NG2S419 Module 9: Managing Human Factors | PDF

USW Aviation Academy Engineering Training Module: NG2S419 Managing Human Factors PART- 66 Module 9 NG2S419 Module 9 Revision 8 Page 1 of 164 Oct 2024 USW Aviation Academy Engineering Training DISCLAIMER Module Notes The information contained within this document is for TRAINING USE ONLY. These training notes should not be used for carrying out any work or procedure on ANY aircraft. You must always use the correct aircraft maintenance manual or equipment manufacturer’s handbook. You should abide by the rules set out by your regulatory authority and as laid down in the company policy where you are working. All reports, documentation, etc. must be in compliance with your organisation. For Health and Safety always follow the guidance laid down by the equipment manufacturer, company policy, national safety policies and national governments. COPYRIGHT NOTICE The distribution or reproduction of these notes in any form is forbidden without the express permission of the University of South Wales. QUALITY NOTICE The University of South Wales is committed to delivering a quality learning experience. Should you have any feedback about this document, please contact the University of South Wales Aviation Academy training team. Approval References CAA UK.147.0111 and EASA.147.0194 Bethan Llewellyn Training Manager Aviation Academy University of South Wales Treforest Campus Pontypridd CF37 1DL NG2S419 Module 9 Revision 8 Page 2 of 164 Oct 2024 USW Aviation Academy Engineering Training Annual Review Log Revision Revision Changes Made Name Number Date 01 Aug 2017 Initial Issue M Bates 02 Jun 2019 Added sleep time M Bates 03 Jul 2020 Adult/Child Model added M Bates 04 Jul 2021 Spelling Errors M Bates 05 Jul 2022 Annual Review Added M Bates 06 Jul 2023 Review and clarification M Bates Updated CHIRP and ideal blood pressure. 07 Jul 2024 M Bates Added Risk Mitigation Methods Added Safety Management to Chapter 9.9 08 Oct 2024 M Bates to satisfy EASA and UKCAA updates. NG2S419 Module 9 Revision 8 Page 3 of 164 Oct 2024 USW Aviation Academy Engineering Training Table of Contents DISCLAIMER......................................................................................................... 2 Foreword.............................................................................................................. 10 9.1 Introduction to Human Factors....................................................................... 11 What is Human Factors?.................................................................................. 11 The Need to Take Human Factors into Account............................................... 11 The SHEL Model............................................................................................... 12 Incidents Attributable to Human Factors/Human Error...................................... 13 ‘Murphy’s’ Law.................................................................................................. 16 Swiss Cheese Model........................................................................................ 16 The Error Chain................................................................................................ 18 Accident Statistics............................................................................................. 19 Conclusion........................................................................................................ 19 9.2 Human Performance and Limitations............................................................. 20 Vision................................................................................................................ 20 The Cornea................................................................................................... 21 The Iris and Pupil.......................................................................................... 21 The Lens....................................................................................................... 21 The Retina..................................................................................................... 22 Hearing............................................................................................................. 26 Outer Ear....................................................................................................... 26 Middle Ear..................................................................................................... 26 Inner Ear....................................................................................................... 27 Performance and Limitations of the Ear........................................................ 27 Damage to the Ears...................................................................................... 28 Hearing Loss................................................................................................. 29 Ear Protection................................................................................................ 30 Other Senses................................................................................................ 30 Other factors.................................................................................................. 31 Information Processing..................................................................................... 34 An Information Processing Model.................................................................. 34 Sensory Receptors and Sensory Stores....................................................... 34 Attention and Perception................................................................................... 36 NG2S419 Module 9 Revision 8 Page 4 of 164 Oct 2024 USW Aviation Academy Engineering Training Factors Affecting Attention............................................................................ 37 Decision Making............................................................................................ 38 Memory............................................................................................................. 39 Information Processing.................................................................................. 41 Perception Examples.................................................................................... 42 Forgetting...................................................................................................... 44 Feedback...................................................................................................... 44 Response Time............................................................................................. 44 Acrophobia, Claustrophobia and Physical Access............................................ 45 9.3 Social Psychology.......................................................................................... 47 Responsibility: Individual and Group................................................................. 47 Motivation and De-motivation........................................................................... 48 Maslow’s Hierarchy of Needs........................................................................ 49 Liaison/Dissemination of Information............................................................. 53 Peer Pressure................................................................................................... 53 Experiments in Conformity............................................................................ 53 ‘Culture’ Issues................................................................................................. 55 Safety Culture................................................................................................ 55 Social Culture................................................................................................ 57 Team Working................................................................................................... 57 Management, Supervision and Leadership....................................................... 58 The Management Role.................................................................................. 58 The Supervisory Role.................................................................................... 59 Characteristics of a Leader........................................................................... 60 9.4 Factors Affecting Performance....................................................................... 62 Fitness and Health............................................................................................ 62 Personal Health............................................................................................. 62 Stress: Domestic and Work Related................................................................. 64 Causes and Symptoms................................................................................. 64 Time Pressure and Deadlines........................................................................... 67 Effects of Time-pressure and Deadlines....................................................... 68 Managing Time Pressure and Deadlines...................................................... 69 Workload: Overload and Underload.................................................................. 70 NG2S419 Module 9 Revision 8 Page 5 of 164 Oct 2024 USW Aviation Academy Engineering Training Arousal.......................................................................................................... 70 Factors Determining Workload...................................................................... 71 Sleep, Fatigue and Shift work........................................................................... 74 What is Sleep?.............................................................................................. 74 Circadian Rhythms........................................................................................ 75 Fatigue.......................................................................................................... 76 Shift Work...................................................................................................... 77 Advantages and Disadvantages of Shift Work.............................................. 78 Working at Night............................................................................................ 78 Rolling Shift Patterns..................................................................................... 79 Continuity of Tasks and Shift Handovers...................................................... 79 Sleep, Fatigue, Shift Work and the Maintenance Engineer........................... 79 Alcohol, Medication, Drug Abuse...................................................................... 80 Alcohol.......................................................................................................... 80 Drugs............................................................................................................. 81 Medication..................................................................................................... 81 9.5 Physical Environment..................................................................................... 84 Noise................................................................................................................. 84 Fumes............................................................................................................... 85 Illumination........................................................................................................ 87 Climate and Temperature................................................................................. 88 Motion............................................................................................................... 90 Vibration............................................................................................................ 90 Confined Spaces............................................................................................... 91 Working Environment........................................................................................ 92 9.6 Tasks.............................................................................................................. 94 Physical Work................................................................................................... 94 Planning........................................................................................................ 94 Physical Tasks.............................................................................................. 95 Repetitive Tasks............................................................................................... 98 Visual Inspection............................................................................................... 99 Complex Systems........................................................................................... 101 9.7 Communication............................................................................................. 104 NG2S419 Module 9 Revision 8 Page 6 of 164 Oct 2024 USW Aviation Academy Engineering Training Communication Within and Between Teams.................................................. 104 Modes of Communication............................................................................ 105 Verbal & Written Communication................................................................ 105 Non-verbal Communication......................................................................... 106 Communication within Teams..................................................................... 106 Communication between Teams................................................................. 107 Flow of Communication.................................................................................. 108 Downwards.................................................................................................. 108 Upwards...................................................................................................... 109 Sideways..................................................................................................... 109 Shift Handovers.............................................................................................. 110 Work Logging and Recording......................................................................... 111 Keeping Up to Date, Currency........................................................................ 113 Maintaining Currency.................................................................................. 113 Currency Responsibilities............................................................................ 113 Dissemination of Information.......................................................................... 114 Poorly Disseminated Information Cause Accident....................................... 115 9.8 Human Error................................................................................................. 116 Error Models and Theories............................................................................. 116 Design versus Operator-Induced Errors...................................................... 116 Variable versus Constant Errors.................................................................. 117 Reversible versus Irreversible Errors.......................................................... 117 Slips, Lapses and Mistakes......................................................................... 118 Violations..................................................................................................... 119 Skill-Based Behaviours & Associated Errors............................................... 119 Rule Based Behaviours & Associated Errors.............................................. 120 Knowledge-Based Behaviours & Associated Errors.................................... 120 The Swiss Cheese Model............................................................................ 120 Types of Error in Maintenance Tasks............................................................. 121 Errors during Regular and Less Frequent Maintenance Tasks................... 122 Violation in Aircraft Maintenance................................................................. 122 Errors Due to Individual Practices and Habits............................................. 124 Errors Associated with Visual Inspection..................................................... 124 NG2S419 Module 9 Revision 8 Page 7 of 164 Oct 2024 USW Aviation Academy Engineering Training Reason’s Study of Aviation Maintenance Engineering................................ 125 Implications of Errors (i.e. Accidents).............................................................. 125 Mandatory Occurrence Reporting Scheme (MORS)................................... 126 Confidential Human Factors Incident Reporting Programme (CHIRP)........ 126 Just Culture................................................................................................. 126 Avoiding and Managing Errors........................................................................ 128 9.9 Safety Management..................................................................................... 130 Hazards on the Workplace.............................................................................. 130 Recognising and Avoiding Hazards............................................................. 130 Safety Management........................................................................................ 136 Safety Risk Management............................................................................ 136 Identification of Safety Issues...................................................................... 137 Assessment of Safety Issues...................................................................... 137 Definition and Programming of Safety Actions............................................ 138 Implementation and Follow-Up.................................................................... 139 Safety Performance Measurement.............................................................. 139 Occurrence Reporting and Just Culture.......................................................... 139 Dealing with Emergencies.............................................................................. 140 Actions to be taken in an Emergency.......................................................... 140 9.10 The Dirty Dozen.......................................................................................... 141 Risk Mitigation Methods (EASA Only)............................................................. 145 Distraction................................................................................................... 145 Communication........................................................................................... 145 Environment................................................................................................ 146 Competence and Confidence...................................................................... 146 Third Party................................................................................................... 146 Fatigue........................................................................................................ 146 Psychosocial Risks...................................................................................... 147 Tools & Techniques..................................................................................... 147 The Adult and the Child Model (NOT ON EXAM)........................................... 148 Early Life Decisions..................................................................................... 149 Lack of Communication............................................................................... 150 Complacency............................................................................................... 152 NG2S419 Module 9 Revision 8 Page 8 of 164 Oct 2024 USW Aviation Academy Engineering Training Lack of Knowledge...................................................................................... 152 Distraction................................................................................................... 152 Lack of Teamwork....................................................................................... 152 Fatigue........................................................................................................ 152 Lack of Resources....................................................................................... 153 Pressure...................................................................................................... 153 Lack of Assertiveness................................................................................. 154 Stress.......................................................................................................... 154 Lack of Awareness...................................................................................... 155 Norms.......................................................................................................... 155 Conclusion.................................................................................................. 155 9.11 General Safety Precautions........................................................................ 156 Hand-Overs and Shift Changes...................................................................... 157 Summary..................................................................................................... 159 9.12 Dealing with Emergencies.......................................................................... 160 Electrocution................................................................................................... 160 Handling a Fire................................................................................................ 160 In a Building:................................................................................................ 160 In an Aircraft/Engine:................................................................................... 161 Aircraft Recovery......................................................................................... 161 Actions on Handling a Security Alert............................................................... 161 References......................................................................................................... 163 Appendix 1......................................................................................................... 164 NG2S419 Module 9 Revision 8 Page 9 of 164 Oct 2024 USW Aviation Academy Engineering Training Foreword Much of the information presented here is taken from: CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors. This is written mainly from the perspective of the Licenced Engineer and addresses human performance and limitations. CAP 716 Aviation Maintenance Human Factors. This is written mainly from the perspective of the organisation maintenance management within Part 145 organisations. These documents are available to download at the CAA website: www.caa.co.uk Also see the Easy Access Rules for Continuing Airworthiness (Regulation (EU) No 1321/2014) available at: Easy Access Rules for Continuing Airworthiness (Regulation (EU) No 1321/2014) (europa.eu) Please regurlarly check these websites for updates to these documents. NG2S419 Module 9 Revision 8 Page 10 of 164 Oct 2024 USW Aviation Academy Engineering Training 9.1 Introduction to Human Factors “An understanding of the importance of human factors to aircraft maintenance engineering is essential to anyone considering a career as a licensed aircraft engineer. Human factors impinges on everything an engineer does in the course of their job in one way or another, from communicating effectively with colleagues to ensuring they have adequate lighting to carry out their tasks. Knowledge of this subject has a significant impact on the safety standards expected of the aircraft maintenance engineer.” (Civil Aviation Authority, 2002) In the early days of flight, when flying was very much “seat of your pants”, aircraft were structurally very unsafe. Most accidents were caused by mechanical failures. However, with the vast improvements in aircraft engineering, human factor incidents are now responsible for approximately 70% of all accidents. The study of Human Factors has, therefore, become a large part of the industry and training is required in an attempt to limit these failings and make flying safer. What is Human Factors? Human Factors is concerned with the relationship between the worker and his surroundings. Its study, called Ergonomics, is concerned with safety and improving output in the workplace. The “person” is comprised of two parts, the physical body and the mental psychological part. Surroundings are comprised of non-working environment and working environment. The working environment can be further split into: Physical side – lighting, temperature, noise, dust, etc. Psychological side – pay, management, workmates, shift patterns, etc. Our main concern is the relationship between the person and his/her work environment, however, the physical condition and mental attitude of a person does not depend on this alone, but is affected by hereditary factors, historical factors and the non-work environment. There is not much that can be done about the hereditary factors but historical factors, such as education, can be overcome with appropriate training. Also note that we cannot ignore the domestic environment either. Things such as divorce, sickness of children and other forms of domestic stress are a large part of workers’ life as well. The Need to Take Human Factors into Account "Human factors" refers to the study of human capabilities and limitations in the workplace. Human factors researchers study system performance. That is, they study the interaction of maintenance personnel, the equipment they use, the written and verbal procedures and rules they follow, and the environmental NG2S419 Module 9 Revision 8 Page 11 of 164 Oct 2024 USW Aviation Academy Engineering Training conditions of any system. The aim of human factors is to optimise the relationship between maintenance personnel and systems with a view to improving safety, efficiency and well-being” (Civil Aviation Authority, 2002) Initially, Human Factors dealt with pilots and air traffic controllers to improve their performance and increase safety. This has now been expanded to include the Aircraft Maintenance Engineer to increase performance, reliability and safety. Today the study of Human Factors includes: Anthropometrics – Can the pilot reach the switch? Human Perception Dynamics – Flashing lights, audio warnings. Biometrics and Physiology – Age, fatigue. Cognition – How we learn and remember things. Work Design – Procedures and workflow. Facility Design – Physical workplace, lighting, space. Work Related Stress – Unattainable goals, not enough time to complete task. Environment – Weather. Workplace Interactions – Communication between individuals and teams. Workplace Diversity – The differences between us. Human Factors refers to the study of human capabilities and limitations in the workplace. Human Factor researchers study system performance. These include: The interaction of maintenance personnel. The equipment that they use. The written and verbal procedures that they follow. The environmental conditions of any system. The SHEL Model H S L E L Figure 1 - The SHEL Model NG2S419 Module 9 Revision 8 Page 12 of 164 Oct 2024 USW Aviation Academy Engineering Training The model was developed to help understand the various interactions between the engineer and his/her workplace surroundings. The L in the middle represents the worker and the other letters feed in and out of the worker. They are: S – Software. Manuals, checklists, software data on computers, documentation, etc. H – Hardware. Tools, test equipment, aircraft, engines, ground equipment, components, etc. E – Environment. Physical environment such as temperature, lighting, location, rain, noise, etc. Work environment such as shift pattern, management, peer pressure, job satisfaction, social background, etc. L – Liveware. The people that you interact with in the performance of your job. Qualifications, work attitudes, enough personnel to carry out the task, shift handovers, etc. Incidents Attributable to Human Factors/Human Error In two surveys (1940 and 1975) it was found that about 70% of all accidents were attributable to human error. A study was carried out in 1986, in the USA by Sears, looking at significant accident causes in 93 aircraft accidents. These were as follows: Causes/ major contributory factors % of accidents in which this was a factor Pilot deviated from basic operational procedures 33% Inadequate cross-check by second crew member 26% Design faults 13% Maintenance and inspection deficiencies 12% Absence of approach guidance 10% Captain ignored crew inputs 10% Air traffic control failures or errors 9% Improper crew response during abnormal conditions 9% Insufficient or incorrect weather information 8% Runways hazards 7% Air traffic control/crew communication deficiencies 6% Improper decision to land 6% Note: The right-hand column shows the percentage of accidents that are a factor of the left hand column. Some overlap. NG2S419 Module 9 Revision 8 Page 13 of 164 Oct 2024 USW Aviation Academy Engineering Training Figure 2 - Aloha Airlines 1988 Aloha Airlines is an example of one such incident: “The accident involving Aloha flight 243 in April 1988 involved 18 feet of the upper cabin structure suddenly being ripped away in-flight due to structural failure. The Boeing 737 involved in this accident had been examined, as required by US regulations, by two of the engineering inspectors. One inspector had 22 years’ experience and the other, the chief inspector, had 33 years’ experience. Neither found any cracks in their inspection. Post-accident analysis determined there were over 240 cracks in the skin of this aircraft at the time of the inspection. The ensuing investigation identified many human-factors-related problems leading to the failed inspections. As a result of the Aloha accident, the US instigated a programme of research looking into the problems associated with human factors and aircraft maintenance, with particular emphasis upon inspection.” (Civil Aviation Authority, 2002) Below is an incident that happened on BA5390 on 10th June 1990. NG2S419 Module 9 Revision 8 Page 14 of 164 Oct 2024 USW Aviation Academy Engineering Training Figure 3 - BA flight 5390 “On June 10th, 1990 in the UK, a BAC1-11 (British Airways flight 5390) was climbing through 17,300 feet on departure from Birmingham International Airport when the left windscreen, which had been replaced prior to flight, was blown out under the effects of cabin pressure when it overcame the retention of the securing bolts, 84 of which, out of a total of 90, were smaller than the specified diameter. The commander was sucked halfway out of the windscreen aperture and was restrained by cabin crew whilst the co-pilot flew the aircraft to a safe landing at Southampton Airport. The Shift Maintenance Manager (SMM), short-handed on a night shift, had decided to carry out the windscreen replacement himself. He consulted the Maintenance Manual (MM) and concluded that it was a straightforward job. He decided to replace the old bolts and, taking one of the bolts with him (a 7D), he looked for replacements. The storeman advised him that the job required 8Ds, but since there were not enough 8Ds, the SMM decided that 7Ds would do (since these had been in place previously). However, he used sight and touch to match the bolts and, erroneously, selected 8Cs instead, which were longer but thinner. He failed to notice that the countersink was lower than it should be, once the bolts were in position. He completed the job himself and signed it off, the procedures not requiring a pressure check or duplicated check. There were several human factors issues contributing to this incident, including perceptual errors made by the SMM when identifying the replacement bolts, poor lighting in the stores area, failure to wear spectacles, circadian effects, working practices, and possible organisational and design factors.” (Civil Aviation Authority, 2002) NG2S419 Module 9 Revision 8 Page 15 of 164 Oct 2024 USW Aviation Academy Engineering Training ‘Murphy’s’ Law If anything can go wrong, then it will go wrong. If there is a task that can be performed incorrectly then someone will do it incorrectly. In an ideal world the designer of the aircraft/systems should make most tasks ‘Murphy Proof’ (the hydraulic quick release connections are all different sizes (pressure, suction and return)), for example, so they cannot be fitted incorrectly. Another example is the battery connector on an aircraft - it can only be fitted the correct way round avoiding incorrect polarity connection. This type of solution is not always possible – so we have to be forever aware that a task can often be performed wrongly. If the engineer is aware of this and is aware of the dangers and the checks to carry out to minimise these possible errors then accidents can be reduced. Some of these errors can be overcome from a manufacturing perspective, e.g. having keyways so that only one LRU can be fitted in that space or keyed plugs so that they only connect in one place. However, not all actions can be controlled in this way. Swiss Cheese Model Accidents generally come about by a number of errors culminating in the final incident. Accidents that occur because of one error are rare and most are the result of several quite unrelated errors. Sometimes things missed years before are, along with a recent error, the cause of the incident. It can best be explained with the aid of a diagram as shown in Figure 4 - Swiss Cheese Model. Record keeping is vital in crash investigation as all work, over the life of the aircraft, can be accessed and examined. The whole history of the aircraft and its components can be traced back to the smallest rivet. In this way if anything goes wrong then records will show the history and, hopefully, where the error lies. The idea is not to use this information to “find the culprit” and punish them, rather it is there to ensure it does not happen again and make flying safer. It is better to learn by our mistakes than keep making the same error. NG2S419 Module 9 Revision 8 Page 16 of 164 Oct 2024 USW Aviation Academy Engineering Training Figure 4 - Swiss Cheese Model Single cause accidents are almost non-existent. Most accidents are the result of a chain of single events occurring in a particular sequence, with many of the single events occurring many times before, but not in sequence. Statistical surveys have shown that for every major accident as many as 600 precursor single events have occurred, not all together but have to be in the right sequence to cause the accident. As shown in Figure 3 - BA flight 5390 a well-known incident is where the pilot of a BAC1-11 was sucked out of the aircraft by the massive decompression airflow through the windscreen when it blew out. He was saved by the quick thinking of the crew who held onto him. The windscreen was fitted by a maintenance engineer using the wrong bolts (84 out of a total of 90 were too small). They did not engage with the threads of the anchor nuts correctly. When the aircraft pressurised at altitude (17,000ft) the windscreen blew out (they are usually fitted from the outside) and the rush or air sucked the pilot out – at least most of his body was out when he was grabbed by the cabin crew by his legs. NG2S419 Module 9 Revision 8 Page 17 of 164 Oct 2024 USW Aviation Academy Engineering Training The windscreen was replaced on the ramp. Time was short. The aircraft had to be turned around. There were other aircraft to be turned around also, and schedules and take-off slots to be met. The shift was not up to full strength. This was an unscheduled defect with no spare capacity to fix it. The pressure was on. The lighting in the stores was poor (when identifying the ‘correct’ bolts). Should planning have ensured that shifts were up to strength? Or should there have been a back-up aircraft (expensive) so the defect could have been rectified later when there was less pressure on time. Should there have been a back-up rectification team? Were the procedures in the AMM specific enough, and available? Was all the correct equipment available – tools, access ladders etc.? Was education at fault? Is it possible that someone might think that the aerodynamic force on the windscreen is high enough to overcome the cabin pressure? There are many other questions to be asked about the accident, which the full enquiry that ensued went into – and recommendations made. Again, there was a chain of errors each occurring in the right sequence at the right time – so an accident was made. The Error Chain In the above example, the accident or incident was preventable and could have been avoided if any one of a number of things had been done differently. In some cases, several individuals were involved and the outcome could have been modified if any one of them had reacted or queried a particular action. In each situation, however, the individuals failed to recognise or react to signs of potential hazards, did not react as expected of them, or allowed themselves to be diverted from giving their attention to the task in hand, leaving themselves open to the likelihood of committing an error. As with many incidents and accidents, all the examples involved a series of human factors problems which formed an Error Chain. NG2S419 Module 9 Revision 8 Page 18 of 164 Oct 2024 USW Aviation Academy Engineering Training If any one of the links in this chain had been broken, by building in measures which may have prevented a problem at one or more of these stages, these incidents may have been prevented. Figure 5 - The Error Chain (Civil Aviation Authority, 2002) Accident Statistics Accidents caused or contributed to by maintenance errors accounted for the following percentages: Sears report 1959 to 1983. 93 accidents investigated of which 12% caused by maintenance errors. Boeing study 1982 to 1991. 232 accidents investigated of which 20% caused by maintenance errors. NTSB study for the year 2000. 14 accidents investigated. 50% caused by maintenance errors. The trend seems to be a rising one. From MOR reports the most common maintenance failure was incorrect fitment of components and Boeing reports that specifically, the most common maintenance error was omission error. Conclusion It has been established that Human Factors in general is the most frequent causal factor in air accidents and that the percentage of accidents related to maintenance is rising. NG2S419 Module 9 Revision 8 Page 19 of 164 Oct 2024 USW Aviation Academy Engineering Training 9.2 Human Performance and Limitations Just as certain mechanical components used in aircraft maintenance engineering have limitations, engineers themselves 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. It is clear that 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. Man can also fail to function properly in certain situations. Physically, humans become fatigued, are affected by the cold, can break bones in workplace accidents, etc. Mentally, humans can make errors, have limited perceptual powers, can exhibit poor judgement due to lack of skills and knowledge, etc. 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. The aircraft engineer is the central part of the aircraft maintenance system. It is therefore very useful to understand how various parts of their body and mental processes function and how performance limitations can influence his effectiveness at work. Vision Vision is probably the most useful sense for man. Most of the information we take in over the course of the day comes from visual input. Indeed, most of the brain’s information comes from visual images. The eye is very sensitive in the right conditions (e.g. clear air, good light, etc.). In fact, the eye has approximately 1.2 million nerve cells leading from the Retinas to the area of the brain responsible for vision, while there are only about 50,000 from the inner ears – making the eye about 24 times more sensitive than the ear. In order to understand vision, it is useful first to know a little about the anatomy of the eye (see Figure 6 - The Human Eye). The eye acts very similar to a camera in that light images are taken in through the cornea and lens and projected, up-side- down, onto the back of the eye (the retina). The brain converts this up-side-down image to an image the right way up. NG2S419 Module 9 Revision 8 Page 20 of 164 Oct 2024 USW Aviation Academy Engineering Training Figure 6 - The Human Eye 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 is achieved by the shape of the cornea bending the incoming light rays. The cornea is responsible for between 70% and 80% of the total focusing ability (refraction) of the eye. The Iris and Pupil The iris (the coloured part of the eye) controls the amount of light that is allowed to enter the eye. It does this by varying the size of the pupil (the dark area in the centre of the iris). The size of the pupil can be changed very rapidly to cater for changing light levels. The amount of light can be adjusted by a factor of 5:1. This means that when the eye experiences a change in light intensity the iris can react quickly to cope with 'reasonable' changes. But if the change is, say, from normal daylight to the dark room of an NDT x-ray investigation room then the eye (retina) will take time to adjust to the new conditions. You might have noticed this when changing from light conditions (where the eye is Light Adapted) to very dark conditions - it takes time (about 7 mins for cones and 30 mins for rods) for the eye to get used to the dark and become Dark Adapted. The Lens After passing through the pupil, the light passes through the lens. Its shape is changed by the muscles (ciliary muscles) surrounding it which results in the final focusing adjustment to place a sharp image onto the retina. The change of shape of the lens is called accommodation. In order to focus clearly on a near object, the lens is thickened. To focus on a distant point, the lens is flattened. The degree NG2S419 Module 9 Revision 8 Page 21 of 164 Oct 2024 USW Aviation Academy Engineering Training of accommodation can be affected by factors such as fatigue or the ageing process. When a person is tired accommodation is reduced, resulting in less sharp vision (sharpness of vision is known as visual acuity). 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 optic nerve. Two types of light sensitive cells are found in the retina - rods and cones. The central area of the retina is known as the fovea and the receptors in this area are all cones. It is here that the visual image is typically focused. Moving outwards, the cones become less dense and are progressively replaced by rods, so that in the periphery of the retina, there are only rods. When we read (this print for example) the image is projected into the centre of the retina where the visual acuity (acuteness of vision) is at its highest. Cones Cones function in good light and are capable of detecting fine detail and are colour sensitive. This means the human eye can distinguish about 1000 different shades of colour. Rods Rods cannot detect colour. They are poor at distinguishing fine detail, but good at detecting movement in the edge of the visual field (peripheral vision). They are much more sensitive at lower light levels. As light decreases, the sensing task is passed from the cones to the rods. This means in poor light levels we see only in black and white and shades of grey. 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 at the same time. Even when viewing with one eye (monocular vision), the constant rapid movement of the eye (saccades) means that the image will 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, tasks such as 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). Factors Affecting Clarity of Sight The eye is very sensitive in the right conditions (e.g. clear air, good light, etc.). As stated earlier, the eye has approximately 1.2 million nerve cells leading from the retinas to the area of the brain responsible for vision, while there are only about NG2S419 Module 9 Revision 8 Page 22 of 164 Oct 2024 USW Aviation Academy Engineering Training 50,000 from the inner ears - making the eye about 24 times more sensitive than the ear. Before considering factors that can influence and limit the performance of the eye, it is necessary to describe visual acuity. Visual acuity is the ability of the eye to discriminate sharp detail at varying distances. An individual with an acuity of 20/20 vision should be able to see at 20 feet that which the so-called ‘normal’ person is capable of seeing at 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 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: o physical imperfections in one or both eyes (short sightedness, long sightedness), o colour blindness (unable to distinguish certain colours. o age. The influence of ingested foreign substances such as: o drugs, o medication, o alcohol, o cigarettes. Environmental factors such as: o amount of light available, o clarity of the air (e.g. dust, mist, rain, etc.). Factors associated with object being viewed such as: o size and contours of the object, o contrast of the object with its surroundings, o relative motion of the object, o distance of the object from the viewer, o the angle of the object from the viewer. Dangers to the eye include foreign objects, fluids and certain light sources. When carrying out any task that might involve flying debris - grinding, milling, wire cutting, drilling, dealing with hazardous fluids etc., then protective goggles (to BS specifications) are to be worn. Ultra violet (UV) light - from the sun and other sources (NDT) - is harmful, as is blue light. Blue light will cause damage to the retina and UV light damages the lens. Wear appropriate safety glasses (to BS standards) when using any light system NG2S419 Module 9 Revision 8 Page 23 of 164 Oct 2024 USW Aviation Academy Engineering Training containing these spectrums (arc welding for example). It is advisable to wear protective sunglasses when working in bright sunlight. Disorders to the retina are called retinopathy. They are usually associated with damage to the blood vessels in the retina and causes impairment or loss of vision. Called diabetic retinopathy if caused by diabetes, solar retinopathy if cause by the sun etc. Long Sightedness Long sightedness (Hypermetropia) is caused by the distortion of the eyeball which shortens the distance between the lens and the fovea. This means that the focussed image falls behind the retina. This will cause blurred vision when looking at relatively close objects unless the combined refractive power of lens and cornea can focus the image correctly. Corrected for using convex lenses (glasses/contact lenses). Figure 7 - A convex lens correcting long sightedness Short Sightedness Short sightedness (myopia) is caused by the lengthening of the distance between the lens and fovea with the result that the image focuses in-front of the retina – unless the eye can be corrected for this with lenses. Distant objects will be out of focus whilst near objects will be clear. Corrected using concave lenses (glasses or contact lenses). Figure 8 - A concave lens correcting short sightedness NG2S419 Module 9 Revision 8 Page 24 of 164 Oct 2024 USW Aviation Academy Engineering Training Colour Blindness Total colour blindness (where a person sees in monochrome only) is rare, but provided the person with this condition is matched to a job that does not require any colour identification then the work can be completed satisfactorily. The most usual form of colour blindness is Daltonism (red blindness) – in which the person has difficulty in distinguishing between reds and greens. Again, job matching is called for and the person should not work in any situation where colour identification is required. Medical opinion should be sought with eye tests to establish the exact extent of the condition. The engineer should contact management and inform them of the condition. Colour blindness is usually hereditary but may occur due to retinal disease. It is more prevalent in men (8%) than women (0.5%). Age As a person gets older the lens becomes less flexible, accommodation becomes poorer and a condition called Presbyopia occurs. This is a form of long sightedness. Can be corrected using glasses/contact lenses. Another symptom usually associated with ageing are cataracts, a clouding of the lens. Fatigue can also cause temporary accommodation problems showing up as blurred vision during close-up work. When carrying out work which requires constant fixed focus work for the eyes it is a good idea to rest them from time to time. If possible, let the eyes go out-of-focus (focus to infinity). Some people call this day-dreaming. Also, to look at green colour (green foliage/green paper) in a moderate light is restful for the eyes. It goes without saying that the eyes are the most important tool in the engineer’s arsenal of non-destructive testing techniques – look after them. Companies should have a policy to ensure that all engineers have an acceptable level of visual acuity and colour discrimination (with glasses/ contact lenses if necessary) appropriate to the job description for each engineer. NG2S419 Module 9 Revision 8 Page 25 of 164 Oct 2024 USW Aviation Academy Engineering Training Hearing The ear performs two quite different functions. It is used to detect sounds by receiving vibrations in the air, and secondly, it is responsible for balance and sensing acceleration. Of these two, the hearing aspect is more pertinent to the maintenance engineer, and thus it is necessary to have a basic appreciation of how the ear works. The hearing system is made up of: The outer ear – from the ear flap to close-to the eardrum. The middle ear – from the eardrum (tympanic membrane) to the cochlea. The inner ear – includes the cochlea and auditory nerve. Noise is modelled as sound waves whose amplitude defines ‘loudness’, and frequency defines ‘pitch’. Figure 9 - Anatomy of the Ear Outer Ear The outer part of the ear directs sounds down the auditory canal, and on to the eardrum. The sound waves will cause the eardrum to vibrate. Middle Ear Beyond the eardrum is the middle ear which transmits vibrations from the eardrum by way of three small bones known as the ossicles, to the fluid of the inner ear. The middle ear also contains two muscles which help to protect the ear from sounds above 80 dB by means of the acoustic or aural reflex, reducing the noise level by up to 20 dB. However, this protection can only be provided for a maximum of about 15 minutes, and does not provide protection against sudden impulse noise such as gunfire. It does explain why a person is temporarily ‘deafened’ for a few seconds after a sudden loud noise. The middle ear is usually filled with air which is refreshed by way of the Eustachian tube which connects this part of the NG2S419 Module 9 Revision 8 Page 26 of 164 Oct 2024 USW Aviation Academy Engineering Training ear with the back of the nose and mouth. However, this tube can allow mucus to travel to the middle ear which can build up, interfering with normal hearing. Inner Ear Unlike the middle ear, the inner ear is filled with fluid. The last of the ossicles in the middle ear is connected to the cochlea. This contains a fine membrane (the basilar membrane) covered in hair-like cells which are sensitive to movement in the fluid. Any vibrations they detect cause neural impulses to be transmitted to the brain via the auditory nerve. The amount of vibration detected in the cochlea depends on the volume and pitch of the original sound. Performance and Limitations of the Ear Noise is modelled as sound waves whose amplitude defines ‘loudness’, and frequency defines ‘pitch’. Frequency is measured in Hz (cycles per second) and loudness is measured in decibels (dB). Table 1 shows intensity levels for various sounds and activities. Approximate Intensity level Activity (Decibels) Rustling of leaves / Whisper 20 Conversation at 2m 50 Typewriter at 1m 65 Car at 15m 70 Lorry at 15m 75 Power Mower at 2m 90 Propeller aircraft at 300m 100 Jet aircraft at 300m 110 Standing near a propeller aircraft 120 Threshold of pain 140 Immediate hearing damage results 150 Table 1 - Typical sound levels The human ear can hear frequencies between 20Hz (lower than the lowest note on a piano for example) and 20,000Hz (higher than the highest note from a piccolo). Normal conversation is between 500 and 3,000Hz. The greatest sensitivity is around 3,000Hz. NG2S419 Module 9 Revision 8 Page 27 of 164 Oct 2024 USW Aviation Academy Engineering Training The sound wave travels down the ear canal in the outer ear and strikes the eardrum. The eardrum will vibrate at a corresponding amplitude and frequency and transmit this movement to 3 small bones in the middle ear (the Ossicles - called the hammer, anvil and stirrup). This movement is transmitted to fluid in the cochlea. Vibrations of the fluid in the cochlea will trigger nerve impulses which pass to the brain via the auditory nerve. The brain ‘hears’ the sound. To allow the air pressure to equalise across the ear drum the inner ear is connected to atmosphere via the Eustachian tube which leads to the throat and nose. There is a small flap valve in the end of the Eustachian tube, which allows air out of the middle ear (when increasing altitude in an aircraft - increasing cabin altitude, decreasing pressure - for example). When descending (decreasing cabin altitude with an attendant increase in pressure) this valve can shut and stop air going into the middle ear. This can cause lack of pressure equalisation across the ear-drum causing distortion and pain. Swallowing or yawning may assist the air to pass the valve. Damage to the Ears Damage to the ears can be caused by poking things into them. Don't, it could cause the ear-drum to become punctured. A punctured or burst ear-drum is called tympanic membrane perforation, it is very painful and can result in hearing loss, bleeding and tinnitus (noises in the ear). A burst ear drum can be caused by sudden pressure changes and violent blowing of the nose. Doctors advise that nothing smaller than an elbow should be inserted into an ear. Damage to the ear due to pressure changes (when flying for example) is called otic barotrauma. (Otic = ear. Bara = pressure. Trauma = damage) or barotitis media. Damage to the ear due to sudden loud noises (an explosion for example) may be temporary and is called acute acoustic trauma). Any wax in the ear that might cause discomfort and possible deafness, should be removed by a medically qualified person. Noise can also damage the ears. The result of Noise Induced Hearing Loss (NIHL) may be temporary at first, but continued exposure to excessive noise will cause permanent damage and hearing loss. NG2S419 Module 9 Revision 8 Page 28 of 164 Oct 2024 USW Aviation Academy Engineering Training The UK ‘Noise at Work’ regulations1 (1989) impose requirements upon employers. They stipulate three levels of noise at which an employer must act: a) 80 decibels (if normal speech cannot be heard clearly at 2 metres), employer must; assess the risk to employees’ hearing, tell the employees about the risks and what precautions are proposed, provide their employees with personal ear protectors and explain their use. b) 85 decibels (if normal speech cannot be heard clearly at 1 metre) employer must; do all that is possible to reduce exposure to the noise by means other than by providing hearing protection, mark zones where noise reaches the second level and provide recognised signs to restrict entry. c) 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 8-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 exceeds the recommended maximum TWA of 85 decibels. Permanent hearing loss may occur if the TWA is above the recommended maximum. It is normally accepted that a TWA noise level exceeding 85 dB for 8 hours is hazardous and potentially damaging to the inner ear. Exposure to noise in excess of 115 decibels without ear protection, even for a short duration, is not recommended. Hearing Loss. The most common form of hearing loss is age related (Presbycusis) and the second most common is noise related (Noise Induced Hearing Loss – NIHL) which is associated with damage to the inner ear, auditory nerve or auditory nerve pathways to the brain. Tinnitus (a ringing or hissing sound in the ear) is caused by broken or damaged hearing receptors. In general hearing protection should be worn by people exposed to prolonged periods of noise in excess of about 80dB. NG2S419 Module 9 Revision 8 Page 29 of 164 Oct 2024 USW Aviation Academy Engineering Training Exposure to noise levels for long periods of 85dB can cause temporary hearing loss. Discomfort can occur with brief exposure periods for noise levels of 120dB. Pain for levels of 130dB and eardrum rupture at 140dB. Ear Protection Noise reduction (attenuation) to the ear can be reduced at source or at the ear. At source means moving the noisy equipment to a place far enough away from personnel so it does not become a problem, or fitting noise mufflers or fitting noise absorbing panelling to interiors of buildings etc. At the ear means some form of ear protection. Noise protectors for the ears may be passive or active. Passive attenuation is the fitting of a noise barrier between the noise source and the ear (ear plugs or headsets for example). Ear plugs provide noise reduction of between 5dB and 40dB depending on the quality of the plug, how well it is fitted and the sound frequency. Headsets provide noise reduction of below 20 and up to 40dB, again depending on the variables as for ear plugs. In general ear plugs/headsets provide little protection at the lower frequencies. Active Noise Reduction (ANR) headsets are more effective (and more expensive) at the lower frequencies and work by using a small microphone near each ear to measure the ambient noise. Electronic circuitry determines the prevalent lower noise frequency of the ambient noise and generates an anti-phase signal of the same amplitude which is sent to a tiny speaker in the ear-cup. When the generated signal combines with the ambient signal they cancel each other out and the result is silence. Battery powered. WARNING. Wearing ear defenders can be dangerous as a person will not hear sounds that can warn of danger – an approaching vehicle, a warning bell, a warning shout, running machinery etc. So, a person wearing ear defenders must be significantly visually more aware of his/her environment. A routine test for hearing is the ability to hear an average conversational voice in a quiet room at a distance of 6ft (2m). Failing this test would require a proper hearing test (audiogram). All noise whether load or not, or continuous or intermittent tends to distract and reduces concentration and performance. Other Senses Touch The sensation of touch, to any part of the body, will send a signal to the brain to tell us if that surface is hot, cold, vibrating, smooth, sharp, wet, dry etc. NG2S419 Module 9 Revision 8 Page 30 of 164 Oct 2024 USW Aviation Academy Engineering Training The hands can check (carefully) if the heater of a Pitot static probe is working. The hands can feel, where there might not be a visual indication, if a passenger seat cushion is wet. This sense of touch or feel also lets the brain know if injury has occurred. Prick your finger on a pin and you will soon be 'told' about it. Touch sensors (nerve endings) are distributed all over the body but are concentrated in the hands - more particularly in the finger and thumb ends. The sensors are situated in the outer layers of the skin and when stimulated by touch send signals to the brain. The brain interprets the signal as hot, dry, cold, wet, smooth, rough, furry etc. Touch can also help the brain to judge balance. Smell Not such an obvious tool to the engineer, but some oils have a distinctive smell – as well as fuels. Fire is usually sensed by smell first - which is a good thing because if we sensed it by feel first then it might be too late. The sense of smell changes from day to day and from one situation to another. Sensitivity to certain smells is different between men and women, and different between individuals – sensitivity also decreases with age. A ‘smell’ is no more than molecules floating in the air. They are breathed in through the nose and are caught by mucus on fine olfactory hairs situated high up in the roof of the nasal cavity. The molecules dissolve in the mucus and stimulate the hairs to send signals to the brain. We smell! Colds and flu can affect these sensors so that the sense of smell is reduced. Other factors Working at altitude At high altitude, the partial pressure across the tissues of the lungs is reduced (due to the lack of atmospheric pressure) and less oxygen diffuses into the blood. This can cause hypoxia (insufficient oxygen getting to the body tissues). Hypoxia causes reduced mental capability, affects the eyes causing poor vision and if severe enough can cause unconsciousness and death. At high altitudes death can occur in a few minutes if the cabin de-pressurises – unless something is done quickly. For example, decision time for pilots at 45,000ft is about 12 seconds. Passenger aircraft are pressurised so the cabin pressure is equivalent to an altitude of about 6,000 feet whilst the aircraft altitude can be as high as, say 30,000 feet. Some military aircraft are pressurised to higher cabin altitudes (less actual pressure acting on the body and lungs) but with a continuous oxygen supply. NG2S419 Module 9 Revision 8 Page 31 of 164 Oct 2024 USW Aviation Academy Engineering Training Breathing The action of the muscles on the chest wall and the lungs diaphragm causes an IN breath. Relaxing the muscles allows the air to be pushed out. The air that is breathed in is high in oxygen. Transportation of oxygen into the blood stream and transportation out of the blood stream of carbon dioxide means that the air leaving the lungs is high in carbon dioxide and low in oxygen. Breathing rate is automatically controlled by the brain. When oxygen levels drop in the blood then the breathing rate is increased. This can cause hyperventilation. The circulation system The heart pumps de-oxygenated blood from the body to the lungs and pumps oxygenated blood from the lungs to the body. It is an automatic function that is controlled by the brain. With exercise, the heart rate is increased to provide more oxygenated blood to the body to allow for the increased metabolic rate. The actual blood cells are suspended in a fluid called blood plasma. It consists of a solution of various inorganic salts of sodium, potassium, calcium etc. with various trace elements. Plasma also contains a high concentration of protein. Figure 10 - The Circulation System NG2S419 Module 9 Revision 8 Page 32 of 164 Oct 2024 USW Aviation Academy Engineering Training The Heart The function of this organ is not too unlike that of a hydraulic pump – to pump fluid around a system. For a hydraulic system the pump pumps fluid around the aircraft hydraulic services, for the heart, it pumps blood around the body. As it contracts so it pumps blood out into the circulatory system and as it relaxes so it takes blood in. Pumping the blood out creates a peak in the pressure (Systolic pressure) and taking the blood in creates a reduced pressure (Diastolic pressure). Blood pressure is measured in millimetres of mercury (mmHg) with a pressure reading taken for both the systolic and diastolic pressures. These readings can be taken at home using a simple home measuring kit or at a medical centre. High pressure (hypertension) can have detrimental effects and upper limits are considered to be: SYSTOLIC 140 mmHg DIASTOLIC 90 mmHg Spoken as 140 over 90 and written as 140/90. Ideal pressure is considered to be 120/80. Every-one’s blood pressure is different and can vary during the day with the lowest readings obtained at night whilst asleep. Taking caffeine in tea or coffee can temporarily raise blood pressure, as can smoking. High blood pressure generally does not produce symptoms but can lead to a higher risk of heart attack and stroke and complications with the heart, kidneys and brain. High blood pressure is affected by your life style with contributory factors to include such things as: Being over-weight. Having a high cholesterol level. Drinking too much alcohol. Eating too much salt. Not eating enough fruit and vegetables (5 a day fruit & veg). Not exercising enough. Smoking. Stress. Age also has an effect; blood pressure tends to increase as you get older. NG2S419 Module 9 Revision 8 Page 33 of 164 Oct 2024 USW Aviation Academy Engineering Training Decompression Sickness If the atmospheric pressure on the body is reduced too quickly then the nitrogen in the blood comes out of solution (forms gas bubbles). These gas bubbles get into joints and can be painful - they can get into the brain and can be fatal. This condition is commonly called 'The Bends' (back pain experienced by divers). Its medical name is Aeroembolism. Fortunately, decompression sickness is rare but it can be a problem if the body has been pressurised shortly before a flight. Information Processing The previous sections have described the basic functions and limitations of some of the senses used by aircraft maintenance engineers in the course of their work. This section examines the way the information gathered by the senses is processed by the brain. The limitations of the human information processing system are also considered. “Information processing is the process of receiving information through the senses, analysing it and making it meaningful.” (Civil Aviation Authority, 2002) An Information Processing Model Information processing can be represented as a model. This captures the main elements of the process, from receipt of information via the senses, to outputs such as decision making and actions. One such model is shown in Figure 11 - Human Information Processing 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 sensory stores (sensory memory). Visual information is stored for up to half a second in iconic memory and sounds are stored for slightly longer (up to 2 seconds) in echoic memory. This enables us to remember a sentence as a sentence, rather than merely as an unconnected string of isolated words, or a film as a film, rather than as a series of disjointed images. NG2S419 Module 9 Revision 8 Page 34 of 164 Oct 2024 USW Aviation Academy Engineering Training Figure 11 - Human Information Processing NG2S419 Module 9 Revision 8 Page 35 of 164 Oct 2024 USW Aviation Academy Engineering Training Attention and Perception Having detected information, our mental resources are concentrated on specific elements - this is attention. Figure 12 - Attention Although our attention can be switched from one item to another very quickly, it can only deal with one item at a time. Attention can come in the following forms: Selective attention – occurs when a person is monitoring several sources of input, with greater attention being given to one or more sources which appear more important, therefore, selective attention is not mutually exclusive. A person can be consciously attending to one source whilst still sampling other sources in the background. Psychologists refer to this as the ‘cocktail party effect’ whereby you can be engrossed in a conversation with one person but your attention is temporarily diverted if you overhear your name being mentioned at the other side of the room, even though you were not aware of listening in to other people’s conversations. Distraction is the negative side of selective attention. Divided attention – is common in most work situations, where people are required to do more than one thing at the same time. Usually, one task suffers at the expense of the other, more so if they are similar in nature. This type of situation is also sometimes referred to as time sharing. Focused attention – is merely the skill of focussing one’s attention upon a single source and avoiding distraction. Sustained attention – as its name implies, refers to the ability to maintain attention and remain alert over long periods of time, often on one task. Most of the NG2S419 Module 9 Revision 8 Page 36 of 164 Oct 2024 USW Aviation Academy Engineering Training research has been carried out in connection with monitoring radar displays, but there is also associated research which has concentrated upon inspection tasks. Perception involves the organisation and interpretation of sensory data in order to make it meaningful, discarding non-relevant data, i.e. transforming data into information. Perception is a highly-sophisticated mechanism and requires existing knowledge and experience to know what data to keep and what to discard, and how to associate the data in a meaningful manner. An example of this is sight. The light enters the eye and is focused in two dimensions and upside-down on the retina. However, the brains perception of the world is the right way up and in three dimensions. Factors Affecting Attention Stress Attention is influenced by your arousal levels and your stress levels. This will usually increase a person's arousal and affect the perception process. With high stress levels, sampling rate is increased but to a narrower range of stimuli. This means that in a stressed condition a person is likely to miss something that is important because the attention is reduced to a limited number of channels. Figure 13 - Graph of Performance Against Stress NG2S419 Module 9 Revision 8 Page 37 of 164 Oct 2024 USW Aviation Academy Engineering Training Mental Workload This may be increased by: Stress levels of the operator. Task complexity. Time constraints. Available skills for the task. Mental attitude (subjective state of mind of the operator). Mental workload is considered to be the relationship between the imposed demands of a task and the availability of channel capacity to meet those demands. Figure 13 shows the relationship between arousal and performance. With a certain amount of arousal performance improves, but as the arousal increases so stress increases and performance suffers. If the workload is too low then concentration is reduced, boredom can set in and we can lose interest. The mental channels are not monitored as they should be and mistakes and accidents can happen. So there is a happy medium to be achieved. Overload may come in two forms - qualitative - where the task is complex and too difficult to perform, and - quantitative - where the tasks are simple enough but there are too many of them. Decision Making Once our body has used its various senses to collate this information we need to make decisions based on those stimuli. In other words, we are decision making. This may range from deciding to do nothing, to deciding to act immediately in a very specific manner. A fire alarm bell, for instance, may trigger a well-trained sequence of actions without further thought (i.e. evacuate); alternatively, an unfamiliar siren may require further information to be gathered before an appropriate course of action can be initiated. We are not usually fully aware of the processes and information which we use to make a decision. Tools can be used to assist the process of making a decision. For instance, in aircraft maintenance engineering, many documents (e.g. maintenance manuals, fault diagnosis manuals), and procedures are available to supplement the basic decision-making skills of the individual. Thus, good decisions are based on knowledge supplemented by written information and procedures, analysis of observed symptoms, performance indications, etc. It can be dangerous to believe that existing knowledge and prior experience will always be sufficient in every situation as will be shown in the section entitled ‘Information Processing Limitations’. NG2S419 Module 9 Revision 8 Page 38 of 164 Oct 2024 USW Aviation Academy Engineering Training Finally, once a decision has been made, an appropriate action can be carried out. Our senses receive feedback of this and its result. This helps to improve knowledge and refine future judgement by learning from experience. Memory In very general terms memory involves the Registration of information into the memory, the Storage of that information and the Retrieval of the stored information. Memory can be divided into Ultra Short Term Memory (sensory stores). Short Term Memory (working memory) and Long Term Memory. Long term memory can be divided into Motor Memory, Semantic Memory and Episodic Memory. Ultra-Short Term Memory. The sensory receptors (eyes, ears etc.) send information to separate memories in the sensory store. Visual information is stored for up to ½ second in the Iconic Memory and sounds are stored for up the 2 seconds in the Echoic Memory. Working or Short Term Memory. This area of the brain allows the storage of information for short periods – long enough for us to use it. If nothing is done to try to remember something more permanently then, 10 to 20 seconds later, the information is lost. Looking up the index in a book to find a page number for a particular subject would use this part of the memory. Once the page is found and the information on that page is studied the page number is forgotten. Unless actively rehearsed information in working memory is lost in about 20 seconds. This memory is strictly limited in size. The number of items that can be retained can be increased by ‘clustering’ or ‘chunking’. For example, the numbers ‘0’ and ‘1’ and ‘8’ and ‘1’ can be remembered better by putting together in one cluster - 0181. 0181 is remembered as one cluster as opposed to 0, 1, 8 and1 being remembered as 4 items. Encoding information in this memory enables things to be remembered more easily – i.e. giving each piece of data a code which is more easily remembered. Rehearsal is also a way of extending the memory. Motor Memory or Motor Programmes. Associated with skills. ‘Motor’ in this sense means the movement of hands, feet, legs etc. Skills with hands are called motor skills. NG2S419 Module 9 Revision 8 Page 39 of 164 Oct 2024 USW Aviation Academy Engineering Training When performing a new task, a great deal of control processing of information will be required by the brain. But after practice the task can be carried out automatically. A task is 'learnt' in 3 stages: Stage 1. Cognitive stage. Careful thought has to be put into each action using the central processor of the brain. Stage 2. Associative stage. All of the separate components of a task become integrated and less central processing is required. Stage 3. Automatic stage. The task is so well known that little or no thought is required to perform it correctly. The driving of a car is a very good example. When first we take the controls we have to think very hard what to do. Each stage has to be thought through carefully with attention being paid to the next stage to come. Memory Short Term or Ultra Short Long Term Working Term Memory Memory Memory Motor Semantic Programmes Memory Episodic Memory Figure 14 – Memory With a little practise, each separate element of the driving process is learnt but we have to be careful when moving from the 1st element to the 2nd etc. When well-rehearsed, we can get into a car and have very little conscious thought of how to drive. We can concentrate our central processing powers on the road conditions etc. Having a detailed conversation about technical subjects is easy with only a monitoring function of the brain lift to check the process of driving. Motor memory can produce errors. Making a cup of tea and pouring the hot water into the milk jug and forgetting to put the tea in the tea pot for example. NG2S419 Module 9 Revision 8 Page 40 of 164 Oct 2024 USW Aviation Academy Engineering Training Episodic Memory. This part of the memory system deals with ‘episodes’. Specific events are stored here. A day at the sea-side, jacking an aircraft etc. It is interesting to note that this part of the memory is not static. The actual event can be changed in the mind over a period of time. This means that when asked to recall an incident, any two people will often give conflicting reports. An important note here. If you are witness to an accident/incident, getting photographs (if possible) and writing it down immediately is more valuable than the best memory in the world when interrogated some time later. The best method of checking long term memory is to use Check Lists – before flight inspection check list, autopilot test check list etc. The most common amnesia (memory loss) affects episodic memory. Semantic Memory. This includes the knowledge of the things we have to do. The understanding of facts, words, phrases, numbers etc. We know the meaning of things from this memory area. Data in this memory, once stored successfully, is never lost. If we are unable to remember anything in this part of the memory, then it is because we cannot retrieve it (recall). It is there but we can't find it. Can be very frustrating. This can be difficult for accident investigators when collecting eye witness evidence. Information Processing Information gets to the brain via the senses - eyes, ears, nose, feel, etc. This stimulus is held in the sensory store for a short time. The information is then sent to the attentional mechanism of the brain. From the attentional mechanism the information is sent to the perception area. Here the information is actually perceived. It is perceived in the form of shapes, colours, sounds etc., which we recognise as a particular message, pattern etc. It involves the organisation and interpretation of the sensory data into something meaningful – using existing knowledge and knowing what data to keep and what to reject. Once the information has been perceived it is sent to the central processor. Here decisions are made on what to do - or what not to do. Information is sent to, and/or received from short term and long term memory to help in the decision making process, and an action is finally decided upon. NG2S419 Module 9 Revision 8 Page 41 of 164 Oct 2024 USW Aviation Academy Engineering Training Figure 15 - Information Processing Motor memory gets involved for those skills where little active decision making is required. It gets data from the attentional mechanisms which is monitored by the central processor. Perception Examples In the following example, the top line is filled with letters and the bottom line is filled with numbers. Figure 16 - Context If you look closely, however, you can see that the B in the top line and the 13 in the bottom line are, in fact, the same. NG2S419 Module 9 Revision 8 Page 42 of 164 Oct 2024 USW Aviation Academy Engineering Training The next diagram (Figure 17) shows the same length line drawn in three different ways. Upon measurement, they are the same length, but to the eye they appear different. Figure 17 - The Muller-Layer Illusion In aviation maintenance, it is often necessary to consult documents with which the engineer can become very familiar. It is possible that an engineer can scan a document and fail to notice that subtle changes have been made. He sees only what he expects to see (expectation). To illustrate how our eyes can deceive us when quickly scanning a sentence, read quickly the sentence below A bird in the the hand Figure 18 – Expectation If you have seen this before then you know what to expect, if not then you may have missed the fact that the sentence contains two “the” in it. NG2S419 Module 9 Revision 8 Page 43 of 164 Oct 2024 USW Aviation Academy Engineering Training Forgetting Not being able to remember something may be because: a) the information was not stored in the first place, or b) it was not stored correctly, or c) it was not possible to retrieve the information even though it is in the memory. Information in short term memory is particularly susceptible to interference. It is always better to use manuals/check list etc. or aides-memoires than to rely on memory alone. Feedback In this context it is information ‘fed-back’ to us (the brain) on an action we have performed. It is correctly called negative feedback and occurs in many mechanical, electrical and electronic control systems on aircraft – which as an engineer you should know about. Feedback can include many things e.g.: The feel of a switch when it operates correctly. The sound (click) of the switch when it operates correctly. The visual feed-back when the light comes on. Steering a car is a good example. The wheel is turned to start to go round a bend. The image to the eye (and possibly sideways accelerations to the ears) shows how the car is positioned on the road at the start of the bend. This feed-back information is sent to the brain so as to initiate responses to correct for any over- steer or under-steer. This process will happen continuously as the car proceeds round the bend. Response Time In very general terms response time and accuracy are inversely proportional to each other. In other words, the faster the response the less accurate it is likely to be. If you slipped from a ladder at height your response would be immediate and you would grab at anything, whether it broke your nails, cut your fingers or broke the item you grabbed at. If you had warning the ladder was about to slip and there was something to grab onto, then the action of doing the grabbing would be more accurate and hopefully no damage to fingers, finger nails etc. and a firmer hold. If arousal levels are high e.g., frightened of heights and working at the top of a ladder then response times are shorter, and less accurate. Auditory stimuli (a shout ‘Look out’) is more likely to attract our attention than visual stimuli - and hence the likely action to be in error. (Ducking when the warning means ‘get out of my way quickly’ for example). NG2S419 Module 9 Revision 8 Page 44 of 164 Oct 2024 USW Aviation Academy Engineering Training If we expect a stimulus and are prepared for it, and it happens, we are likely to give the correct response quickly. If we are prepared for a certain stimulus and something else happens then we are very likely to make an incorrect response. For example, sitting at traffic lights at red, waiting for them to change from red to red/amber (in the UK) and then to green. If, when the red/amber came on, the lights then changed back to red the most likely response would be to drive the car forward because a green light was expected. Another example of incorrect response is the testing of a Pitot static probe heater. It is switched on and tested carefully with the fingers. If it is switched on we expect it to be very hot - if in fact it is not working and we are about to test it we 'tap' our fingers quickly on the probe, withdrawing them immediately expecting it to be hot (an incorrect response to a cold object), but it is the response to the stimuli we expected. The examples show a marked difference in the outcome of the response to an expected stimulus. The first one is inherently unsafe, whilst the second is inherently safe. In general, the older we get the slower the response times. Acrophobia, Claustrophobia and Physical Access Acrophobia is a fear of heights. Much of the work carried out on aircraft can be undertaken on or near ground level, but on large aircraft some work has to go on as high as 30 or 40 feet above ground level (9m to 12m) or more. For some people work at this height would not be possible. People working at heights should not suffer unduly from acrophobia and should take all necessary precautions to ensure their own safety and the safety of others around (and beneath) them. These precautions should include: The use of correct docking equipment. The correct maintenance platforms, steps etc., correctly positioned, assembled and maintained. All safety barriers in place around the work platform. Safety harness attached to engineer and aircraft structure/dock. Non-movement of platforms with anyone on-board. Safety barriers at all open aircraft exits where there are no steps. Suitable warning notices displayed. NG2S419 Module 9 Revision 8 Page 45 of 164 Oct 2024 USW Aviation Academy Engineering Training The maintenance team should be informed if anyone has a phobia and the team (including management) should give support to that person. Tasks should be so allocated and arranged that phobia suffers are not put to the test. Treatment of phobias is undertaken by counselling and may include the use of virtual reality techniques. Claustrophobia is a fear of closed-in spaces. If work is to be carried out in a confined space such as a small enclosed structure, or in a fuel tank then a person suffering from claustrophobia should not undertake that task (job matching). Some-one else should do it. Even then, if the area is confined with difficult access, a look-out person should be in attendance with continuous communication with the person inside. Provision should be made, prior to entry into the area, for quick evacuation in the event of a fire warning etc. Provision should also be made for adequate lighting, ventilation, cooling, heating etc. If extrication of the person inside could be difficult then a harness and rope (leading to outside of the area) should be used. All systems, of course, should be made safe, and if fumes might be a problem then breathing apparatus must be used. NG2S419 Module 9 Revision 8 Page 46 of 164 Oct 2024 USW Aviation Academy Engineering Training 9.3 Social Psychology The psychology of the workplace (human behaviour in the workplace) can have a very significant effect on an individual. It is impossible to quantify in terms of what effects who, and by how much. Responsibility: Individual and Group The overall working environment starts with the World Environment and works down through the various stages – Industry Regulation – Management – Supervision – Immediate Environment – The Engineer. Each stage affects the engineer in some respect and in an ideal world each stage should be ‘perfect’. An event (happening at any stage) may affect one person adversely whilst the same event may not effect another at all. The amount of adverse effect will also depend on how often a particular event happens and on the state of mind of the person at the time. For example; there being no soap in the washroom – it may affect some more than others – it gets a bit tiresome when there has not been a

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