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the canals catches up with the canal walls. The hairs are no longer bent, and the brain receives the false impression that turning has stopped. Thus, the position of the hair cells and the resulting sensation during a prolonged, constant turn in either direction results in the false sensation of no turn. 

When the aircraft returns to straight-and-level flight, the fluid in the canal moves briefly in the opposite direction. This sends a signal to the brain that is falsely interpreted as movement in the opposite direction. In an attempt to correct the falsely perceived turn, the pilot may reenter the turn placing the aircraft in an out-of-control situation. 

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 or 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. [Figure 3-6] As a result, during takeoff and while accelerating, the pilot may sense a steeper than normal climb resulting in a tendency to nose-down. 

## 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 or her current position. Acceleration is 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. 

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## Illusions Leading to Spatial Disorientation 

The sensory system responsible for most of the illusions leading to spatial disorientation is the vestibular system. Visual illusions can also cause spatial disorientation. 

## Vestibular Illusions 

## The Leans 

A condition called “the leans” can result when a banked attitude, to the left for example, may be entered too slowly to set in motion the fluid in the “roll” semicircular tubes. [Figure 3-5]  An abrupt correction of this attitude sets the fluid in motion, creating the illusion of a banked attitude to the right. The disoriented pilot may make the error of rolling the aircraft into the original left banked attitude, or if level flight is maintained, feel compelled to lean in the perceived vertical plane until this illusion subsides. 

