Module 9 (Human Factors) Sub Module 9.2 (Human Performance and Limitations)_Rev 1_Sep 2013

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Module 9 – HUMAN FACTORS Category – A/B1/B2 Sub Module 9.2 – Human Performance and Limitations

ISO 9001:2008 Certified For Training Purpose Only

PTC/CM/B Basic/M9/01 Rev. 00

9.2 Mar 2014

PIA Training Centre (PTC)

MODULE 9

Sub Module 9.2

HUMAN PERFORMANCE AND LIMITATIONS






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Contents

SECTION 1: INTRODUCTION --------------------------------------------------- 1

SECTION 2: VISION --------------------------------------------------------------- 2

2.1 VISION ----------------------------------------------------------------------- 2

THE BASIC FUNCTION OF THE EYE --------------------------------------- 2

THE CORNEA ------------------------------------------------------------------ 2

THE IRIS AND PUPIL --------------------------------------------------------- 2

THE LENS ----------------------------------------------------------------------- 2

THE RETINA-------------------------------------------------------------------- 3

2.2 FACTORS AFFECTING CLARITY OF SIGHT ---------------------------- 4

PHYSICAL FACTORS ---------------------------------------------------------- 5

OTHER VISUAL PROBLEMS INCLUDE: ----------------------------------- 5

FOREIGN SUBSTANCES ----------------------------------------------------- 5

ENVIRONMENTAL FACTORS ----------------------------------------------- 6

2.3 THE NATURE OF THE OBJECT BEING VIEWED --------------------- 7

2.4 COLOUR VISION ----------------------------------------------------------- 7

2.5 VISION AND THE AIRCRAFT MAINTENANCE ENGINEER --------- 8

SECTION 3: HEARING ------------------------------------------------------------ 9

3.1 THE BASIC FUNCTION OF THE EAR -------------------------------------- 9

OUTER EAR -------------------------------------------------------------------- 9

MIDDLE EAR ------------------------------------------------------------------- 9

INNER EAR --------------------------------------------------------------------- 9

3.2 PERFORMANCE AND LIMITATIONS OF THE EAR -------------------- 11

3.3 IMPACT OF NOISE ON PERFORMANCE -------------------------------- 11

3.4 HEARING IMPAIRMENT --------------------------------------------------- 13

3.5 HEARING PROTECTION ---------------------------------------------------- 13

3.6 PRESBYCUSIS ---------------------------------------------------------------- 143.7 HEARING AND THE AIRCRAFT MAINTENANCE ENGINEER -------- 14

SECTION 4: INFORMATION PROCESSING --------------------------------- 15

4.1 INFORMATION PROCESSING -------------------------------------------- 15

4.2 AN INFORMATION PROCESSING MODEL ----------------------------- 15

4.3 SENSORY RECEPTORS AND SENSORY STORES ----------------------- 15

SECTION 5: ATTENTION AND PERCEPTION ------------------------------ 17

5.1 ATTENTION AND PERCEPTION: HOW DOES IT WORK? -- --------- 18

5.2 DECISION MAKING --------------------------------------------------------- 19

5.3 SITUATION AWARENESS -------------------------------------------------- 19SECTION 6: LEARNING AND MEMORY ------------------------------------ 21

6.1 SHORT TERM OR WORKING MEMORY -------------------------------- 21

6.2 LONG TERM MEMORY (LTM) -------------------------------------------- 21

6.3 MOTOR MEMORY ---------------------------------------------------------- 22

6.4 SHORT TERM MEMORY AID --------------------------------------------- 22

SUMMARY ------------------------------------------------------------------------ 23

SECTION 7: DECISION MAKING, MEMORY, AND MOTOR

PROGRAMMES ------------------------------------------------------------------ 24

SECTION 8: CLAUSTROPHOBIA AND PHYSICAL ACCESS -------------- 25






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SECTION 1: INTRODUCTION

The intention of this chapter is to provide an overview of those

key physical and mental human performance characteristicswhich are likely to affect an aircraft maintenance engineer in hisworking environment, such as his vision, hearing, informationprocessing, attention and perception, memory, judgment anddecision making.

The human performance can be affected by physical conditionand psychological condition. The physical condition can be dueto the health of the person’s body, the environment of home andworkplace. The mental state of the person, i.e. the psychologycondition, can be due to hereditary factors, historical factors andmental pressures. We may not be able to do much about

hereditary factors. Historical factors such as education due to apoor early environment can be worked on and improved.However, we can certainly look at the factors which affecthuman performance mentally and physically, i.e.:

The performance of the human body’s senses.

The effect of the environment of the workplace on theperson.

While the precise range of human capabilities and limitationsmight not be as well-defined as the performance range ofmechanical or electrical components, the same principles applyin that human performance is likely to degrade and eventually‘fail’ under certain conditions (e.g. stress).

Mechanical components in aircraft can, on occasion, suffercatastrophic failures. Man, can also fail to function properly incertain situations. Physically, humans become fatigued, are

affected by the cold, can break bones in workplace accidents,etc. Mentally, humans can make errors, have limited perceptualpowers, can exhibit poor judgment due to lack of skills andknowledge, etc.

In addition, unlike mechanical components, human performanceis also affected by social and emotional factors. Thereforefailure by aircraft maintenance engineers can also be to thedetriment of aircraft safety.

The aircraft engineer is the central part of the aircraftmaintenance system. It is therefore very useful to have an

understanding of how various parts of his body and mentalprocesses function and how performance limitations caninfluence his effectiveness at work.

To have a better idea of how a person reacts with his/hersurroundings it is necessary to understand at least something ofhow the body works.

The parts that will be covered are those that directly affect aperson’s ability to perform maintenance tasks namely:

Vision

Hearing






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SECTION 2: VISION

2.1 VISION

Of the senses, this is the most useful to man. We are morearoused to anger or passion by visual inputs than by says feelor smell.

The eye receives light information from the outside world andpasses it to the brain. The eye acts very similarly to a camera.Light is taken in through the cornea and lens, a clear window atthe front of the eyeball. The cornea acts as a focusing deviceand is responsible for between 70-80% of the total focusingability of the eye. The iris will control the amount of lightentering the eye by altering the size of the hole in the middle. It

can change shape very quickly to cater for changing light levelsi.e. luminance level up to 5 times.

THE BASIC FUNCTION OF THE EYE

In order to understand vision, it is useful first to know a littleabout the anatomy of the eye (see Figure 4.1). The basicstructure of the eye is similar to a simple camera with anaperture (the iris), a lens, and a light sensitive surface (theretina). Light enters the eye through the cornea, then passesthrough the iris and the lens and falls on the retina. Here thelight stimulates the light-sensitive cells on the retina (rods and

cones) and these pass small electrical impulses by way of theoptic nerve to the visual cortex in the brain. Here, the electricalimpulses are interpreted and an image is perceived.

THE CORNEA

The cornea is a clear ‘window’ at the very front of the eye. The

cornea acts as a fixed focusing device. The focusing isachieved by the shape of the cornea bending the incoming lightrays. The cornea is responsible for between 70% and 80% ofthe total focusing ability (refraction) of the eye.

THE IRIS AND PUPIL

The iris (the coloured part of the eye) controls the amount oflight that is allowed to enter the eye. It does this by varying thesize of the pupil (the dark area in the centre of the iris). The sizeof the pupil can be changed very rapidly to cater for changinglight levels. The amount of light can be adjusted by a factor of

5:1.

THE LENS

After passing through the pupil, the light passes through thelens. Its shape is changed by the muscles (cillary muscles)surrounding it which results in the final focusing adjustment toplace a sharp image onto the retina. The change of shape of thelens is called accommodation. In order to focus clearly on anear object, the lens is thickened. To focus on a distant point,the lens is flattened. The degree of accommodation can beaffected by factors such as fatigue or the ageing process.






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THE RETINA

The retina is located on the rear wall of the eyeball. It is made

up of a complex layer of nerve cells connected to the opticnerve. Two types of light sensitive cells are found in the retina -rods and cones. The central area of the retina is known as thefovea and the receptors in this area are all cones. It is here thatthe visual image is typically focused. Moving outwards, thecones become less dense and are progressively replaced byrods, so that in the periphery of the retina, there are only rods.

At the point at which the optic nerve joins the back of the eye, a‘blind spot’ occurs. This is not evident when viewing things withboth eyes (binocular vision), since it is not possible for theimage of an object to fall on the blind spots of both eyes at the

same time. Even when viewing with one eye (monocular vision),the constant rapid movement of the eye (saccades) means thatthe image will not fall on the blind spot all the time. It is onlywhen viewing a stimulus that appears very fleetingly (e.g. a lightflashing), that the blind spot may result in something not beingseen. In maintenance engineering, tasks such as close visualinspection or crack detection should not cause such problems,as the eye or eyes move across and around the area of interest(visual scanning).

Figure 2.1 the Human Eye






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2.2 FACTORS AFFECTING CLARITY OF SIGHT

The eye is very sensitive in the right conditions (e.g. clear air,

good light, etc.). In fact, the eye has approximately 1.2 millionnerve cells leading from the retinas to the area of the brainresponsible for vision, while there are only about 50,000 fromthe inner ears - making the eye about 24 times more sensitivethan the ear.

Before considering factors that can influence and limit theperformance of the eye, it is necessary to describe visual acuity.When a person is tired accommodation is reduced, resulting inless sharp vision (sharpness of vision is known as visual acuity).Cones function in good light and are capable of detecting finedetail and are colour sensitive. This means the human eye can

distinguish about 1000 different shades of colour. Rods cannotdetect colour. They are poor at distinguishing fine detail, butgood at detecting movement in the edge of the visual field(peripheral vision). They are much more sensitive at lower lightlevels. As light decreases, the sensing task is passed from thecones to the rods. This means in poor light levels we see only inblack and white and shades of grey. Visual acuity is the abilityof the eye to discriminate sharp detail at varying distances.

An individual with acuity of 20/20 vision should be able to see at20 feet that which the so-called ‘normal’ person is capable ofseeing at this range. It may be expressed in meters as 6/6

vision. The figures 20/40 mean that the observer can read at 20feet what a ‘normal’ person can read at 40 feet.

Various factors can affect and limit the visual acuity of the eye.These include:

Physical factors such as:

Physical imperfections in one or both eyes(short sightedness, long sightedness)

Age

The influence of ingested foreign substances such as:

Drugs

Medication

Alcohol

Cigarettes

Environmental factors such as:

Amount of light available

Clarity of the air (e.g. dust, mist, rain, etc.)

Factors associated with object being viewed such as:

Size and contours of the object

Contrast of the object with its surroundings

Relative motion of the object Distance of the object from the viewer

The angle of the object from the viewer






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PHYSICAL FACTORS

Long sight - known as Hypermetropia - is caused by a shorter

than normal eyeball which means that the image is formedbehind the retina. If the cornea and the lens cannot use theircombined focusing ability to compensate for this, blurred visionwill result when looking at close objects.

Fig 2.2 a convex lens will overcome long sightedness bybending light inwards before it reaches the cornea.

Short sight - known as Myopia - is where the eyeball is longerthan normal, causing the image to be formed in front of theretina (Figure 2.3). If the accommodation of the lens cannotcounteract this then distant objects are blurred.

Fig 2.3 a concave lens will overcome short-sightedness bybending light outwards before it reaches the cornea

OTHER VISUAL PROBLEMS INCLUDE:

Cataracts - clouding of the lens usually associated with

ageing

Astigmatism - a misshapen cornea causing objects toappear irregularly shaped

Glaucoma - a build up in pressure of the fluid within theeye which can cause damage to the optic nerve andeven blindness

Migraine - severe headaches that can cause visualdisturbances

Finally as a person grows older, the lens becomes less flexiblemeaning that it is unable to accommodate sufficiently. This isknown as presbyopia and is a form of long sightedness.Consequently, after the age of 40, spectacles may be requiredfor near vision, especially in poor light conditions. Fatigue canalso temporarily affect accommodation, causing blurred visionfor close work.

FOREIGN SUBSTANCES

Vision can be adversely affected by the use of certain drugs andmedications, alcohol, and smoking cigarettes. With smoking,

carbon monoxide which builds up in the bloodstream allows lessoxygen to be carried in the blood to the eyes. This is known ashypoxia and can impair rapidly the sensitivity of the rods.

Alcohol can have similar effects, even hours after the last drink.






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ENVIRONMENTAL FACTORS

Vision can be improved by increasing the lighting level, but onlyup to a point as the law of diminishing returns operates. Also,increased illumination could result in increased glare. Olderpeople are more affected by the glare of reflected light thanyounger people. Moving from an extremely bright environmentto a dimmer one has the effect of vision being severely reduceduntil the eyes get used to less light being available. This isbecause the eyes have become light adapted. If an engineerworks in a very dark environment for a long time, his eyesgradually become dark adapted allowing better visual acuity.This can take about 7 minutes for the cones and 30 minutes forthe rods. As a consequence, moving between a bright hanger(and the inside of an aircraft) to a dark apron area at night canmean that the maintenance engineer must wait for his eyes to

adjust (adapt). In low light conditions, it is easier to focus if youlook slightly to one side of an object. This allows the image tofall outside the fovea and onto the part of the retina which hasmany rods.

Any airborne particles such as dust, rain or mist can interferewith the transmission of light through the air, distorting what isseen. This can be even worse when spectacles are worn, asthey are susceptible to getting dirty, wet, misted up orscratched. Engineers who wear contact lenses (especially hardor gas-permeable types) should take into account the advicefrom their optician associated with the maximum wear time -

usually 8 to 12 hours - and consider the effects which extendedwear may have on the eyes, such as drying out and irritation.This is particularly important if they are working in anenvironment which is excessively dry or dusty, as airborne

particles may also affect contact lens wear. Goggles should beworn where necessary.






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2.3 THE NATURE OF THE OBJECT BEING VIEWED

Many factors associated with the object being viewed can alsoinfluence vision. We use information from the objects we arelooking at to help distinguish what we are seeing. These areknown as visual cues. Visual cues often refer to the comparisonof objects of known size to unknown objects. An example of thisis that we associate small objects with being further away.Similarly, if an object does not stand out well from itsbackground (i.e. it has poor contrast with its surroundings), it isharder to distinguish its edges and hence its shape. Movementand relative motion of an object, as well as distance and angleof the object from the viewer, can all increase visual demands.

2.4 COLOUR VISION

Although not directly affecting visual acuity, inability to seeparticular colours can be a problem for the aircraft maintenanceengineer. Amongst other things, good colour vision formaintenance engineers is important for:

Recognizing components

Distinguishing between wires

Using various diagnostic tools

Recognizing various lights on the airfield (e.g. warninglights)

Colour defective vision is usually hereditary, although may alsooccur as a temporary condition after a serious illness.






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There are degrees of colour defective vision, some peoplesuffering more than others. Individuals may be able todistinguish between red and green in a well-lit situation but not

in low light conditions. Colour defective people typically see thecolours they have problems with as shades of neutral grey.

Ageing also causes changes in colour vision. This is a result ofprogressive yellowing of the lens, resulting in a reduction incolour discrimination in the blue-yellow range. Colour defectivevision and its implications can be a complex area and careshould be taken not to stop an engineer from performing certaintasks merely because he suffers from some degree of colourdeficient vision. It may be that the type and degree of colourdeficiency is not relevant in their particular job. However, ifabsolutely accurate colour discrimination is critical for a job, it is

important that appropriate testing and screening be put in place.Colour-defective vision (normally referred to incorrectly ascolour blindness) affects about 8% of men but only 0.5% ofwomen. The most common type is difficulty in distinguishingbetween red and green. More rarely, it is possible to confuseblues and yellows.

2.5 VISION AND THE AIRCRAFT MAINTENANCE ENGINEER

It is important for an engineer, particularly one who is involvedin inspection tasks, to have adequate vision to meet the task

requirements. As discussed previously, age and problemsdeveloping in the eye itself can gradually affect vision. Withoutregular vision testing, aircraft maintenance engineers may notnotice that their vision is deteriorating.

In the UK, the CAA has produced guidance which states:

“A reasonable standard of eyesight is needed for any aircraftengineer to perform his duties to an acceptable degree. Manymaintenance tasks require a combination of both distance andnear vision. In particular, such consideration must be madewhere there is a need for the close visual inspection ofstructures or work related to small or miniature components.

The use of glasses or contact lenses to correct any visionproblems is perfectly acceptable and indeed they must be wornas prescribed. Frequent checks should be made to ensure thecontinued adequacy of any glasses or contact lenses. Inaddition, colour discrimination may be necessary for anindividual to drive in areas where aircraft maneuver or wherecolour coding is used, e.g. in aircraft wiring. Organizationsshould identify any specific eyesight requirement and put inplace suitable procedures to address these issues.”

Often, airline companies or airports will set the eyesightstandards for reasons other than aircraft maintenance safety,e.g. for insurance purposes, or for driving on the airfield.Ultimately, what is important is for the individual to recognizewhen his vision is adversely affected, either temporarily orpermanently and to consider carefully the possibleconsequences should they continue to work if the task requiresgood vision.






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SECTION 3: HEARING

3.1 THE BASIC FUNCTION OF THE EAR

The ear performs two quite different functions. It is used todetect sounds by receiving vibrations in the air, and secondly, itis responsible for balance and sensing acceleration. Of thesetwo, the hearing aspect is more pertinent to the maintenanceengineer, and thus it is necessary to have a basic appreciationof how the ear works.

As can be seen in Figure 8, the ear has three divisions: outerear, middle ear and inner ear. These act to receive vibrationsfrom the air and turn these signals into nerve impulses that thebrain can recognize as sounds.

OUTER EAR

The outer part of the ear directs sounds down the auditorycanal, and on to the eardrum. The sound waves will cause theeardrum to vibrate.

MIDDLE EAR

Beyond the eardrum is the middle ear which transmits vibrationsfrom the eardrum by way of three small bones known as theossicles, to the fluid of the inner ear. The middle ear alsocontains two muscles which help to protect the ear from soundsabove 80 dB by means of the acoustic or aural reflex, reducingthe noise level by up to 20 dB. However, this protection can onlybe provided for a maximum of about 15 minutes, and does notprovide protection against sudden impulse noise such asgunfire.

It does explain why a person is temporarily ‘deafened’ for a fewseconds after a sudden loud noise. The middle ear is usuallyfilled with air which is refreshed by way of the eustachian tubewhich connects this part of the ear with the back of the nose

and mouth. However, this tube can allow mucus to travel to themiddle ear which can build up, interfering with normal hearing.

INNER EAR

Unlike the middle ear, the inner ear is filled with fluid. The last ofthe ossicles in the middle ear is connected to the cochlea. Thiscontains a fine membrane (the basilar membrane) covered inhair-like cells which are sensitive to movement in the fluid. Anyvibrations they detect cause neural impulses to be transmittedto the brain via the auditory nerve.






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Figure 2.4 the Human Ear






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3.2 PERFORMANCE AND LIMITATIONS OF THE EAR

The performance of the ear is associated with the range ofsounds that can be heard - both in terms of the pitch(frequency) and the volume of the sound. Volume (or intensity)of sound is measured in decibels (dB). Table 2.1 below showsintensity levels for various sounds and activities.

Table 2.1 Typical sound levels for various activities

3.3 IMPACT OF NOISE ON PERFORMANCE

Noise can have various negative effects in the workplace. Itcan:

Be annoying (e.g. sudden sounds, constant loud sound,etc.)

Interfere with verbal communication between individualsin the workplace

Cause accidents by masking warning signals ormessages

Be fatiguing and affect concentration, decision making,etc.

Damage workers’ hearing (either temporarily orpermanently)

The amount of vibration detected in the cochlea depends on thevolume and pitch of the original sound. The audible frequencyrange that a young person can hear is typically between 20 and20,000 cycles per second (or Hertz), with greatest sensitivity atabout 3000 Hz.

Intermittent and sudden noise is generally considered to bemore disruptive than continuous noise at the same level. Inaddition, high frequency noise generally has a more adverseaffect on performance than lower frequency. Noise tends toincrease errors and variability, rather than directly affect workrate.






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3.4 HEARING IMPAIRMENT

Hearing loss can result from exposure to even relatively shortduration noise. The degree of impairment is influenced mainlyby the intensity of the noise. Such damage is known as NoiseInduced Hearing Loss (NIHL). The hearing loss can betemporary - lasting from a few seconds to a few days - orpermanent. Temporary hearing loss may be caused byrelatively short exposure to very loud sound, as the hair-likecells on the basilar membrane take time to ‘recover’. Withadditional exposure, the amount or recovery graduallydecreases and hearing loss becomes permanent. Thus, regularexposure to high levels of noise over a long period maypermanently damage the hair like cells in the cochlea, leading toirreversible hearing impairment.

The UK ‘Noise at Work’ regulations (1989) impose requirementsupon employers. They stipulate three levels of noise at which anemployer must act:

a) 85 decibels (if normal speech cannot be heard clearly at 2meters), employer must:

Assess the risk to employees’ hearing

Tell the employees about the risks and what precautionsare proposed

Provide their employees with personal ear protectorsand explain their use

b) 90 decibels (if normal speech cannot be heard clearly at1meter) employer must:

Do all that is possible to reduce exposure to the noise bymeans other than by providing hearing protection

Mark zones where noise reaches the second level andprovide recognized signs to restrict entry

c) 140 decibels (noise causes pain)

The combination of duration and intensity of noise can bedescribed as noise dose. Exposure to any sound over 80 dBconstitutes a noise dose, and can be measured over the day asan 8 hour Time Weighted Average sound level (TWA).

Permanent hearing loss may occur if the TWA is above therecommended maximum.

3.5 HEARING PROTECTION

Hearing protection is available, to a certain extent, by using earplugs or ear defenders. It is good practice to reduce noise levelsat source, or move noise away from workers. Often this is not apractical option in the aviation maintenance environment.Hearing protection should always be used for noise, of anyduration, above 115 dB. Referring again to Table 1, this meansthat the aviation maintenance engineer will almost always need

to use some form of hearing protection when in reasonablyclose proximity (about 200 - 300m) to aircraft whose enginesare running.






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3.6 PRESBYCUSIS

Hearing deteriorates naturally as one grows older. This isknown as presbycusis. This affects ability to hear high pitch

sounds first, and may occur gradually from the 30’s onwards.When this natural decline is exacerbated by Noise InducedHearing Loss, it can obviously occur rather sooner.

3.7 HEARING AND THE AIRCRAFT MAINTENANCEENGINEER

The UK CAA makes the following recommendations regardinghearing:

“The ability to hear an average conversational voice in a quietroom at a distance of 2 meters (6 feet) from the examiner isrecommended as a routine test. Failure of this test would

require an audiogram to be carried out to provide an objectiveassessment. If necessary, a hearing aid may be worn butconsideration should be given to the practicalities of wearing theaid during routine tasks demanded of the individual.”

It is very important that the aircraft maintenance engineerunderstands the limited ability of the ears to protect themselvesfrom damage due to excessive noise. Even though engineersshould be given appropriate hearing protection and trained in itsuse, it is up to individuals to ensure that they actually put this togood use. It is a misconception that the ears get used toconstant noise: if this noise is too loud, it will damage the earsgradually and insidiously. Noise levels can be reduced(attenuated) by up to 20 decibels using ear plugs and 40decibels using ear muffs. However, using ear protection willtend to adversely interfere with verbal communication. Despitethis, it must be used consistently and as instructed to beeffective.






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SECTION 4: INFORMATION PROCESSING

4.1 INFORMATION PROCESSING

The previous sections have described the basic functions andlimitations of two of the senses used by aircraft maintenanceengineers in the course of their work. This section examines theway the information gathered by the senses is processed by thebrain. The limitations of the human information processingsystem are also considered.

4.2 AN INFORMATION PROCESSING MODEL

Information processing can be represented as a model. Thiscaptures the main elements of the process, from receipt of

information via the senses, to outputs such as decision makingand actions. One such model is shown in Figure 2.5.

4.3 SENSORY RECEPTORS AND SENSORY STORES

Physical stimuli are received via the sensory receptors (eyes,ears, etc.) And stored for a very brief period of time in sensorystores (sensory memory). Visual information is stored for up tohalf a second in iconic memory and sounds are stored forslightly longer (up to 2 seconds) in echoic memory. Thisenables us to remember a sentence as a sentence, rather thanmerely as an unconnected string of isolated words, or a film as

a film, rather than as a series of disjointed images.






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Fig 2.5 A functional model of human information






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SECTION 5: ATTENTION AND PERCEPTION

Having detected information, our mental resources areconcentrated on specific elements - this is attention. Althoughattention can move very quickly from one item to another, it canonly deal with one item at a time. Attention can take the form of:

Selective attentions

Divided attention

Focused attention

Sustained attention

Selective attention occurs when a person is monitoring severalsources of input, with greater attention being given to one ormore sources which appear more important. A person can beconsciously attending to one source whilst still sampling othersources in the background. Psychologists refer to this as the‘cocktail party effect ’ whereby you can be engrossed in aconversation with one person but your attention is temporarilydiverted if you overhear your name being mentioned at theother side of the room, even though you were not aware oflistening in to other people’s conversations. Distraction is thenegative side of selective attention.

Divided attention is common in most work situations, wherepeople are required to do more than one thing at the same time.Usually, one task suffers at the expense of the other, more so ifthey are similar in nature. This type of situation is alsosometimes referred to as time sharing.

Focused attention is merely the skill of focusing one’s attentionupon a single source and avoiding distraction.

Sustained attention as its name implies, refers to the ability tomaintain attention and remain alert over long periods of time,often on one task. Most of the research has been carried out inconnection with monitoring radar displays, but there is alsoassociated research which has concentrated upon inspectiontasks.

Attention is influenced by arousal level and stress. This canimprove attention or damage it depending on thecircumstances.

Perception involves the organization and interpretation of

sensory data in order to make it meaningful, discarding non-relevant data, i.e. transforming data into information. Perceptionis a highly sophisticated mechanism and requires existingknowledge and experience to know what data to keep and whatto discard, and how to associate the data in a meaningfulmanner.






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5.1 ATTENTION AND PERCEPTION: HOW DOES IT WORK?

A proportion of ‘sensed’ data may be lost without being‘perceived’. An example with which most people are familiar isthat of failing to perceive something which someone has said toyou, when you are concentrating on something else, eventhough the words would have been received at the ear withoutany problem. The other side of the coin is the ability of theinformation processing system to perceive something (such asa picture, sentence, concept, etc.) even though some of thedata may be missing. The danger, however, is that people canfill in the gaps with information from their own store ofknowledge or experience, and this may lead to the wrongconclusion being drawn.

There are many well-known visual ‘illusions’ which illustrate thelimits of human perception. Figure 2.6 shows how theperceptual system can be misled into believing that one line islonger than the other, even though a ruler will confirm that theyare exactly the same.

Figure 2.7 illustrates that we can perceive the same thing quitedifferently (i.e. the letter “B” or the number “13”). This shows theinfluence of context on our information processing.

In aviation maintenance it is often necessary to consultdocuments with which the engineer can become very familiar. It

is possible that an engineer can scan a document and fail tonotice that subtle changes have been made. He sees only whathe expects to see (expectation). To illustrate how our eyes candeceive us when quickly scanning a sentence, read quickly thesentence below in Figure 2.8.

Expectation can also affect our memory of events. The studyoutlined above was extended such that subjects were asked, aweek later, whether they recalled seeing glass on the road afterthe collision. (There was no glass). The group, who had beentold that they would see a crash, recalled seeing glass; theother group recalled seeing no glass.

Fig 2.6 the Muller –Layer Illusion

Fig 2.7 the importance of context

Fig 2.8 the effects of expectation






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At first, most people tend to notice nothing wrong with thesentence. Our perceptual system sub-consciously rejects theadditional “THE”.

5.2 DECISION MAKING

Having recognized coherent information from the stimulireaching our senses, a course of action has to be decided upon.In other words decision making occurs. This may range fromdeciding to do nothing, to deciding to act immediately in a veryspecific manner. A fire alarm bell, for instance, may trigger awell-trained sequence of actions without further thought (i.e.evacuate); alternatively, an unfamiliar siren may require furtherinformation to be gathered before an appropriate course ofaction can be initiated.

We are not usually fully aware of the processes and informationwhich we use to make a decision. Tools can be used to assistthe process of making a decision. For instance, in aircraftmaintenance engineering, many documents (e.g. maintenancemanuals, fault diagnosis manuals), and procedures areavailable to supplement the basic decision making skills of theindividual. Thus, good decisions are based on knowledgesupplemented by written information and procedures, analysisof observed symptoms, performance indications, etc. It can bedangerous to believe that existing knowledge and priorexperience will always be sufficient in every situation as will beshown in the section entitled ‘Information ProcessingLimitations’.

Finally, once a decision has been made, an appropriate actioncan be carried out. Our senses receive feedback of this and itsresult. This helps to improve knowledge and refine future

judgment by learning from experience.

5.3 SITUATION AWARENESS

Although not shown explicitly in Figure, the process of attention,perception and judgment should result in awareness of thecurrent situation. Situation awareness has traditionally beenused in the context of the flight deck to describe the pilot’sawareness of what is going on around him, e.g. where he isgeographically, his orientation in space, what mode the aircraftis in, etc. In the maintenance engineering context, it refers tothe:

Perception of important elements, e.g. seeing loosebolts or missing parts, hearing information passedverbally

Comprehension of their meaning, e.g. why is it likethis? Is this how it should be?

Projection of their status into the future, e.g. futureeffects on safety, schedule, airworthiness

As with decision making, feedback improves situationawareness by informing us of the accuracy of our mentalmodels and their predictive power. The ability to project systemstatus backward, to determine what events may have led to anobserved system state, is also very important in aircraftmaintenance engineering, as it allows effective fault finding and

diagnostic behaviour. Situation awareness for the aircraftmaintenance engineer can be summarized as the:






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Status of the system the engineer is working on

The relationship between the reported defect and theintended rectification

The possible effect of this work on other systems

The effect of this work on that being done by others andthe effect of their work on this work






9.2-21 Mar 2014


SECTION 6: LEARNING AND MEMORY

By definition, learning is the acquisition of knowledge or skillthrough instruction or experience. Essentially, learning involves

the storage of information within memory as this information isacquired through experience. The human brain must be able tostore information for as long as several decades (Long TermMemory ) or for as short a time as several seconds (Short TermMemory ). Additionally the human brain is able to transferinformation from short term to long term memory through theprocess of memory consolidation. For example one canremember a telephone number long enough to dial it or it canbe transferred to long term memory for future use.

6.1 SHORT TERM OR WORKING MEMORY

From the example of the telephone number we can see that ifnothing is done to try to remember then a minute or so later theinformation is lost. Looking up the index in a book to find apage number for a particular item would use this part of thememory. Once the page is found the number is forgotten.Unless actively rehearsed, information in working memory islost in about 20 to 30 seconds. Acoustic information is easier toremember than visual because it is easier to rehearse a soundthan memorise written data.

6.2 LONG TERM MEMORY (LTM)

Long term memory can be divided into Semantic Memory and

Episodic Memory. Semantic memory is where the knowledgeis retained in the long term, where the general meaning ratherthan the specific sounds of the original message areremembered.

Information once transferred from the short term memory isnever lost. If we are unable to remember anything in this part ofthe memory it is because we cannot retrieve it (recall). It isthere but we cannot find it. Confusions in LTM recollection are

much more likely to involve mixing up words rather than similarmeanings. The word ‘car’ may be recalled instead of ‘van’ forexample.

Episodic memory is that part of the memory system that dealswith ‘episodes’ , specific events in your life. This part of thememory changes over the years. It can result in similar eventsbecoming mixed, even into one event. This is due to theretention rate over time - approximately only 35% isremembered after one day and 22% after 31 days. This alsomeans that when asked to recall an incident, any two people willoften give conflicting reports. Therefore, if you are a witness to

an accident or incident, write your evidence down immediately,even take photos. This will help in the interrogation later on,when your memory is weaker on the facts you hoped to retain.Of the two types of long term memory, semantic is said to lastlonger, whereas episode is more accurate.






9.2-22 Mar 2014


6.3 MOTOR MEMORY

This memory is associated with the skill of controlling hand,feet, leg movement etc. When performing or learning a newtask a great deal of central processing of information will berequired by the brain. Firstly in the short term memory and thenwith rehearsal, stored in the long term memory.

Learning to drive a car is a good example. At the first stage oflearning, the Cognitive Stage, we have to understand what eachcontrol in the car is for and how it works. The first time we setoff in the car we have to think hard about what we are doing.The second stage of learning is the Associative Stage. With alittle practice each element of the driving process is learnt, forexample the use of the clutch whilst gear changing, yet still

looking ahead.

When well rehearsed, all the elements of car driving becomeautomatic and you are aware little conscious thought is used.This is the automatic stage of learning when the task no longerrequires thought to perform it. Whilst driving, a conversationcan be carried out with only a monitoring function of the brainleft to check the process of driving.

6.4 SHORT TERM MEMORY AID

To improve your chances of remembering information,particularly numbers, it is sometimes useful to break the numberdown into more easily handled ‘chunk’ . For example, thenumber 19391914999365 would not be easy to remember whentaken as a whole. However, we can break this number downinto more manageable chunks:

1939 – Start of World War 2.

1914 – Start of World War 1.

999 – Emergency phone number in the UK.

365 – Days in a year.

By remembering the smaller chunks, we are far more likely to

remember the larger number.






9.2-23 Mar 2014


SUMMARY

There are three parts to memory: encoding, storage andretrieval.

Most material that is forgotten is lost in the first fewhours after learning. If something is still rememberedafter a few days, it is unlikely to be forgotten.

STM has a very small capacity, but the amount in thestorage can be increased by chunking. Material in STMwill be lost within 20-30 seconds if it is not rehearsed.

LTM has a more or less unlimited capacity but theproblem is how to retrieve the information.

Memory is not like a video-recording. When weremember we reconstruct the event on the basis ofinformation available in schemata. Material can bedistorted in memory.

Memory is most efficient where there are lots of recallcues. When there are few recall cues it may help to bein the same physical or mental state as you were whenyou first came across the material.

What we do before and after learning something may

interfere with later recall. This is especially true if theinterfering material is of a similar nature to the learntmaterial and is presented just before or just after it.

Physical shock can disturb memory, especially forevents that occurred up to half an hour before the shock.

Highly emotionally charged material might be repressed.This does not make total loss but is in thesub-conscious. It may affect behaviour.

Acoustic information is easier to remember as the brainfinds it easier to rehearse a sound than a visual image.






9.2-24 Mar 2014


SECTION 7: DECISION MAKING, MEMORY, AND MOTORPROGRAMMES

Attention and perception shortcomings can clearly impinge on

decision making. Perceiving something incorrectly may meanthat an incorrect decision is made, resulting in an inappropriateaction. Figure 2.5 also shows the dependence on memory tomake decisions. It was explained earlier that sensory and short-term memory has limited capacity, both in terms of capacity andduration. It is also important to bear in mind that human memoryis fallible, so that information:

May not be stored

May be stored incorrectly

May be difficult to retrieve

All these may be referred to as forgetting, which occurs wheninformation is unavailable (not stored in the first place) orinaccessible (cannot be retrieved). Information in short-termmemory is particularly susceptible to interference, an exampleof which would be trying to remember a part number whilsttrying to recall a telephone number.

It is generally better to use manuals and temporary aides-memoirs rather than to rely upon memory, even in

circumstances where the information to be remembered orrecalled is relatively simple. For instance, an aircraftmaintenance engineer may think that he will remember a torquesetting without writing it down, but between consulting themanual and walking to the aircraft (possibly stopping to talk to

someone on the way), he may forget the setting or confuse it(possibly with a different torque setting appropriate to a similartask with which he is more familiar). Additionally, if unsure of theaccuracy of memorized information, an aircraft maintenance

engineer should seek to check it, even if this means goingelsewhere to do so. Noting something down temporarily canavoid the risk of forgetting or confusing information. However,the use of a personal note book to capture such information ona permanent basis can be dangerous, as the information in itmay be come out-of-date.






9.2-25 Mar 2014


SECTION 8: CLAUSTROPHOBIA AND PHYSICAL ACCESS

Although not peculiar to aircraft maintenance engineering,working in restricted space and at heights is a feature of this

trade. Problems associated with physical access are notuncommon. Maintenance engineers and technicians often haveto access, and work in, very small spaces (e.g. in fuel tanks),cramped conditions (such as beneath flight instrument panels,around rudder pedals), elevated locations (on cherry-pickers orstaging), sometimes in uncomfortable climatic or environmentalconditions (heat, cold, wind, rain, noise). This can beaggravated by aspects such as poor lighting or having to wearbreathing apparatus.

There are many circumstances where people may experience

various levels of physical or psychological discomfort when inan enclosed or small space, which is generally considered to bequite normal. When this discomfort becomes extreme, it isknown as claustrophobia. It is quite possible that susceptibilityto claustrophobia is not apparent at the start of employment. Itmay come about for the first time because of an incident whenworking within a confined space, e.g. panic if unable to extricateoneself from a fuel tank. If an engineer suffers an attack ofclaustrophobia, they should make their colleagues andsupervisors aware so that if tasks likely to generateclaustrophobia cannot be avoided, at least colleagues may beable to assist in extricating the engineer from the confined

space quickly, and sympathetically.

Engineers should work in a team and assist one another ifnecessary, making allowances for the fact that people come inall shapes and sizes and that it may be easier for one person to

access a space, than another. However, this should not be usedas an excuse for an engineer who has put on weight, to excusehimself from jobs which he would previously have been able todo with greater ease!

Claustrophobia can be defined as abnormal fear of being in anenclosed space.

Documents

Module 9 (Human Factors) Sub Module 9.2 (Human Performance and Limitations)_Rev 1_Sep 2013