Eyes
During flight in visual meteorological conditions (VMC), the eyes are the major orientation source and usually provide accurate and reliable information. Visual cues usually prevail over false sensations from other sensory systems. When these visual cues are taken away, as they are in IMC, false sensations can cause the pilot to quickly become disoriented.
The only effective way to counter these false sensations is to recognize the problem, disregard the false sensations, and while relying totally on the flight instruments, use the eyes to determine the aircraft attitude. The pilot must have an understanding of the problem and the self-confidence to control the aircraft using only instrument indications.
Ears
The inner ear has two major parts concerned with orientation, the semicircular canals and the otolith organs. The semicircular canals detect angular acceleration of the body while the otolith organs detect linear acceleration and gravity. The semicircular canals consist of three tubes at right angles to each other, each located on one of the three axes: pitch, roll, or yaw. Each canal is filled with a fluid called endolymph fluid. In the center of the canal is the cupola, a gelatinous structure that rests upon sensory hairs located at the end of the vestibular nerves.
When the ear canal is moved in its plane, the relative motion of the fluid moves the cupola, which, in turn, stimulates the sensory hairs to provide the sensation of turning. This effect can be demonstrated by taking a glass filled with water and turning it slowly. The wall of the glass is moving, yet the water is not. If these sensory hairs were attached to the glass, they would be moving in relation to the water, which is still standing still.
The ear was designed to detect turns of a rather short duration. After a short period of time (approximately 20 seconds), the fluid accelerates due to friction between the fluid and the canal wall. Eventually, the fluid will move at the same speed as the ear canal. Since both are moving at the same speed, the sensory hairs detect no relative movement and the sensation of turning ceases. This can also be illustrated with the glass of water. Initially, the glass moved and the water did not. Yet, continually turning the glass would result in the water accelerating and matching the speed of the wall of the glass.
The pilot is now in a turn without any sensation of turning. When the pilot stops turning, the ear canal stops moving but the fluid does not. The motion of the fluid moves the cupola and therefore, the sensory hairs in the opposite direction. This creates the sensation of turning in the opposite direction even though the turn has stopped.
The otolith organs detect linear acceleration and gravity in a similar way. Instead of being filled with a fluid, a gelatinous membrane containing chalk-like crystals covers the sensory hairs. When the pilot tilts his/her head, the weight of these crystals causes this membrane to shift due to gravity and the sensory hairs detect this shift. The brain orients this new position to what it perceives as vertical. Acceleration and deceleration also cause the membrane to shift in a similar manner. Forward acceleration gives the illusion of the head tilting backward.
Nerves
Nerves in the body's skin, muscles, and joints constantly send signals to the brain, which signals the body's relation to gravity. These signals tell the pilot his/her current position. Acceleration will be felt as the pilot is pushed back into the seat. Forces created in turns can lead to false sensations of the true direction of gravity, and may give the pilot a false sense of which way is up.
Uncoordinated turns, especially climbing turns, can cause misleading signals to be sent to the brain. Skids and slips give the sensation of banking or tilting. Turbulence can create motions that confuse the brain as well. Pilots need to be aware that fatigue or illness can exacerbate these sensations and ultimately lead to subtle incapacitation.
Vestibular: The central cavity of the bony labyrinth of the ear, or the parts of the membranous labyrinth that it contains.
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