Vestibular System

Semicircular Canals:

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• The ear consists of the inner, middle, and outer ear [Figure 1]
• The three canals in the inner ear, arranged at approximately right angles to each other, detect the roll, pitch, and yaw axis
• 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
• It is the movement of these hairs within the fluid that causes sensations of motion [Figure 2]
• Because of the friction between the fluid and the canal, it may take about 15-20 seconds for the fluid in the ear canal to reach the same speed as the canal's motion
• A mismatch between what the fluid/hairs are telling your body and what is happening can lead to disorientation
  • To illustrate what happens during a turn, visualize the aircraft in straight and level flight
  • With no acceleration of the aircraft, the hair cells are upright, and the body senses that no turn has occurred
  • Placing the aircraft into a turn puts the semicircular canal and its fluid into motion, with the fluid within the semicircular canal lagging behind the accelerated canal walls [Figure 3]
  • This lag creates a relative movement of the fluid within the canal
  • The canal wall and the cupula move in the opposite direction from the motion of the fluid
  • The brain interprets the movement of the hairs to be a turn in the same direction as the canal wall
  • The body correctly senses the turn
  • If the turn continues at a constant rate for several seconds or longer, the motion of the fluid in 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 will result in the false sensation of no turn, which can result in illusions such as the graveyard spiral
  • When the aircraft returns to straight-and-level flight, the fluid in the canal moves briefly in the opposite direction
  • The brain receives a false signal, 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; a condition called the Coriolis illusion

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Otolith Organs:

• 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 their 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
• As a result, during takeoff and while accelerating, the pilot may sense a steeper than normal climb resulting in a tendency to nose-down, also called the somatogravic illusion

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Ear Block:

• Ear Block Causes:

  • As the aircraft cabin pressure decreases during ascent, the expanding air in the middle ear pushes the Eustachian tube open, and by escaping down it to the nasal passages, equalizes in pressure with the cabin pressure
  • During a climb, middle ear air pressure may exceed the pressure of the air in the external ear canal, causing the eardrum to bulge outward and the Eustachian tube to bulge outward
  • Either an upper respiratory infection, such as a cold or sore throat, or a nasal allergic condition can produce enough congestion around the Eustachian tube to make equalization difficult
    • Consequently, the difference in pressure between the middle ear and aircraft cabin can build up to a level that will hold the Eustachian tube closed, making equalization difficult if not impossible
    • The problem is commonly referred to as an "ear block"

• Ear Block Effects:

  • An ear block produces severe ear pain, and loss of hearing that can last from several hours to several days
    • This ear pain may have significant impacts on a pilot's ability to communicate
  • Rupture of the eardrum can occur in flight or after landing
  • Fluid can accumulate in the middle ear and become infected

• Ear Block Prevention & Recovery:

  • An ear block is prevented by not flying with an upper respiratory infection or nasal allergic condition
    • Adequate protection is usually not provided by decongestant sprays or drops to reduce congestion around the eustachian tubes
    • Oral decongestants have side effects that can significantly impair pilot performance
  • During climbs and descents, a pilot must periodically open the Eustachian tube by swallowing, yawning, tensing muscles in the throat or the Valsalva maneuver
    • Descents can be more challenging to relieve since the partial vacuum tends to constrict the walls of the Eustachian tube

Vestibular System Knowledge Quiz:

Conclusion:

• The feelings caused by changing altitudes (really pressures) differ from person to person, and often regular fliers will become used to it
  • More than the actual change of pressure is the rate at which it changes, resulting in ears popping when climbing/descending at about 750-1000 feet per minute
• Consult a physician if any problems occur and as always, remember the IMSAFE checklist for medications to see that you're fit to fly
• Pilot's experiencing an ear block are vulnerable to mixed signals from the inner ear, leading to both losses in situational awareness and illusions in flight
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References:

• Federal Aviation Administration - Pilot/Controller Glossary
• Aeronautical Information Manual (8-1-2b) Effects of Altitude
• AmericanHearing.org
• Instrument Flying Handbook (1-4) Ear
• Pilot Handbook of Aeronautical Knowledge