hearing & balance chapter 4. aim – chapter 4 to understand the physiology of the outer, middle...

Hearing & BalanceChapter 4

Aim – Chapter 4To understand the physiology of the outer, middle and inner ear and how it gives rise to hearing and balance.

To appreciate the limitations and disorientations with respect to aviation

Objectives – Chapter 41. To learn the basic function of the ear and how it

gives rise to hearing and balance

2. To recognise the various illusions and disorienting factors as a result of the ear physiology and how to manage them

1. The Ear & Vestibular System

Anatomy of the Ear• The Outer Ear

• Pinna• Auditory Canal• Ear Drum (Tympanic

Membrane)

• The Middle Ear• Ossicles (Hammer, Anvil,

Stirrup)• Eustachian Tube

• The Inner Ear• Round/Oval Window• Cochlea• Semi-Circular Canals


The Outer Ear• Pinna/Auditory Canal

• Shaped cartilage gathers sound waves to be passed through the Auditory Canal toward the ear drum

• Ear Drum• Sound waves vibrate the

tympanic membrane, setting the ossicles in motion, which continues to the

• Humans detect sound waves between 20-20KHz


The Middle Ear• Ossicles

• Three auditory bones (Hammer, Anvil, Stirrup) receive sound waves directly from the tympanic membrane.

• Through muscular action, they can amplify and attenuate wave intensity to enhance and protect hearing respectively

• Vibrations are converted into mechanical energy, which is passed on to the cochlea for processing.


The Middle Ear (cont.)• Eustachian Tube

• Serves to maintain the middle ear at ambient atmospheric pressure via access to the nasal passage

• Ear drum is kept air tight

• Swelling, mucous or inflammation reduces this function, causing discomfort in changing atmospheric pressures (especially descent)


The Inner Ear• Cochlea

• Snail shaped cavity that receives vibrations from the middle ear via the oval window.

• Hair-like cells protrude into the cochlea fluid path and are positioned at various locations so as to resonate under various frequencies.

• As the hair cells resonate, a nerve impulse is generated to be interpreted by the brain


Sound Tolerance (Safe Work Aus)Table 1: Equivalent Noise Exposures

LAeq,8h = 85 dB(A)

Noise Level dB(A) Typical source Exposure Time

80 Kerbside Heavy traffic 16 hours1

82 12hours1

85 Front-end loader 8 hours

88 4 hours

91 Lawn-mower 2 hours

94 1 hour

97 30 minutes

100 15 minutes

102 Aero Commander Take-Off 7.5 minutes

106 3.8 minutes

109 Chain saw 1.9 minutes

112 57 seconds

115 28.8 seconds

118 14.4 seconds

121 Rock Drill 7.2 seconds

124 3.6 seconds

127 Rivet Hammer 1.8 seconds

130 Jet Engine at 50m 0.9 seconds

A table demonstrating the length of time one can be exposed to various dB without hearing protectors before potential damage. Note: 80dB is equivalent to being kerbside in heavy traffic. 130dB is being exposed to a jet engine at a proximity of 30m

• Decibels are not like normal numbers - they can’t be added or subtracted in the normal way.

• According to the decibel scale, an increase of 3 dB represents twice as much sound energy. Hence, the length of time one could be exposed to the noise is reduced by half for every 3 dB increase


Balance• Each inner ear contains a vestibular

system, consisting of two distinct structures: • the semicircular canals –

Posterior, Superior and Inferior – and;

• the otolithic organs, which are small sacs located in the vestibule

• Both the semicircular canals and otolithic organs are filled with a fluid known Endolymph, the movement of which stimulates nerve endings and creates a message for the brain to inform of a change in aircraft attitude.


Linear Acceleration/Gravity• Sensory hair-like fibres project into the

otolithic membrane to respond to gravity and linear acceleration/deceleration via the movement of Endolymph fluid. Note: There is no differentiation!

• When the head is upright, the hair cells remain at a "resting" frequency of nerve impulses.

• Forward acceleration also results in backward displacement of the otolithic membrane.

• When an adequate visual reference is not available, pilots can experience a false sensation of backward tilt


Angular Movement• Each of the semicircular canals are

positioned in one of the three dimensions (X,Y,Z) to sense changes in angular acceleration of roll, pitch, and yaw. They are not designed to detect linear changes in motion.

• At the base of each canal lives a chamber known as the cupula. The cupula has a group of fine hairs that extend into the fluid

• As these hairs move, their Vestibular nerve endings produce a neurochemical transmission that the brain interprets as rotation in that dimension


Angular Movement - example

1. In Straight & Level flight, no angular acceleration is occurring and so the hairs remain stationary. The body (correctly) senses no motion

2. When the semicircular canal is moved during clockwise acceleration [right turn], fluid within it lags behind the canal walls, creating a relative anti-clockwise movement. The body (correctly) senses a clockwise turn is being made.


Angular Movement – example (cont.)

3. If a clockwise turn continues at a constant rate, the fluid eventually catches up. At this point the hairs are no longer bent, and the body senses (incorrectly) that turning has stopped

4. When the turn rate is slowed, or stopped, the fluid moves briefly in the opposite direction. This movement leads the body to (incorrectly) sense a turn in the opposite direction. In an attempt to correct the perceived turn, the pilot may re-commence a turn in the original direction.

2. Disorientation & Illusions

Air Sickness• Occurs when the central nervous system (CNS) receives conflicting

messages from the body’s visual sense and other supplementary senses (i.e. vestibular and pro-prioceptive) or when the vestibular system is over-stimulated

• Hints to avoid air sickness:• Avoid areas of known turbulence • Eat lightly & avoid fatty foods• Fly as smoothly as possible and

reduce IAS to/below Va in turbulence

• Focus on the horizon; avoid head movements inside the cockpit

• Maintain supply of fresh air; turn off heating devices

• Keep flying if you start to feel ill


The Leans• Occurs when a pilot’s body fails to perceive angular motion

• During continuous straight & level flight, the pilot’s body senses the attitude in its correct sense. However during a constant AoB, the body may in fact sense the aircraft is still straight & level.

• Once the body detects a slow roll back toward S&L, he or she will make a quick recovery and resume what he/she believes is straight-and-level flight.

• Trust your instruments!


The Graveyard Spiral• A prolonged coordinated, constant-rate turn, can create the illusion of not

turning. During recovery to level flight, the pilot will experience the sensation of turning in the opposite direction

• If disoriented, the pilot may seek to regain the turn to restore the fluid movement in the inner ear.

• As the aeroplane turns, it descends

• In the absence of any turning sensation, the pilot may experience a level descent and raise the nose to level off - further tightening the spiral and rate of descent


Somatogravic Illusion• Without a visual reference, the body cannot differentiate between the

forces of gravity and changes in linear motion.

• Two illusions are possible:

1) Nose-Up Pitch Illusion• During linear acceleration, the

Endolymph fluid in the otolithic organ lags behind, influencing the hairs in the same direction.

• Nerve cells send a message to the brain that the head has been tilted backward.

• The pilot senses a nose-up moment and may be inclined to (dangerously) lower the nose


Somatogravic Illusion (cont.)

1) Nose-Down Pitch Illusion• Conversely, during linear

deceleration, the Endolymph fluid and hairs advance forward

• The nerve cells send a message to the brain that the head has been tilted forward.

• The pilot senses a descent and may be inclined to pick the nose up

Head Down

Deceleration


Coriolis Illusion• Can be the most dangerous and disorienting of the Vestibular illusions

• During prolonged turns, fluid in the semicircular canals has equalised, hence no further sense of movement.

• The pilot may now move his/her head in the cockpit to complete a task (e.g. radio, transponder) thereby disrupting the movement of fluid in all three canals.

• The combined effect from all three canals creates a disorienting sense of rotation in the three planes - yaw, pitch, and roll.

• The pilot experiences an overwhelming head-over-heels tumbling sensation.


Load Factor ‘g’• “Apparent gravity” - Mathematically, the ratio of lift to weight (LF=L/W).

Expressed as multiples of gravity (9.8m/s 2)

• During positive g, the body is forced into the seat. Oxygen supply can be temporarily lost to vitals such as the brain and eyes, causing grey-out, tunnel vision and/or black-out.

• Blood also becomes heavier and harder to pump, causing low blood pressure. Physiological effects can result at higher load factors (usually above +3.5g). Unconsciousness can result from prolonged +g

High g’s can result from steep turn, where the ‘resultant load’ is a product of weight and the centrifugal force.


Load Factor ‘g’ (cont.)• During negative g (‘-g’), the body is forced out of the seat.

• A phenomenon known as ‘Red-Out’ can occur • Caused by excessive negative g’s, the lower-eyelid can travel upside-

down to obscure vision. There is no muscle to prevent this from occurring.

• G-Induced Loss Of Consciousness (g-loc) can result if +g-loading is ignored, or where no warning exists.

• This table illustrates the typical consequences of prolonged g-loading on the body.

Questions?

hearing & balance chapter 4. aim – chapter 4 to understand the physiology of the outer, middle...

Documents

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