Module 9.2 Human Factors PDF

MODULE 9 HUMAN FACTORS Licence category A, B1, B2, B2L, and B3 9.2 Human performance and limitations Module 9.2 Human performance and limitations Certification statement and objectives These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex II...

MODULE 9 HUMAN FACTORS Licence category A, B1, B2, B2L, and B3 9.2 Human performance and limitations Module 9.2 Human performance and limitations Certification statement and objectives These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, including the amendment Regulation (EU) 2023/989, and the associated Knowledge Levels as specified below: Part-66 Knowledge Levels Objective Ref. A, B1, B2, B2L, B3 Human performance and 9.2 2 limitations Vision; Hearing; Information processing; Attention and perception; Memory; Claustrophobia and physical access 2-2 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Table of contents Human performance and limitations _______________ 4 Memory ______________________________________ 34 Ultra-short-term memory _______________________ 34 Vision_________________________________________ 6 Short-term memory ___________________________ 34 Components of the eye _________________________ 6 Long-term memory ____________________________ 36 Common eye defects ___________________________ 8 Motor programmes ____________________________ 36 Blind spot ___________________________________ 10 Situation awareness ___________________________ 38 Factors affecting the clarity of sight _______________ 12 Information processing limitations ________________ 38 The nature of the object being viewed _____________ 13 Colour vision _________________________________ 14 Claustrophobia, physical access and other phobias _ 40 Vision and the aircraft maintenance engineer _______ 15 Physical access and claustrophobia ______________ 40 Fear of heights _______________________________ 42 Hearing ______________________________________ 16 Other phobias ________________________________ 42 Components of the ear _________________________ 16 Performance and limitations of the ear _____________ 18 Impact of noise on performance __________________ 18 Hearing impairment ___________________________ 18 High and low tone deafness _____________________ 22 Hearing protection ____________________________ 22 Presbycusis _________________________________ 22 Information processing _________________________ 24 An information processing model _________________ 24 Sensory receptors and sensory stores _____________ 24 Cognition ___________________________________ 26 Attention and perception ________________________ 28 Attention ____________________________________ 28 Perception __________________________________ 28 Decision making ______________________________ 29 Confirmation bias _____________________________ 30 Perception illusions ____________________________ 30 Attention and perception shortcomings ____________ 31 2-3 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Human performance and limitations This subsection intends to provide an overview of critical The aircraft engineer is the central part of the aircraft physical and mental human performance characteristics likely maintenance system. It is beneficial to understand how various to affect an aircraft maintenance engineer in their working parts of their body and mental processes function and how environment, such as vision, hearing, information processing, performance limitations can influence their effectiveness at attention and perception, memory, judgment and decision- work. making. Just as specific mechanical components used in aircraft maintenance engineering have limitations, engineers have certain capabilities and limitations that must be considered when looking at the maintenance engineering ‘system’. For instance, rivets used to attach aluminium skin to a fuselage can withstand forces that act to pull them apart. These rivets will eventually fail if enough force is applied to them. While the precise range of human capabilities and limitations might not be as well-defined as the performance range of mechanical or electrical components, the same principles apply in that human performance is likely to degrade and eventually ‘fail’ under certain conditions, e.g. stress. Mechanical components in aircraft can, on occasion, suffer catastrophic failures. Humans can also fail to function correctly in certain situations. Physically, humans become fatigued, are affected by the cold, and can break bones in workplace accidents. Mentally, humans can make errors, have limited perceptual powers, and exhibit poor judgment due to lacking skills and knowledge. In addition, unlike mechanical components, human performance is also affected by social and emotional factors. Therefore, failure by aircraft maintenance engineers can also be to the detriment of aircraft safety. 2-4 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations The aircraft engineer is the central part of the aircraft maintenance system. It is therefore very useful to have an understanding of how various parts of their body and mental processes function and how performance limitations can influence thier effectiveness at work. 2-5 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Vision To understand vision, it is helpful first to know a little about the The lens is thickened to focus clearly on a near object. The anatomy of the eye pictured below. The structure of the eye is lens is flattened to focus on a distant point. Factors such as similar to a camera, with an aperture, the iris, a lens, and a fatigue or ageing can affect the degree of accommodation. light-sensitive surface, the retina. Light enters the eye through the cornea, then passes through the iris and the lens and falls When a person is tired, accommodation is reduced, on the retina. Here the light stimulates the light-sensitive cells resulting in less sharp vision. Sharpness of vision is on the retina, rods and cones, and these pass tiny electrical known as visual acuity. impulses by the optic nerve to the visual cortex in the brain. Here, the electrical impulses are interpreted, and an image is The retina perceived. The retina is located on the rear wall of the eyeball. It comprises a complex layer of nerve cells connected to the Components of the eye optic nerve. Two types of light-sensitive cells are found in The cornea the retina – rods and cones. The central area of the retina The cornea is a clear ‘window’ at the very front of the eye. is known as the fovea, and the receptors in this area are all The cornea acts as a fixed focusing device. The shape of cones. It is here that the visual image is typically focused. the cornea bending the incoming light rays achieves the The cones become less dense, moving outwards and are focusing. The cornea is responsible for between 70% and progressively replaced by rods so that only rods are in the 80% of the eye’s total focusing ability (refraction). retina’s periphery. The iris and pupil Cones function in good light and are capable of detecting The iris, the coloured part of the eye, controls the light fine detail and are colour sensitive. This means the entering the eye. It does this by varying pupil size, the dark human eye can distinguish about 1,000 different shades area in the centre of the iris. The pupil size can be changed of colour. Rods cannot detect colour. They are poor at rapidly to cater to changing light levels. The amount of light distinguishing fine detail, but good at detecting movement can be adjusted by a factor of 5:1. in the edge of the visual field, peripheral vision. They are much more sensitive at lower light levels. As light The lens decreases, the sensing task is passed from the cones to After passing through the pupil, the light passes through the the rods. This means in poor light levels we see only in lens. Its shape is changed by the muscles (ciliary muscles) black and white and shades of grey. surrounding it, resulting in the final focusing adjustment to place a sharp image onto the retina. The change of shape of the lens is called accommodation. 2-6 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations The human eye 2-7 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Common eye defects The figures below depict the effects of extreme short- The diagrams below assume a relaxed eye, where the focal sightedness, long-sightedness and astigmatism on the paths of length of the eye’s interior lens is at a maximum. The eye’s parallel input rays, characteristic of objects at effectively infinite effort to view closer objects involves muscle tension, resulting distances, like looking at the moon. In these extreme cases, in the interior lens being more rounded and, therefore, of neither the long-sighted nor the short-sighted eye would see shorter focal length with greater refracting power. In the cases the moon clearly. depicted here, accommodation might be sufficient to bring the moon into focus for the long-sighted eye but would work against Short sight (near-sight) – known as myopia – is where the the short-sighted eye. eyeball is longer than expected, causing the image to be formed in front of the retina. If the accommodation of the lens If the object to be viewed is brought closer to the eye, you would cannot counteract this, then distant objects are blurred. expect the image to be worse for the long-sighted eye and better for the short-sighted eye. An example of an intermediate A concave lens overcomes short-sightedness by bending light distance might be instructive. outwards before it reaches the cornea. The lower figure below shows the nature of lenses, which would The short-sighted eye refracts the light too much, focusing on correct the vision by providing the appropriate amount of total a position before it reaches the image-detecting retina. The refraction to bring the parallel rays from a distant object to a long-sighted eye does not refract the light enough and cannot point on the retina. That is the typical strategy for prescribing bring the rays to focus by the time they reach the retina. glasses or contact lenses: correct the eye for clear vision at a great distance and then rely upon accommodation to allow the Long sight (far sight) – known as hypermetropia or person to see clearly at intermediate distances and up to a hyperopia – is caused by a shorter-than-usual eyeball which standard close-focus point. means that the image is formed behind the retina if the cornea and the lens cannot use their combined focusing ability to compensate for this; blurred vision results when looking at close objects. A convex lens overcomes long-sightedness by bending light inwards before it reaches the cornea. 2-8 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations 2-9 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Blind spot It occurs at the point where the optic nerve enters the retina (between the rods and cones). Facial features such as the nose also contribute to this problem. Hold the picture away and focus on the circle with your right eye. Move the page slowly towards your face; at some point, the triangle disappears – the blind spot. 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 with both eyes (binocular vision) since it is not possible for the image of an object to fall on the blind spots of both eyes simultaneously. Even when viewing with one eye (monocular vision), the constant rapid movement of the eye (saccades) means that the image does not fall on the blind spot all the time. It is only when viewing a stimulus that appears very fleetingly (e.g. a light flashing) that the blind spot may result in something not being seen. In maintenance engineering, close visual inspection or crack detection should not cause such problems, as the eye or eyes move across and around the area of interest (visual scanning). 2-10 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Standard test for the blind spot 2-11 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Factors affecting the clarity of sight The eye is very sensitive in clear air and good light conditions. Environmental factors such as: The eye has approximately 1.2 million nerve cells, from the retinas to the area of the brain responsible for vision. There are amount of light available; and only about 50,000 from the inner ears – making the eye about clarity of the air, e.g. dust, mist, rain. 24 times more sensitive than the ear. Factors associated with the object being viewed such as: Before considering factors that can influence and limit the size and contours of the object; performance of the eye, it is necessary to describe visual the contrast of the object with its surroundings; acuity. the relative motion of the object; Visual acuity is the ability of the eye to discriminate distance of the object from the viewer; and sharp detail at varying distances. the angle of the object from the viewer. An individual with an acuity of 20/20 vision should be able to Each of these factors is now examined in some detail. see at 20 feet, which the so-called ‘normal’ person can see at Other visual problems include: this range. It may be expressed in metres as 6/6 vision. The figures 20/40 mean that the observer can read at 20 feet what cataracts – a clouding of the lens usually associated with a ‘normal’ person can read at 40 feet. ageing; astigmatism – a misshapen cornea causing objects to Various factors can affect and limit the visual acuity of the eye. appear irregularly shaped; These include: glaucoma – a build-up in pressure of the fluid within the Physical factors such as: eye which can cause damage to the optic nerve and even blindness; and physical imperfections in one or both eyes (short- migraine – severe headaches that can cause visual sightedness, long-sightedness); and disturbances. age. Finally, as a person ages, the lens becomes less flexible, The influence of ingested foreign substances such as: meaning it cannot accommodate sufficiently. Consequently, after age 40, spectacles may be required for near vision, drugs; especially in poor light conditions. Fatigue can also temporarily medication; affect accommodation, causing blurred vision for close work. alcohol; and This is known as presbyopia and is a form of long-sightedness. cigarettes. 2-12 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Foreign substances Vision can be adversely affected by certain drugs and Any airborne particles such as dust, rain or mist can interfere medications, alcohol, and smoking cigarettes. With smoking, with light transmission, distorting what is seen. This can be carbon monoxide, which builds up in the bloodstream, allows even worse when spectacles are worn; they can get dirty, wet, less oxygen to be carried in the blood to the eyes. This is known misted, or scratched. Engineers who wear contact lenses as hypoxia and can rapidly impair the rods’ sensitivity. Alcohol (especially hard or gas-permeable types) should consider the can have similar effects, even hours after the last drink. advice from their optician associated with the maximum wear time – usually 8 to 12 hours – and consider the effects extended Environmental factors wear may have on the eyes, such as drying out and irritation. Vision can be improved by increasing the lighting level, but only This is particularly important if they are working in an up to a point, as the law of diminishing returns operates. Also, excessively dry or dusty environment, as airborne particles may increased illumination could result in increased glare. Older also affect contact lens wear. Goggles should be worn where people are more affected by the reflected light glare than necessary. younger people. Moving from a bright environment to a dimmer one has the effect of severely reduced vision until the eyes get The nature of the object being viewed used to less available light. This is because the eyes have Many factors associated with the object being viewed can also become light-adapted. If engineers work in a dark influence vision. We use information from the objects we are environment for a long time, their eyes gradually become dark looking at to help distinguish what we see. The movement and adapted, allowing better visual acuity. Consequently, moving relative motion of an object and the distance and angle of the between a bright hanger, or the inside of an aircraft, to a dark object from the viewer can all increase visual demands. These apron area at night can mean that the maintenance engineer are known as visual cues. Visual cues often refer to comparing must wait for their eyes to adjust; it can take about seven objects of known size to unknown objects. An example is that minutes for the cones and 30 minutes for the rods. we associate small objects with being further away. Similarly, if an object does not stand out well from its background (i.e. it has It is easier to focus in low light conditions if you look slightly to poor contrast with its surroundings), it is harder to distinguish one side of an object. This allows the image to fall outside the its edges and hence its shape. fovea and onto the part of the retina with many rods. 2-13 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Colour vision Although not directly affecting visual acuity, the aircraft It may be that the type and degree of colour deficiency are not maintenance engineer’s inability to see particular colours can relevant in their particular job. However, if accurate colour be problematic. Amongst other things, good colour vision for discrimination is critical for a job, it is essential that appropriate maintenance engineers is important for: testing and screening are put in place. recognising components; Colour loss at night distinguishing between wires; At night central vision is poor under low illumination. Better using various diagnostic tools; and results are obtained by looking slightly to one side of the object recognising various lights on the airfield. rather than directly at it. This permits better use of the peripheral vision by using rods instead of the central cones. Colour-defective vision is usually hereditary, although it may This effect can be demonstrated by counting a group of faint also be temporary after a serious illness. lights in the distance when looking directly at them. Then by looking some 10° to one side, it is possible to see more lights. Colour defective vision (normally referred to incorrectly as colour blindness, ‘Daltonism’) affects about 8% of men but Some people with perfect day vision may be myopic (near- only 0.5% of women. The most common type is difficulty in sighted) at night. Night myopia is little recognised but can distinguishing between red and green. More rarely, it is present a significant hazard, particularly because of the false possible to confuse blues and yellows. confidence instilled from having good vision by day. There are degrees of defective colour vision; some suffer more The reason for night myopia lies in the differing frequency of than others. Colour-defective people typically see the colours colours that prevail by night and the varying ability of the eyes they have problems with as shades of neutral grey. Individuals lens to focus them. Red and orange predominate by day, and may be able to distinguish between red and green in a well-lit a lens, whether natural or artificial, which can easily focus these situation but not in low-light conditions. wavelengths can be found wanting when it tries to focus on the more violet colours that prevail at night. In dim conditions, the Ageing also causes changes in colour vision. This results from lens has enough elasticity to focus the light from near objects progressive lens yellowing, resulting in a reduction in colour (thus near-sightedness) but cannot focus properly on objects discrimination in the blue-yellow range. Colour-defective vision further away. and its implications can be a complex area. Care should be taken not to stop engineers from performing specific tasks merely because they suffer from colour-deficient vision. 2-14 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Vision and the aircraft maintenance engineer It is essential for an engineer, particularly one who is involved in inspection tasks, to have adequate vision to meet the task requirements. As discussed previously, age and problems developing in the eye itself can gradually affect vision. Without regular vision testing, aircraft maintenance engineers may not notice their vision deteriorating. A reasonable standard of eyesight is needed for any aircraft engineer to perform their duties to an acceptable degree. Many maintenance tasks require a combination of both distance and near vision. In particular, such consideration must be made where there is a need for the close visual inspection of structures or work related to undersized or miniature components. Using glasses or contact lenses to correct any vision problems is perfectly acceptable, and indeed they must be worn as prescribed. Frequent checks should be made to ensure the continued adequacy of any glasses or contact lenses. In addition, colour discrimination may be necessary for an individual to drive in areas where aircraft manoeuvre or where colour coding is used, e.g. in aircraft wiring. Organisations should identify any specific eyesight requirement and put in place suitable procedures to address these issues. Often, airline companies or airports set the eyesight standards for reasons other than aircraft maintenance safety, e.g. for insurance purposes or for driving on the airfield. Ultimately, it is essential for the individual to recognise when their vision is adversely affected, either temporarily or permanently, and to consider the possible consequences carefully should they continue to work if the task requires good vision. 2-15 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Hearing The ear performs two entirely different functions. It detects The middle ear is usually filled with air refreshed by the sounds by receiving air vibrations; secondly, it is responsible eustachian tube that connects this part of the ear with the for balance and sensing acceleration. Of these two, the hearing back of the nose and mouth. However, this tube can allow aspect is more pertinent to the maintenance engineer, and thus mucus to travel to the middle ear, which can build up, it is necessary to have a basic appreciation of how the ear interfering with normal hearing. works. Inner ear As seen in the figure below, the ear has three divisions: the Unlike the middle ear, the inner ear is filled with fluid. The outer ear, middle ear and inner ear. These act to receive last of the ossicles in the middle ear is connected to the vibrations from the air and turn these signals into nerve cochlea. This contains a delicate membrane (the basilar impulses that the brain can recognise as sounds. membrane) covered in hair-like cells that are sensitive to fluid movement. Any vibrations they detect cause neural Components of the ear impulses to be transmitted to the brain via the auditory nerve. Outer ear The outer part of the ear directs sounds down the auditory The amount of vibration detected in the cochlea depends canal and onto the eardrum. The sound waves cause the on the volume and pitch of the original sound. eardrum to vibrate in response to sound waves. Middle ear Beyond the eardrum is the middle ear, which transmits vibrations from the eardrum through three tiny bones, known as the ossicles, to the inner ear’s fluid. The middle ear also contains two muscles which help to protect the ear from sounds above 80 dB using the acoustic or aural reflex, reducing the noise level by up to 20 dB. However, this protection can only be provided for about 15 minutes and does not protect against impulse noise such as gunfire. It does explain why a person is temporarily ‘deafened’ for a few seconds after a sudden loud noise. 2-16 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations The human ear 2-17 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Performance and limitations of the ear Hearing impairment The ear’s performance is associated with the range of sounds Hearing loss can result from exposure to even relatively short- heard in pitch and volume. duration noise. The degree of impairment is influenced mainly by the intensity of the noise. Such damage is known as noise- The audible frequency range that a young person can hear induced hearing loss (NIHL). The hearing loss can be is typically between 20 and 20,000 cycles per second (or temporary – lasting from a few seconds to a few days – or Hertz), with greatest sensitivity at about 3000 Hz. permanent. Temporary hearing loss may be caused by relatively short exposure to loud sound, as the hair-like cells on The volume of sound is measured in decibels (dB). The table the basilar membrane take time to recover. below shows intensity levels for various sounds and activities. With additional exposure, recovery gradually decreases, and Impact of noise on performance hearing loss becomes permanent. Thus, regular exposure to Noise can have various adverse effects in the workplace. It can: high noise levels over a long period may permanently damage be annoying – sudden sounds or constant loud noise; the hair-like cells in the cochlea, leading to irreversible hearing interfere with verbal communication between coworkers; impairment. cause accidents by masking warning signals or messages; be tiring and affect concentration and decision-making; and damage workers’ hearing, either temporarily or permanently. Noise tends to increase errors and variability rather than directly affect work rate. Intermittent and sudden noise is generally considered more disruptive than continuous noise at the same level. In addition, high-frequency noise generally adversely affects performance more than lower frequency. 2-18 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations A jet engine at take off is just above the ‘threshold of pain’ of 120 dB Hearing threshold is 0 dB, or 10-12 W/m2 Intensity levels of common sounds 2-19 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Noise at work regulations stipulate three levels of noise at which an employer must act: It is normally accepted that a TWA noise level exceeding 85 dB for 8 hours is hazardous and potentially damaging to the 85 decibels (if normal speech cannot be heard clearly inner ear. Exposure to noise in excess of 115 decibels at two metres), the employer must: without ear protection, even for a short duration, is not recommended. - assess the risk to employees’ hearing; - tell the employees about the risks and what precautions are proposed; and - provide their employees with personal ear protectors and explain their use. 90 decibels (if normal speech cannot be heard clearly at one metre), the employer must: - do all that is possible to reduce exposure to noise by means other than providing hearing protection; and - mark zones where noise reaches the second level, and provide recognised signs to restrict entry. 140 decibels (noise causes pain). The combination of duration and intensity of noise can be described as noise dose. Exposure to any sound over 80 dB constitutes a noise dose and can be measured over the day as an eight-hour time- weighted average sound level (TWA). For example, a person subjected to 95 decibels for 3.5 hours, then 105 decibels for 0.5 hours, then 85 decibels for 4 hours, results in a TWA of 93.5 which exceed the recommended maximum TWA of 90 decibels. Permanent hearing loss may occur if the TWA is above the recommended maximum. 2-20 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations A noise level meter 2-21 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations High and low tone deafness The average human ear is sensitive to frequencies between Presbycusis 20 Hz and 20,000 Hz, particularly in the 1,000 Hz to 4,000 Hz Hearing naturally deteriorates as one grows older. This is range and progressively less sensitive at higher and lower known as presbycusis. This affects the ability to hear high frequencies. This is important when measuring noise since two pitch sounds first and may occur gradually from the 30s sounds of equal intensity, but different frequencies may appear onwards. When this natural decline is exacerbated by noise- subjectively to be of different loudness. induced hearing loss, it can occur sooner. The cochlea has 23,000 nerve cells, each with about 100 The ability to hear an average conversational voice in a quiet sensory hairs. These hairs sense the vibration of the ossicles. room at a distance of 2 metres (6 feet) from the examiner is There are two sizes of hair; long, which detect low frequencies, recommended as a routine test. Failure of this test would and short, which detect high frequencies. Deterioration of the require an audiogram to be carried out to provide an objective sensory hairs occurs with overexposure to high levels of noise. assessment. If necessary, a hearing aid may be worn, but consideration should be given to the practicalities of wearing Hearing protection the aid during routine tasks. To a certain extent, hearing protection is available using ear plugs or defenders. The aircraft maintenance engineer must understand the limited ability of the ears to protect themselves from damage due to Noise levels can be reduced (attenuated) by up to 20 excessive noise. Even though engineers should be given decibels using ear plugs and 40 decibels using ear muffs. appropriate hearing protection and trained in its use, it is up to However, using ear protection will tend to adversely interfere individuals to ensure that they put this to good use. It is a with verbal communication. Despite this, it must be used misconception that the ears get used to constant noise: if this consistently and as instructed to be effective. noise is too loud, it will damage the ears gradually and insidiously. Hearing protection should always be used for noise, of any duration, above 115 dB. Reducing noise levels at the source or moving noise away from workers is good practice. Often this is not a practical option in the aviation maintenance environment. Referring to the sound level table, the aviation maintenance engineer will almost always need hearing protection when reasonably close – about 200-300 m – to aircraft whose engines are running. 2-22 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Ear muffs Ear plugs This sign displayed in a hangar or workshop indicates that ear protection must be worn 2-23 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Information processing The previous sections have described the essential functions As information is perceived, it is therefore stored in sensory and limitations of two of the senses used by aircraft memory automatically. Unlike other types of memory, sensory maintenance engineers. This section examines how the brain memory cannot be prolonged via rehearsal. processes the information gathered by the senses. Sensory memory is an ultra-short-term memory and decays or An information processing model degrades very quickly, typically in the region of 200 – 500 Information processing can be represented as a model. This milliseconds (1∕5 – ½ a second) after the perception of an item, captures the main elements of the process, from receipt of and certainly less than a second (although echoic memory is information via the senses to outputs such as decision-making now thought to last a little longer, up to perhaps three or four and actions. seconds). Indeed, it lasts for such a short time that it is often considered part of the process of perception. Still, it represents Sensory receptors and sensory stores an essential step for storing information in short-term memory. Physical stimuli are received via the sensory receptors and stored briefly in sensory stores or memory. The sensory memory for visual stimuli is sometimes known as the iconic memory, the memory for aural stimuli is known as Sensory memory is the shortest-term element of memory. It is the echoic memory, and that for touch is the haptic memory. the ability to retain impressions of sensory information after the The sense of smell may be more closely linked to memory than original stimuli have ended. It acts as a buffer for stimuli the other senses. The olfactory bulb and olfactory cortex, where received through the senses of sight, hearing, smell, taste and smell sensations are processed, are physically very close to the touch, which are retained accurately but briefly. The stimuli hippocampus and amygdala, which are involved in memory detected by our senses can be deliberately ignored, in which processes, separated by just 2 or 3 synapses. Thus, smells case they disappear almost instantaneously, or perceived, in may be more quickly and strongly associated with memories which case they enter our sensory memory. For example, the and emotions than the other senses. Memories of a smell may ability to look at something and remember what it looked like persist for longer, even without constant reconsolidation. with just a second of observation is an example of sensory memory. Information is passed from the sensory memory into short-term memory via attention, the cognitive process of selectively This does not require any conscious attention and, indeed, is concentrating on one aspect of the environment while ignoring usually considered to be outside of conscious control. The brain other things, which effectively filters the stimuli to only those of is designed only to process information that is useful at a later interest given time. date and to allow the rest to pass by unnoticed. 2-24 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations A functional model of human information processing 2-25 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Cognition The standard information processing model has three major components: Think back to a time when you were in a crowded room and a lot of people were talking. Were you paying attention to sensory memory; what each person was saying? Probably not, but what working memory (i.e., short-term memory); and happened when someone in the room said your name? I bet long-term memory. your ears perked up. Sensory memory, the first level of memory, allows us to take a Even though you may not have actively perceived the words ‘snapshot’ of our environment and store this information for a of the many conversations around you, your brain did. Most short period. The environment makes various available sources of that information was quickly discarded because your brain of information; light, sound, smell, heat, and cold, but the brain quickly judged you didn’t need it. The common pattern of only understands electrical energy. So, the body has special hearing your name, though, triggered something your brain sensory receptor cells that transduce this external energy to a might have thought was important and caused you to pay signal the brain can understand. Through transduction, a attention (the conscious act of recognizing a perceived memory is created. This memory is very short; less than half a stimulus) so you could move that information to your working second for vision, about three seconds for hearing, and about memory and decide what to do with it from there. ten seconds for touch. Once perceived, paying attention to something allows Perception is the process of recognising different stimuli. Can information to pass from sensory memory into working you imagine trying to process information about every single memory. Thus, attention serves as a filter for stimuli from our stimulus you experience? Your brain would quickly become environment. By selectively determining what will ‘get through’ overloaded. Fortunately, sensory memory is a buffer for stimuli for further examination and what will not, attention allows us to received through the five senses. A sensory memory exists for focus on only the necessary stimuli. each sensory channel: iconic memory for visual stimuli, echoic memory for aural stimuli and haptic memory for touch. A stimulus must first be perceived. For the information to be further processed, attention is vital. Attention is a cognitive process that allows us to focus on particular environmental stimuli. 2-26 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations 2-27 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Attention and perception Attention Sustained attention, as its name implies, refers to the Our mental resources are concentrated on specific elements ability to maintain attention and remain alert over long having detected information – this is attention. periods, often on one task. Most research has been carried out concerning monitoring radar displays, but Attention can be thought of as the concentration of mental associated research has concentrated on inspection effort on sensory or mental events. tasks. Although attention can move quickly from one item to another, Attention is influenced by arousal level and stress. This can it can only deal with one item simultaneously. Attention can improve attention or damage it depending on the take the form of: circumstances. Selective attention occurs when a person monitors Perception several input sources, with greater attention given to one Perception involves organising and interpreting sensory data to or more sources that appear more important. A person make it meaningful, discarding the irrelevant, and transforming can consciously attend to one source whilst still data into information. Perception is a highly sophisticated sampling other sources in the background. mechanism and requires existing knowledge and experience to Psychologists refer to this as the cocktail party effect, know what data to keep and discard and how to associate the whereby you can be engrossed in a conversation with data meaningfully. one person, but your attention is temporarily diverted if you overhear your name being mentioned at the other Perception can be defined as the process of assembling side of the room, even though you were not aware of sensations into a usable mental representation of the world. listening in to other people’s conversations. Distraction Perception creates faces, melodies, works of art and illusions is the negative side of selective attention. out of the raw material of sensation. Divided attention is typical in most work situations, where people must do more than one thing Examples of the perceptual process: simultaneously. Usually, one task suffers at the expense the image formed on the retina is inverted and two- of the other, more so if they are similar. This type of dimensional, yet we see the world the right way up and situation is also sometimes referred to as time sharing. in three dimensions; and Focused attention is merely focusing on a single eyes detect a constantly changing pattern of images as source and avoiding distraction. we move, yet we perceive things around us to have a set location rather than move chaotically. 2-28 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Decision making Having recognised coherent information from the stimuli Finally, appropriate action can be taken once a decision has reaching our senses, a course of action has to be decided been made. Our senses receive feedback on this and its result. upon. In other words, decision-making occurs. This helps to improve knowledge and refine future judgment by learning from experience. Decision making is the generation of alternative courses of action based on available information, knowledge, prior experience, expectation, context, goals, etc. and selecting one preferred option. It is also described as thinking, problem solving and judgment. This may range from deciding to do nothing to acting immediately in a particular manner. A fire alarm bell, for instance, may trigger a well-trained sequence of actions without further thought, i.e., evacuation. Alternatively, an unfamiliar siren may require further information to be gathered before initiating an appropriate course of action. We are not usually fully aware of the processes and information we use to make decisions. Tools can be used to assist the process of making a decision. For instance, in aircraft maintenance engineering, many documents – maintenance manuals, fault diagnosis manuals – and procedures are available to supplement the individual’s basic decision-making skills. Thus, sound decisions are based on knowledge supplemented by written information and procedures, analysis of observed symptoms, and performance indications. As will be shown in the ‘Information processing limitations’ section, it can be dangerous to believe that existing knowledge and prior experience are always sufficient in every situation. 2-29 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Confirmation bias A proportion of ‘sensed’ data may be lost without being It is often necessary to consult documents with which an ‘perceived’. An example with which most people are familiar is engineer can become familiar in aviation maintenance. It is failing to perceive something someone has said to you when possible that an engineer can scan a document and fail to you concentrate on something else, even though the words notice that subtle changes have been made. They only see would have been received at the ear without any problem. The what they expect to see (expectation). To illustrate how our other side of the coin is the ability of the information processing eyes can deceive us when quickly scanning a sentence, read system to perceive something, such as a picture, sentence, or the sentence on the importance of context quickly. concept, even though some of the data may be missing. The danger, however, is that people can fill in the gaps with At first, most people tend to notice nothing wrong with the information from their store of knowledge or experience, which sentence. Our perceptual system sub-consciously rejects the may lead to the wrong conclusion. additional “THE”. Once we have formed a mental model of a situation, we As an illustration of how expectation, can affect our often seek information which will confirm this model and, not judgment, the same video of a car accident was shown to consciously, reject information which suggests that this two groups of subjects. One group was told in advance that model is incorrect. they were to be shown a video of a car crash; the other was told that the car had been involved in a ‘bump’. Both groups Perception illusions were asked to judge the speed at which the vehicles had Many well-known visual ‘illusions’ illustrate the limits of human collided. The first group assessed the speed as significantly perception. The Muller-Lyer illusion shows how the perceptual higher than the second group. system can be misled into believing that one line is longer than the other, even though a rule confirms that they are precisely Expectations can also affect our memory of events. The study the same. outlined above was extended such that subjects were asked, a week later, whether they recalled seeing glass on the road after The importance of context illustrates that we can perceive the the collision. (There was no glass). The group who had been same thing quite differently, i.e., the letter B or the number 13. told they would see a crash recalled seeing glass; the other This shows the influence of context on our information group recalled seeing no glass. processing. 2-30 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations The Muller-Lyer illusion The effects of expectation The importance of context 2-31 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Attention and perception shortcomings Attention and perception shortcomings can impinge on Additionally, if unsure of the accuracy of memorised decision-making. Misperceiving something may mean an information, the engineer should seek to check it, even if this incorrect decision is made, resulting in an inappropriate action. means going elsewhere to do so. Temporarily noting something The figure below shows the dependence on memory to make down can avoid the risk of forgetting or confusing information. decisions. It was explained earlier that sensory and short-term However, using a personal notebook to capture such memories are limited in terms of capacity and duration. It is also information permanently can be dangerous, as its information important to bear in mind that human memory is fallible, so that may become outdated. information: may not be stored; may be stored incorrectly; and may be difficult to retrieve. In the B737 double engine oil loss incident, the AAIB report stated: All these may be called forgetting, which occurs when information is unavailable (not stored in the first place) or “Once the controller and fitter had got to T2 and found that inaccessible (cannot be retrieved). Information in short-term this supportive material [task cards and AMM extracts] was memory is particularly susceptible to interference, an example not available in the work-pack, they would have had to return of which would be trying to remember a part number whilst to base engineering or to have gone over to the line trying to recall a telephone number. maintenance office to get it. It would be, in some measure, understandable for them to have a reluctance to re-cross the It is generally better to use manuals and temporary aide- exposed apron area on a winter’s night to obtain a memoires rather than rely upon memory, even when the description of what they were fairly confident they knew information to be remembered or recalled is relatively simple. anyway. However, during the course of the night, both of them had occasion to return to the base maintenance hangar For instance, an aircraft maintenance engineer may think they a number of times before the task had been completed. will remember a torque setting without writing it down. Still, Either could, therefore, have referred to or even drawn the between consulting the manual and walking to the aircraft task descriptive papers before the job was signed off. The (possibly stopping to talk to someone on the way), they may question that should be addressed, therefore, is whether forget the setting or confuse it, possibly with a different torque there might be any factors other than overconfidence in their setting appropriate to a similar task with which they are more memories, bad judgment or idleness which would dispose familiar. them to pass up these opportunities to refresh their memories on the proper and complete procedures.” 2-32 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Summary of information processing 2-33 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Memory Memory is critical to our ability to act consistently and to learn Short-term memory new things. Without memory, we could not capture a ‘stream’ Short-term memory receives a proportion of the information of information reaching our senses, draw on the experience, received into sensory stores and allows us to store information and apply this knowledge when making decisions. long enough to use it (hence the idea of ‘working memory’). It can store only a relatively small amount of information at one Memory can be considered to be the storage and retention time, i.e., five to nine (often referred to as 7 ±2) items, for a of information, experiences and knowledge, as well as the short duration, typically 10 to 20 seconds. The following ability to retrieve this information. example shows that short-term memory capacity can be enhanced by splitting information into ‘chunks’ (a group of Memory depends on three processes: related items). registration – the input of information into memory; The telephone number 01222555234, can be stored as 11 storage – the retention of information; and discrete digits, in which case it is unlikely to be remembered. retrieval – the recovery of stored information. Alternatively, it can be stored in chunks of related information. In the UK, 01222 may be stored as one chunk, It is possible to distinguish between three forms of memory: 555 as another, and 234 as another, using only three chunks and therefore, more likely to be remembered. In mainland ultra-short-term memory (or sensory memory); Europe, the same telephone number would probably be short-term memory (often referred to as working stored as 01 22 25 55 23 4, using six chunks. The size of the memory); and chunk will be determined by the individual’s familiarity with long-term memory. the information (based on prior experience and context), Ultra-short-term memory thus in this example, a person from the UK might recognise Ultra-short-term memory has already been described when 0208 as the code for London, but a person from mainland examining the role of sensory stores. It lasts up to two Europe might not. seconds (depending on the sense) and is used as a buffer, giving us time to attend to sensory input. Short-term memory can be extended through rehearsal – mental repetition of the information – or encoding the information meaningfully – e.g., associating it with something, as in the example above. 2-34 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations 2-35 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Long-term memory Motor programmes The capacity of long-term memory appears to be unlimited. It If a task is performed often enough, it may eventually become is used to store information that is not currently being used, automatic, and the required skills and actions are stored in including: long-term memory. These are known as motor programmes and are ingrained routines that have been established through knowledge of the physical world and objects within it and practice. Using a motor programme reduces the load on the how these behave; central decision-maker. An often-quoted example is that of personal experiences; driving a car. At first, each action, such as gear changing, is beliefs about people, social norms, and values; demanding, but eventually, the separate actions are combined motor programmes, problem-solving skills and plans for into a motor programme. They can be performed with little or achieving various activities; and no awareness. abilities, such as language comprehension. These motor programmes allow us to carry out simultaneous Information in long-term memory can be divided into two types: activities, such as conversing whilst driving. Semantic memory stores general, factual knowledge about the world, such as concepts, rules, and language. It is information that is not tied to where and when the knowledge was initially acquired. Episodic memory refers to the memory of specific events, such as past experiences (including people, events and objects). We can usually place these things within a particular context. It is believed that episodic memory is heavily influenced by a person’s expectations of what should have happened; thus, two people’s recollections of the same event can differ. 2-36 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Sections of the memory 2-37 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Situation awareness The process of attention, perception and judgment should As with decision-making, feedback improves situation result in awareness of the current situation. awareness by informing us of the accuracy of our mental models and their predictive power. The ability to project system Situation awareness is the synthesis of an accurate and up- status backwards, to determine what events may have led to to-date ‘mental model’ of one’s environment and state, and an observed system state, is also very important in aircraft the ability to use this to make predictions of possible future maintenance engineering, as it allows effective fault finding and states. diagnostic behaviour. Situation awareness has traditionally been used in the context Situation awareness for the aircraft maintenance engineer can of the flight deck to describe the pilot’s awareness of what is be summarised as: happening around them, where they are geographically, their orientation in space and what mode the aircraft is in. In the the status of the system the engineer is working on; maintenance engineering context, it refers to: the relationship between the reported defect and the intended rectification; the perception of important elements, e.g. seeing loose the possible effect of this work on other systems; and bolts or missing parts, hearing information passed the effect of this work on that being done by others and verbally; vice versa. the comprehension of their meaning, e.g. why is it like this? Is this how it should be?; and Information processing limitations the projection of their status into the future, e.g. future The basic elements of human information processing have now effects on safety, schedule, and airworthiness. been explored. It is important to appreciate that these elements have limitations. Consequently, like other skilled professionals, An example is an engineer seeing (or perceiving) blue the aircraft engineer requires support such as references to streaks on the fuselage. Their comprehension may be that written material, e.g., manuals. the lavatory fill cap could be missing or the drain line leaking. If their situation awareness is good, they may appreciate that This suggests that in aircraft maintenance engineering, the such a leak could allow blue water to freeze, leading to entire team needs to have situation awareness – not just of airframe or engine damage. what they are doing individually, but of their colleagues’ activities as well. 2-38 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations 2-39 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Claustrophobia, physical access and other phobias Although not peculiar to aircraft maintenance engineering, Engineers should work in a team and assist one another if working in restricted spaces and at heights is a feature of this necessary, making allowances for the fact that people come in trade. Problems associated with physical access are not all shapes and sizes and that it may be easier for one person uncommon. Maintenance engineers and technicians often to access a space than another. However, this should not be have to access and work in very small spaces (e.g. in fuel used as an excuse for an engineer who has put on weight to tanks), cramped conditions (such as beneath flight instrument excuse himself from jobs they would previously have been able panels, around rudder pedals), elevated locations (on cherry- to do with greater ease! pickers or staging), sometimes in uncomfortable climatic or environmental conditions (heat, cold, wind, rain, noise). This can be aggravated by poor lighting or having to wear breathing apparatus. Physical access and claustrophobia In many circumstances, people may experience various levels of physical or psychological discomfort when in an enclosed or small space, which is generally considered normal. When this discomfort becomes extreme, it is known as claustrophobia. Claustrophobia can be defined as abnormal fear of being in an enclosed space. Susceptibility to claustrophobia may not be apparent at the start of employment. It may come about for the first time because of an incident when working within a confined space, e.g. panic if unable to extricate oneself from a fuel tank. If an engineer suffers an attack of claustrophobia, they should make their colleagues and supervisors aware so that if tasks likely to generate claustrophobia cannot be avoided, at least colleagues may be able to assist in extricating the engineer from the confined space quickly, and sympathetically. 2-40 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations There are many circumstances where people may experience various levels of physical or psychological discomfort when in an enclosed or small space, which is generally considered to be quite normal. When this discomfort becomes extreme, it is known as claustrophobia 2-41 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024 Module 9.2 Human performance and limitations Fear of heights Working at significant heights can also be problematic for some “Inspection of the rivets required inspectors to climb on aircraft maintenance engineers, especially when doing ‘crown’ scaffolding and move along the upper fuselage carrying a inspections (top of the fuselage). Some engineers may be quite bright light with them; in the case of an eddy current at ease in situations like these, whereas others may be so inspection, the inspectors needed a probe, a meter, and a uncomfortable that they are far more concerned about the light. At times, the inspector needed ropes attached to the height and holding on to the access equipment than they are rafters of the hangar to prevent falling from the airplane when about the job. In such situations, harnesses and safety ropes it was necessary to inspect rivet lines on top of the fuselage. must be used appropriately. These will not necessarily remove Even if the temperatures were comfortable and the lighting the fear of heights but will help reassure the engineer and allow was good, the task of examining the area around one rivet them to concentrate on the task. after another for signs of minute cracks while standing on Ultimately, if an engineer finds working high up brings on scaffolding or on top of the fuselage is very tedious. After phobic symptoms such as severe anxiety and panic, they examining more and more rivets and finding no cracks, it is should avoid such situations for safety’s sake. However, as with natural to begin to expect that cracks will not be found.” claustrophobia, support from team members can be helpful. Other phobias Shortly before the Aloha accident, during maintenance, the There is a name for almost every fear. Some are rare, others inspector needed ropes attached to the rafters of the hangar to are common, and many are irrational, but many are rational prevent falling from the aircraft when it was necessary to fears. There are far too many to list here. inspect rivet lines on top of the fuselage. Although unavoidable, this would not have been conducive to ensuring that the See http://phobialist.com for the complete list. inspection was carried out meticulously, nor was it, as the subsequent accident investigation revealed. The NTSB investigation report stated: “Managers and supervisors should attempt to make the job as comfortable and secure as reasonably possible (e.g. providing knee pad rests, ensuring that staging does not wobble, providing ventilation in enclosed spaces, etc.) and allow for frequent breaks if practicable.” 2-42 Copyright 2024 © Joramco Academy FOR TRAINING PURPOSE ONLY Issue 2 – June 2024