Endolymph  Tube  Cupola  No turning  No sensation.  Start of turn  Sensation of turning  as moving fluid deflects  hairs.  Constant rate turn  No sensation after fluid accelerates to same speed as tube wall.  Turn stopped  Sensation of turning in  opposite direction as moving  fluid deflects hairs in opposite  direction.  Figure 3-5.  Angular acceleration. 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 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. 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.  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 Illusions Leading to Spatial  Disorientation Nerves Normal  Accelerating  Head tilted back  Figure 3-6.  Linear acceleration. 3-5