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Vestibular System

Introduction:

  • The ear and its functions are a crucial part of the pilot's situational awareness and therefore decision making process [Figure 1]
  • The two parts of concern are the semicircular canals and the otolith organs
    • The semicircular canals detect angular acceleration of the body
    • The otolith organs detect linear acceleration and gravity
Ear Diagram
Figure 1: AmericanHearing.org

Semicircular Canals:

  • The 3 canals 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 which 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
  • Mismatch between what the fluid/hairs are telling your body and what is actually 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 that a turn is being made
    • 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
    • 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, a condition called the Coriolis illusion

Instrument Flying Handbook. Figure 1-6, Angular Acceleration
Figure 2: Instrument Flying Handbook, Angular Acceleration

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 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
  • 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

Instrument Flying Handbook. Figure 1-6, Linear Acceleration
Figure 3: Instrument Flying Handbook, Linear Acceleration

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
    • Therefore 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 difficult to relieve due to the fact that the partial vacuum tends to constrict the walls of the Eustachian tube
      • To remedy this often painful condition, which also causes a temporary reduction in hearing sensitivity, pinch the nostrils shut, close the mouth and lips, and blow slowly and gently in the mouth and nose
    • During descent, the pilot must periodically open the eustachian tube to equalize pressure
      • This can be accomplished by swallowing, yawning, tensing muscles in the throat, or if these do not work, by a combination of closing the mouth, pinching the nose closed, and attempting to blow through the nostrils (Valsalva maneuver)
    • 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"
  • Effects:

    • An ear block produces severe ear pain and loss of hearing that can last from several hours to several days
    • Rupture of the ear drum can occur in flight or after landing. Fluid can accumulate in the middle ear and become infected
  • Prevention:

    • 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

Conclusion:

  • A physician should be consulted if any problems occur and as always remember the IMSAFE checklist for medications to see that you're fit to fly

References: