sensations of pitch from linear acceleration, especially on take-off. And vertigo is a broader loss of spatial awareness with a rotating sensation, triggered by various factors including inner ear problems and acceleration.