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11.7.8 Vision, Disorientation, Illusion

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Maintaining Orientation

Are you sitting up or lying down? Leaning one way or another? Three sensory systems give us the information we use to maintain our equilibrium and determine where we are and how were oriented:

  • Visual system: Our eyes, which sense position based on what we see.
  • Vestibular system: Organs found in the inner ear that sense position by the way were balanced.
  • Somatosensory system: Nerves in the skin, muscles, and joints, which, along with hearing, sense position based on gravity, feeling, and sound.

Knowing how each of these systems operates helps explain how spatial disorientation develops, and how to minimize your chances of experiencing it.

Visual System

Ninety per cent of the information we use for point of reference comes from our eyes. The most reliable of our senses, vision overrides conflicting sensations from our other systems. When we fly in visual meteorological conditions (VMC), our vision enables us to keep the airplane properly oriented to Earth by reference to the ground, sky, and horizon. Such is its power that we're rarely aware when our brain receives conflicting signals from other systems. Vision is relatively reliable, but it's prone to illusions, mistakes in processing or interpreting what we see, that can result in spatial disorientation.

Vestibular System

The vestibular system, also called the kinaesthetic senses, is our secondary positioning system, consisting of motion- and gravity-sensing organs. The system is redundant; there's one in each inner ear, each capable of providing the brain with all the information needed to maintain balance. They can, however, be compromised by several factors: when sick, inebriated, hung over, dizzy, or nauseous, our internal gyros don't function properly. Also, this system can only supplement, not replace, vision for maintaining orientation while airborne. Each vestibular apparatus has two structures: semicircular canals and otolith organs.

Inner ear with semicircular canals shown, likening them to the roll, pitch and yaw axis of an aircraft.

Semicircular canals

The semicircular canals each have three perpendicular tubes containing fluid and sensory hairs. As the body moves, the motion of the fluid in the canals provides the brain with roll, pitch, and yaw information. This system can even substitute for sight while on the ground; if you close your eyes, you can still walk, or sense whether you're upright or lying down.

However, there are some limitations, such as when a turn commences in the air, the inertia of the fluid moves in the opposite direction relative to the sensory hairs, and we correctly interpret the turn and its direction. But if the turn continues, the fluid catches up, creating the sensation that the turn has ceased. Therefore, a prolonged constant rate turn results in the false sensation of not turning at all.

When the turn finally does stop, due to inertia the fluid continues moving, creating the sensation of a turn in the opposite direction. Additionally, any bank rate of less than two degrees per second is insufficient to stimulate the fluid in the canals, and will not be felt.

Considering that a standard rate turn is three degrees per second, you can understand how, without visual reference, it's possible to enter a bank that becomes progressively steeper while feeling that the aircraft is flying straight and level.

Otolith organs

The otolith organs are small sacs at the base of the semicircular canals. They are embedded with sensory hairs and contain a gelatinous membrane with chalklike crystals - called otolith. As the head or body moves, the movement of the membrane against the sensory hairs registers gravity.

The forces of acceleration and deceleration also stimulate the otolith and, without visual reference, the body can't tell the difference between the inertial forces resulting from acceleration and the force of gravity. Thus, acceleration may give the sensation of tilting backwards. Deceleration may give the perception of pitching forward.

Somatosensory System

Also called the proprioceptive system; this system is comprised of nerves in the skin, muscles, joints, and internal organs, along with hearing. The nerves sense pressure differentials. This system remains relatively unnoticed on the ground. But while flying, pilots can feel changes in G-forces and pressure as the inertia of their bodies reacts to the motion of the airplane. These sensations are most acutely felt where the body and the airplane meet, namely on the seat, and the ability to correctly interpret these sensations is the source of the term "seat-of-the-pants" flying.

Our binaural hearing can determine our position relative to a sound source. In the air our hearing can also identify conditions such as an over-speeding propeller, air rushing against the airframe, or an engine suddenly going quiet.