Hearing & Balance Chapter 4

Aim – Chapter 4 To 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 4 1.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

1. The Ear & Vestibular System 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 KHz

1. The Ear & Vestibular System 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.

1. The Ear & Vestibular System 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)

1. The Ear & Vestibular System 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

1. The Ear & Vestibular System Sound Tolerance (Safe Work Aus) Table 1: Equivalent Noise Exposures L Aeq,8h = 85 dB(A) Noise Level dB(A) Typical source Exposure Time 80 Kerbside Heavy traffic16 hours hours 1 85 Front-end loader 8 hours 884 hours 91 Lawn-mower 2 hours 941 hour 9730 minutes minutes 102 Aero Commander Take-Off 7.5 minutes minutes 109 Chain saw 1.9 minutes seconds seconds seconds 121 Rock Drill 7.2 seconds seconds 127 Rivet Hammer 1.8 seconds 130Jet Engine at 50m0.9 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

1. The Ear & Vestibular System 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.

1. The Ear & Vestibular System 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

1. The Ear & Vestibular System 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

1. The Ear & Vestibular System 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.

1. The Ear & Vestibular System 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

2. Disorientation & Illusions 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!

2. Disorientation & Illusions 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

2. Disorientation & Illusions 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

2. Disorientation & Illusions 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

2. Disorientation & Illusions 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.

2. Disorientation & Illusions 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.

2. Disorientation & Illusions 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.

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