4		The Nervous System, Ear, Hearing and Balance

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Audible Range of the Human Ear and Measurement of Sound

The frequency of sounds that a young person can hear is generally stated to be between 20 and 20 000 Hertz. However the sound range depends, to a great extent, on intensity (which is measured in decibels). The human voice uses the frequency range of 500 Hz to 3000 Hz.

Sound intensities are expressed in terms of the logarithm of the actual intensities. Below is a table of typical noise levels.

Noise levels for various sounds and locations

	Sound	Noise Level
		(dB)
	Threshold of hearing	0
	Rustle of leaves in gentle breeze	10
	Average whisper (at 4 ft)	20
	Quiet conversation	30
	Office noise	40
	Conversation in noisy factory	50
	Loud street noises (trucks etc.)	60
	Standing close to heavy machinery	80
	Maximum recommended for 8 hours exposure	87
	Maximum recommended for 2 hours exposure	100
	Maximum recommended for 30 minutes exposure	110
	Standing near a piston engine aircraft (noise becoming uncomfortable)	120
	Standing near a jet aircraft (threshold of pain)	140

Hearing Impairment

Hearing difficulties are broadly classified into three categories:

Conductive Deafness

Any damage to the conducting system, the ossicles or the ear drum, will result in a degradation of hearing. It is possible that perforations of the ear drum will result in scarring of the tissue thus reducing its ability to vibrate freely. A blow to the ear may cause damage to the small bones in the middle ear again limiting the transfer of vibrations. Modern surgery may help in some circumstances.

Excessive wax or a tumour in the ear canal can also cause conductive deafness.

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Noise Induced Hearing Loss (NIHL)

Loud noises can damage the very sensitive membrane in the cochlea and the fine structures on this membrane. The loss of hearing may at first be temporary but continued exposure to loud noise in excess of 90 decibels (dB) will result in permanent loss of hearing. The early symptoms are an inability to hear high pitched notes as these notes are normally detected by the finer cells which suffer the greatest damage. Helicopter pilots and military jet pilots tend to suffer from NIHL and, with the advent of personal stereos, there has been an alarming increase of this impairment appearing in youth.

Environmental noise pollution is now a significant factor in the prevalence of NIHL. NIHL is an occupational hazard for those of us in the aviation industry and it is strongly recommended that ear plugs are used conscientiously whenever possible. The most dangerous to the ear is noise of high frequency.

Presbycusis (Loss through Ageing)

Hearing deteriorates with advancing age. In old age, the frequency falls to between 50 and 8000 cycles per second or less. The loss of some hearing is natural as one grows older but if combined with NIHL there may be sufficient impairment to lead to a loss of a flying licence.

It is worth noting that aircraft engineers are warned always to use hearing protection when exposed to noise in excess of 115 dB. As a rough guide such levels occur when normal speech cannot be clearly heard at 2 metres.

Intermittent and sudden noise is generally considered to be more disruptive than continuous noise. In addition, high frequency noise generally has a more adverse effect on performance than lower frequency.

The Ear and Balance

As well as distinguishing sound, the ear is used to detect both angular/linear movement and accelerations. Our primary source of spatial orientation is sight but the ear provides a secondary system, particularly if vision is restricted.

Semicircular Canals

Within the inner ear are three semicircular canals filled with liquid and arranged in three planes at 90º to each other. They detect angular accelerations greater than 0.5°/sec². Within the semicircular canals are fine hair-like cells which bend as the liquid in the canals moves in relation to the walls of the canals. The movement of these hairs generates small electric currents which are passed to the cerebellum (the second smaller division of the brain).

In fact the cerebellum has the ability to predict the loss of balance and compensate. For example as you step onto an escalator muscles will work to push the body forward instinctively to avoid losing balance. Thus the cerebellum has a major part to play in both balance and coordination.

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Figure 4.2 Vestibular apparatus

Figure 4.3 Operation of the otoliths & semicircular canals

Otoliths

The otoliths, literally ‘stones in the ears’ are small grains of chalk embedded in a fleshy medium containing hair cells, located at the base of the semicircular canals. Otoliths detect tilting of the head and linear acceleration and are contained within chambers known as utricles and saccules.

Acceleration greater than 0.1 m/s² causes the hair cells to bend and this bending is interpreted by the brain to determine the new position of the head.

This gives rise to a problem when a pilot experiences linear acceleration/deceleration. As acceleration takes place the otoliths are moved backwards giving the signals to the brain that

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the head is tilting backwards. Thus the pilot feels that he/she is climbing. The reverse takes place during deceleration, giving the pilot the false impression of pitching down. This is known as the somatogravic effect or somatogravic illusion.

The somatogravic effect can be reinforced by information received from the nerve cells in the muscles of the body from pressures due to gravity. This is discussed in more detail in Chapter 10 (Cognition in Aviation). The result of these two quite separate effects combine to lead to an almost overpowering illusion of climb or descent and has led to catastrophic results.

It is worth noting that, should an air driven artificial horizon be fitted to the aircraft, this false feeling of pitch up will be reinforced by the indication of a climb on the instrument resulting from one of the two acceleration errors (the artificial horizon will show a climbing turn to the right having been subjected to a linear acceleration). If the aircraft decelerates, the reverse will apply and the reading from the instrument will reinforce the feeling of an apparent descent.

The semicircular canals and the otoliths together make up the vestibular apparatus which helps to maintain spatial orientation and controls other functions. For example eye movement to maintain a stable picture of the world on the retina when the head is moved.

Problems of Balance and Disorientation

Statistics have shown that spatial disorientation has been a contributory factor in 37% of accidents in general aviation and 12% in commercial transport operations. It is the most dangerous of conditions and over 80% of accidents resulting directly from disorientation are fatal. The most well known example of disorientation among pilots is “The Leans”.

Leans or Somatogyral Illusion.

The vestibular apparatus is not always sufficiently reliable to maintain an accurate model of orientation. This condition is known as the leans or somatogyral illusion. It can occur in all conditions of flight, and can persist for up to an hour after the event causing it.

The two most common circumstances under which the ‘leans’ may be experienced are:

•The pilot commences a very gentle slow turn, so gentle that the movement of the liquid is not enough to cause a detectable bending of the hair cells. Therefore, although in a turn, the balance mechanism senses no change has been made. A subsequent normal return to straight and level flight, will be detected as a turn from the straight and level by the balance mechanism. The aircraft is now, in reality straight and level, but the pilot feels that he is still turning.

•The pilot executes a prolonged turn to such an extent as to allow the hairs to erect in the canals while still in the turn. This gives the pilot the erroneous feeling that he/she is straight and level. As the pilot rolls out, the ends of the hairs move again to give a false impression of a turn when - in fact - the aircraft is level. This condition is illustrated in Figure 4.4 .

In both the above, the pilot will be subjected to two conflicting signals. The visual sense will tell him that he/she is level whereas the vestibular apparatus will tell him he/she is turning.

If in IMC

BELIEVE YOUR INSTRUMENTS

If in VMC

LOOK OUT AT THE HORIZON

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