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Safety Regulation Group CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 www.caa.co.uk Safety Regulation Group CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 22 January 2002 CAP 715 An In...

Safety Regulation Group CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 www.caa.co.uk Safety Regulation Group CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 22 January 2002 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 © Civil Aviation Authority 2002 All rights reserved. Copies of this publication may be reproduced for personal use, or for use within a company or organisation, but may not otherwise be reproduced for publication. To use or reference CAA publications for any other purpose, for example within training material for students, please contact the CAA at the address below for formal agreement. ISBN 0 86039 834 X First Edition, 22 January 2002 Enquiries regarding the content of this publication should be addressed to: Human Factors, Aerodrome, Air Traffic and Licensing Standards Division, Safety Regulation Group, Civil Aviation Authority, Aviation House, Gatwick Airport South, West Sussex, RH6 0YR The latest version of this document is available in electronic format at www.caa.co.uk, where you may also register for e-mail notification of amendments. Published by TSO (The Stationery Office) on behalf of the UK Civil Aviation Authority. Printed copy available from: TSO, PO Box 29, Norwich NR3 1GN www.tso.co.uk/bookshop Telephone orders/General enquiries: 0870 600 5522 E-mail: [email protected] Fax orders: 0870 600 5533 Textphone: 0870 240 3701 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 List of Effective Pages Chapter Page Date Chapter Page Date iii 22 January 2002 Chapter 3 6 22 January 2002 Chapter 6 2 22 January 2002 iv 22 January 2002 Chapter 3 7 22 January 2002 Chapter 6 3 22 January 2002 v 22 January 2002 Chapter 3 8 22 January 2002 Chapter 6 4 22 January 2002 vi 22 January 2002 Chapter 3 9 22 January 2002 Chapter 6 5 22 January 2002 vii 22 January 2002 Chapter 3 10 22 January 2002 Chapter 6 6 22 January 2002 viii 22 January 2002 Chapter 3 11 22 January 2002 Chapter 6 7 22 January 2002 ix 22 January 2002 Chapter 3 12 22 January 2002 Chapter 6 8 22 January 2002 x 22 January 2002 Chapter 3 13 22 January 2002 Chapter 7 1 22 January 2002 xi 22 January 2002 Chapter 3 14 22 January 2002 Chapter 7 2 22 January 2002 xii 22 January 2002 Chapter 3 15 22 January 2002 Chapter 7 3 22 January 2002 Chapter 1 1 22 January 2002 Chapter 3 16 22 January 2002 Chapter 7 4 22 January 2002 Chapter 1 2 22 January 2002 Chapter 3 17 22 January 2002 Chapter 7 5 22 January 2002 Chapter 1 3 22 January 2002 Chapter 3 18 22 January 2002 Chapter 7 6 22 January 2002 Chapter 1 4 22 January 2002 Chapter 3 19 22 January 2002 Chapter 7 7 22 January 2002 Chapter 1 5 22 January 2002 Chapter 3 20 22 January 2002 Chapter 8 1 22 January 2002 Chapter 1 6 22 January 2002 Chapter 3 21 22 January 2002 Chapter 8 2 22 January 2002 Chapter 1 7 22 January 2002 Chapter 4 1 22 January 2002 Chapter 8 3 22 January 2002 Chapter 1 8 22 January 2002 Chapter 4 2 22 January 2002 Chapter 8 4 22 January 2002 Chapter 2 1 22 January 2002 Chapter 4 3 22 January 2002 Chapter 8 5 22 January 2002 Chapter 2 2 22 January 2002 Chapter 4 4 22 January 2002 Chapter 8 6 22 January 2002 Chapter 2 3 22 January 2002 Chapter 4 5 22 January 2002 Chapter 8 7 22 January 2002 Chapter 2 4 22 January 2002 Chapter 4 6 22 January 2002 Chapter 8 8 22 January 2002 Chapter 2 5 22 January 2002 Chapter 4 7 22 January 2002 Chapter 8 9 22 January 2002 Chapter 2 6 22 January 2002 Chapter 4 8 22 January 2002 Chapter 8 10 22 January 2002 Chapter 2 7 22 January 2002 Chapter 4 9 22 January 2002 Chapter 8 11 22 January 2002 Chapter 2 8 22 January 2002 Chapter 4 10 22 January 2002 Chapter 8 12 22 January 2002 Chapter 2 9 22 January 2002 Chapter 4 11 22 January 2002 Chapter 8 13 22 January 2002 Chapter 2 10 22 January 2002 Chapter 4 12 22 January 2002 Chapter 9 1 22 January 2002 Chapter 2 11 22 January 2002 Chapter 4 13 22 January 2002 Chapter 9 2 22 January 2002 Chapter 2 12 22 January 2002 Chapter 4 14 22 January 2002 Chapter 9 3 22 January 2002 Chapter 2 13 22 January 2002 Chapter 4 15 22 January 2002 Chapter 9 4 22 January 2002 Chapter 2 14 22 January 2002 Chapter 4 16 22 January 2002 Appendix A 1 22 January 2002 Chapter 2 15 22 January 2002 Chapter 4 17 22 January 2002 Appendix A 2 22 January 2002 Chapter 2 16 22 January 2002 Chapter 4 18 22 January 2002 Appendix A 3 22 January 2002 Chapter 2 17 22 January 2002 Chapter 4 19 22 January 2002 Appendix A 4 22 January 2002 Chapter 2 18 22 January 2002 Chapter 5 1 22 January 2002 Appendix A 5 22 January 2002 Chapter 2 19 22 January 2002 Chapter 5 2 22 January 2002 Chapter 2 20 22 January 2002 Chapter 5 3 22 January 2002 Chapter 2 21 22 January 2002 Chapter 5 4 22 January 2002 Chapter 3 1 22 January 2002 Chapter 5 5 22 January 2002 Chapter 3 2 22 January 2002 Chapter 5 6 22 January 2002 Chapter 3 3 22 January 2002 Chapter 5 7 22 January 2002 Chapter 3 4 22 January 2002 Chapter 5 8 22 January 2002 Chapter 3 5 22 January 2002 Chapter 6 1 22 January 2002 22 January 2002 Page iii CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Contents List of Effective Pages iii Contents iv Explanatory Note vi Amendment Record vii Foreword viii Acknowledgements x Glossary of Terms xi Chapter 1 Introduction The Need To Take Human Factors Into Account 1 Incidents and Accidents Attributable To Human Factors / Human Error 4 Murphy’s Law 8 Chapter 2 Human Performance and Limitations Human Performance as Part of the Maintenance Engineering System 1 Vision 2 Hearing 8 Information Processing 12 Claustrophobia, Physical Access and Fear of Heights 20 Chapter 3 Social Psychology The Social Environment 1 Responsibility: Individual and Group 2 Motivation and De-motivation 4 Peer Pressure 8 Culture Issues 9 Team Working 12 Management, Supervision and Leadership 14 Maintenance Resource Management (MRM) 18 Chapter 4 Factors Affecting Performance Fitness and Health 1 22 January 2002 Page iv CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Stress: Domestic and Work Related 3 Time Pressure and Deadlines 6 Workload - Overload and Underload 8 Sleep, Fatigue and Shift Work 11 Alcohol, Medication and Drug Abuse 16 Chapter 5 Physical Environment Noise 1 Fumes 2 Illumination 3 Climate and Temperature 5 Motion and Vibration 6 Confined Spaces 7 Working Environment 7 Chapter 6 Tasks Physical Work 2 Repetitive Tasks 4 Visual Inspection 5 Complex Systems 7 Chapter 7 Communication Within and Between Teams 1 Work Logging and Recording 4 Keeping Up-to-Date, Currency 6 Dissemination of Information 6 Chapter 8 Human Error Error Models and Theories 2 Types of Error in Maintenance Tasks 7 Implications of Errors (i.e. Accidents) 10 Avoiding and Managing Errors 12 Chapter 9 Hazards In The Workplace Recognising and Avoiding Hazards 1 Appendix A 1 22 January 2002 Page v CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Explanatory Note 1 Introduction This document is intended to provide an introduction to human factors and human performance and limitations for ab-initio engineers studying for their JAR-66 engineering licenses. The document expands upon the syllabus items listed in Module 9 of JAR-66, but is not a fully comprehensive reference document on human factors in aircraft maintenance. A separate document, CAP 7161, addresses human factors in maintenance from an organisational perspective, within a JAR-145 organisation. 1. Civil Aviation Authority (2002) CAP 716 Aviation Maintenance Human Factors (JAA JAR145) 22 January 2002 Page vi CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Amendment Record Amendment Amendment Date Incorporated by Incorporated on Number Edition 1 22 January 2002 22 January 2002 Page vii CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Foreword 1 1.1 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. 1.2 This document is intended to provide guidance and supporting information in respect of those subjects contained in the human factors syllabus: module 9 in JAR 66 and module 13 in BCAR Section L. In this respect, the order in which subjects are introduced maps on to that in the JAR and BCAR syllabi to enable ready identification of the associated information. This text primarily concerns itself with the individual and his responsibilities. A few other topics, not specifically listed in the syllabus, are also included in order to provide an introduction to the concepts, e.g. Maintenance Resource Management (MRM), organisational culture issues, etc. It is hoped that in studying this publication, prospective aircraft engineers will see that human factors is not just a subject to be passed in an exam, but rather an area that influences how well they do their job, in terms of both safety and quality, and ultimately the airworthiness of the aircraft they maintain. 1.3 There are many publications in existence dealing with aviation human factors, but the majority of these were developed for pilots rather than maintenance personnel. Whilst much of the material in these publications is relevant, in as far as it describes human performance and limitations, the contexts and examples used tend to be less relevant to the maintenance engineer. This publication has been developed specifically for the maintenance engineer and focuses on research from a number of sources and incidents investigated by the CAA. 1.4 How to use this Document This document should be used as a broad starting point for studying human factors as it affects aircraft maintenance engineering. Some further reading will be needed for more detailed information concerning human performance and limitations. Suggested further reading is included throughout the document. 1.5 In order to aid the reader, key terms are highlighted thus. Definitions and explanations are indicated like this: Definition:............ Important points to remember are shown thus: Remember:............ Examples to illustrate points are presented thus: Example:............ 22 January 2002 Page viii CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 1.6 Whilst this text has been prepared for those personnel wishing to qualify as certifying staff under JAR66, it is also relevant to all staff working in aircraft maintentance engineering. Thus whilst the term ‘engineer’ has been used throughout the document, it is generally used in a generic sense to include all aircraft maintenance technicians, fitters, licensed engineers, inspectors and supervisors. (In some cases, it also includes managers, planners, etc.). Where specific reference is made to Licensed Aircraft Engineers (LAEs), this is made clear in the text. 1.7 Also, for all the female engineers reading this text, please forgive the use of the masculine gender throughout this document. ‘He’ should be interpreted as ‘he/she’, and is used merely for ease of reading. 22 January 2002 Page ix CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Acknowledgements 1 1.1 Many sources of information have been used in the course of producing this document, including text books on human factors, ergonomics, occupational psychology and the like (reference to which can be found in footnotes and ‘Further Reading’ in this document), accident and investigation data, such as reports produced by the Air Accidents Investigation Branch (AAIB) (see AAIB web site www.aaib.dtlr.gov.uk) and information from the CAA’s Mandatory Occurrence Reporting Scheme (MORS) (see CAA web site www.srg.caa.co.uk) and the ICAO Human Factors Digests. This document has also drawn on the FAA Human Factors Guide for Aviation Maintenance and various other material from the large body of FAA funded research into human factors and maintenance engineering. These sources can be accessed via the Internet on http://hfskyway.faa.gov 1.2 Acknowledgements are given to all those authors, researchers, editors and participating organisations who contributed to the sources of information used in the preparation of this document. 22 January 2002 Page x CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Glossary of Terms AAIB Air Accidents Investigation Branch ANO Air Navigation Order APU Auxiliary Power Unit ATC Air Traffic Control AWN Airworthiness Notice BCAR British Civil Airworthiness Requirements CAA Civil Aviation Authority CAP Civil Aviation Publication cd candela CHIRP Confidential Human Factors Incident Reporting Programme CRM Crew Resource Management dB decibels FAA Federal Aviation Administration fL footLambert FOD Foreign Object Damage FODCOM Flight Operations Department Communication HFAMI Human Factors in Aviation Maintenance and Inspection HFCAG Human Factors Combined Action Group HFRG Human Factors in Reliability Group HSE Health and Safety Executive Hz Hertz IATA International Air Transport Association ICAO International Civil Aviation Organisation IMIS Integrated Maintenance Information System JAR Joint Aviation Requirement LAE Licensed Aircraft Engineer lm lumen lux lumens/m² MEDA Maintenance Engineering Decision Aid MM Maintenance Manual MORS Mandatory Occurrence Report Scheme MRM Maintenance Resource Management 22 January 2002 Page xi CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 NIHL Noise Induced Hearing Loss NDI Non-Destructive Inspection NDT Non-Destructive Testing NTSB National Transportation Safety Board OJT On-the-job Training REM Rapid Eye Movement SHEL Model Software, Hardware, Environment, Liveware SMM Shift Maintenance Manager SMS Safety Management System TWA Time Weighted Average VWF Vibratory - Induced White Finger 22 January 2002 Page xii CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Chapter 1 Introduction This chapter introduces human factors and explains its importance to the aviation industry. It examines the relationship between human factors and incidents largely in terms of human error and “Murphy’s Law” (i.e. if it can happen, one day it will). 1 The Need To Take Human Factors Into Account 1.1 In the early days of powered flight, the design, construction and control of aircraft predominated. The main attributes of the first pilots were courage and the mastery of a whole new set of skills in the struggle to control the new flying machines. 1.2 As the technical aspects of flight were overcome bit by bit, the role of the people associated with aircraft began to come to the fore. Pilots were supported initially with mechanisms to help them stabilise the aircraft, and later with automated systems to assist the crew with tasks such as navigation and communication. With such interventions to complement the abilities of pilots, aviation human factors was born. 1.3 As stated in the Foreword, an understanding of the importance of human factors to aircraft maintenance engineering is essential to anyone considering a career as a licensed aircraft engineer. This is because human factors will impinge on everything they do in the course of their job in one way or another. 1.4 What is “Human Factors”? 1.4.1 The term “human factors” is used in many different ways in the aviation industry. The term is, perhaps, best known in the context of aircraft cockpit design and Crew Resource Management (CRM). However, those activities constitute only a small percentage of aviation-related human factors, as broadly speaking it concerns any consideration of human involvement in aviation. 1.4.2 The use of the term “human factors” in the context of aviation maintenance engineering is relatively new. Aircraft accidents such as that to the Aloha aircraft in the USA in 19881 and the BAC 1-11 windscreen accident in the UK in June 19902 brought the need to address human factors issues in this environment into sharp focus. This does not imply that human factors issues were not present before these dates nor that human error did not contribute to other incidents; merely that it took an accident to draw attention to human factors problems and potential solutions. 1. NTSB (1989) Aircraft Accident Report - Aloha Airlines, Flight 243, Boeing 737-200, N73711, near Maui, Hawaii, April 28, 1988. NTSB/AAR-89/03. 2. AAIB (1992) Report on the accident to BAC 1-11, G-BJRT over Didcot, Oxfordshire on 10 June 1990. Aircraft Accident Report 1/92. 22 January 2002 Chapter 1 Page 1 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 1.4.3 Before discussing how these accidents were related to human factors, a definition of human factors is required. There are many definitions available. Some authors refer to the subject as ‘human factors’ and some as ‘ergonomics’. Some see “human factors” as a scientific discipline and others regard it as a more general part of the human contribution to system safety. Although there are simple definitions of human factors such as: “Fitting the man to the job and the job to the man”, a good definition in the context of aviation maintenance would be: "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 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”. 1.4.4 Thus, human factors include such attributes as: human physiology; psychology (including perception, cognition, memory, social interaction, error, etc.); work place design; environmental conditions; human-machine interface; anthropometrics (the scientific study of measurements of the human body). 1.5 The SHEL Model 1.5.1 It can be helpful to use a model to aid in the understanding of human factors, or as a framework around which human factors issues can be structured. A model which is often used is the SHEL model, a name derived from the initial letters of its components: Software (e.g. maintenance procedures, maintenance manuals, checklist layout, etc.); Hardware (e.g. tools, test equipment, the physical structure of aircraft, design of flight decks, positioning and operating sense of controls and instruments, etc.); Environment (e.g. physical environment such as conditions in the hangar, conditions on the line, etc. and work environment such as work patterns, management structures, public perception of the industry, etc.); Liveware (i.e. the person or people at the centre of the model, including maintenance engineers, supervisors, planners, managers, etc.). 22 January 2002 Chapter 1 Page 2 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 H S L E L Figure 1 SHEL Model. Source: Edwards, 1972 (as referenced in ICAO Human Factors Digest No 1, Circular 216 (1989)) 1.5.2 Human factors concentrates on the interfaces between the human (the ‘L’ in the centre box) and the other elements of the SHEL model1 (see Figure 1), and - from a safety viewpoint - where these elements can be deficient, e.g.: S: misinterpretation of procedures, badly written manuals, poorly designed checklists, untested or difficult to use computer software H: not enough tools, inappropriate equipment, poor aircraft design for maintainability E: uncomfortable workplace, inadequate hangar space, extreme temperatures, excessive noise, poor lighting L: relationships with other people, shortage of manpower, lack of supervision, lack of support from managers 1.5.3 As will be covered in this document, man - the “Liveware” - can perform a wide range of activities. Despite the fact that modern aircraft are now designed to embody the latest self-test and diagnostic routines that modern computing power can provide, one aspect of aviation maintenance has not changed: maintenance tasks are still being done by human beings. However, man has limitations. Since Liveware is at the centre of the model, all other aspects (Software, Hardware and Environment) must be designed or adapted to assist his performance and respect his limitations. If these two aspects are ignored, the human - in this case the maintenance engineer - will not perform to the best of his abilities, may make errors, and may jeopardise safety. 1.5.4 Thanks to modern design and manufacturing, aircraft are becoming more and more reliable. However, it is not possible to re-design the human being: we have to accept the fact that the human being is intrinsically unreliable. However, we can work around that unreliability by providing good training, procedures, tools, duplicate inspections, etc. We can also reduce the potential for error by improving aircraft design such that, for example, it is physically impossible to reconnect something the wrong way round2. 1. Hawkins, F.H. (1993) Human Factors in Flight. Aldershot: Ashgate 2. Dohertey, S. (1999) Development of a Human Hazard Analysis Method for Crossed Connection Incidents in Aircraft Maintenance. MSc Thesis. Bournemouth University. 22 January 2002 Chapter 1 Page 3 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 One of the main aims of this document is to help all personnel in the engineering maintenance environment (technicians, engineers, planners, managers, etc.) to recognise human performance limitations in themselves and others, and to be able to avoid, detect and rectify errors or error prone behaviour and practices Further Reading: a) Human Factors Digest No. 1. Fundamental Human Factors Concepts. (ICAO Circular 216) b) Human Factors Digest No. 12: Human Factors in Aircraft Maintenance and Inspection. 1995. (ICAO Circular 253) 2 Incidents and Accidents Attributable To Human Factors / Human Error 2.1 In 1940, it was calculated that approximately 70% of all aircraft accidents were attributable to man’s performance, that is to say human error1. When the International Air Transport Association (IATA) reviewed the situation 35 years later, they found that there had been no reduction in the human error component of accident statistics2 (Figure 2). OTHER CAUSES HUMAN FAILURE Flight Crew, ATC, Maintenance, Aircraft Design, etc. Figure 2 The dominant role played by human performance in civil aircraft accidents Source: IATA, 1975 2.2 A study was carried out in 1986, in the USA by Sears3, 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 1. Meier Muler, H. (1940) Flugwehr und Technik, 1:412-414 and 2:40-42. 2. IATA (1975) Safety in Flight Operations. The 20th Technical Conference of IATA, Istanbul. 3. Sears, R.L. A new look at accident contributions and the implications of operational training programmes (unpublished report). Quoted in Graeber and Marx: Reducing Human Error in Aviation Maintenance Operations. (presented at the Flight Safety Foundation 46th Annual International Air Safety Seminar, Kuala Lumpur, Malaysia, 1993). 22 January 2002 Chapter 1 Page 4 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 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 2.3 As can be seen from the list, maintenance and inspection deficiencies are one of the major contributory factors to accidents. 2.4 The UK CAA carried out a similar exercise1 in 1998 looking at causes of 621 global fatal accidents between 1980 and 1996. Again, the area “maintenance or repair oversight / error / inadequate” featured as one of the top 10 primary causal factors. 2.5 It is clear from such studies that human factors problems in aircraft maintenance engineering are a significant issue, warranting serious consideration. 2.6 Examples of Incidents and Accidents 2.6.1 There have been several ‘high profile’ incidents and accidents which have involved maintenance human factors problems. The Human Factors in Aviation Maintenance and Inspection (HFAMI) web site2 lists 24 NTSB accident reports where maintenance human factors problems have been the cause or a major contributory factor. In the UK, there have been several major incidents and accidents, details of which can be found on the AAIB web site3. Some of the major incidents and accidents are summarised below. These are: Accident to Boeing 737, (Aloha flight 243), Maui, Hawaii, April 28 1988; Accident to BAC One-Eleven, G-BJRT (British Airways flight 5390), over Didcot, Oxfordshire on 10 June 1990; Incident involving Airbus A320, G-KMAM at London Gatwick Airport, on 26 August 1993; Incident involving Boeing 737, G-OBMM near Daventry, on 23 February 1995. 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. 1. CAA (1998) CAP 681: Global Fatal Accident Review; 1980-1996. UK Civil Aviation Authority. 2. http://hfskyway.faa.gov 3. www.aaib.dtlr.gov.uk 22 January 2002 Chapter 1 Page 5 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 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. An incident in the UK in August 1993 involved an Airbus 320 which, during its first flight after a flap change, exhibited an undemanded roll to the right after takeoff. The aircraft returned to Gatwick and landed safely. The investigation discovered that during maintenance, in order to replace the right outboard flap, the spoilers had been placed in maintenance mode and moved using an incomplete procedure; specifically the collars and flags were not fitted. The purpose of the collars and the way in which the spoilers functioned was not fully understood by the engineers. This misunderstanding was due, in part, to familiarity of the engineers with other aircraft (mainly 757) and contributed to a lack of adequate briefing on the status of the spoilers during the shift handover. The locked spoiler was not detected during standard pilot functional checks. In the UK in February 1995, a Boeing 737-400 suffered a loss of oil pressure on both engines. The aircraft diverted and landed safely at Luton Airport. The investigation discovered that the aircraft had been subject to borescope inspections on both engines during the preceding night and the high pressure (HP) rotor drive covers had not been refitted, resulting in the loss of almost all the oil from both engines during flight. The line engineer was originally going to carry out the task, but for various reasons he swapped jobs with the base maintenance controller. The base maintenance controller did not have the appropriate paperwork with him. The base maintenance controller and a fitter carried out the task, despite many interruptions, but failed to refit the rotor drive covers. No ground idle engine runs (which would have revealed the oil leak) were carried out. The job was signed off as complete. 2.6.2 In all three of these UK incidents, the engineers involved were considered by their companies to be well qualified, competent and reliable employees. All of the incidents were characterised by the following: There were staff shortages; Time pressures existed; All the errors occurred at night; 22 January 2002 Chapter 1 Page 6 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Shift or task handovers were involved; They all involved supervisors doing long hands-on tasks; There was an element of a “can-do” attitude; Interruptions occurred; There was some failure to use approved data or company procedures; Manuals were confusing; There was inadequate pre-planning, equipment or spares. Source: AAIB, 19881 2.7 Incidents and Accidents - A Breakdown in Human Factors 2.7.1 In all of the examples above, 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, a number of 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. 2.7.2 As with many incidents and accidents, all the examples above involved a series of human factors problems which formed an error chain (see Figure 3). 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. g Accident C rew M an a gem en t M aintenance If we can break just one link of the chain, the accident does not happen Figure 3 The Error Chain. Source: Boeing2 1. King, D. (1988) Learning Lessons the (not quite so) Hard Way; Incidents - the route to human factors in engineering. In: 12th Symposium on Human Factors in Aviation Maintenance. March 1988. 2. Boeing (1993) Accident Prevention Strategies: Commercial Jet Aircraft Accidents World Wide Operations 1982-1991. 22 January 2002 Chapter 1 Page 7 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 2.7.3 Further chapters in this document aim to help the aircraft maintenance engineer to identify where the vulnerable areas might be within the maintenance ‘link’, how to identify them, and to provide an introduction to those human factors practices and principles which should prevent the error chain reaching a catastrophic conclusion. Further Reading: a) Marx, D.A. and Graeber, C. (1994) Human Error in Aircraft Maintenance; Chapter 5. In: Johnston, N., McDonald, N., Fuller, R. (Eds) (1994) Aviation Psychology in Practice. Aldershot: Avebury Aviation. b) Reason, J.T. (1995) The BAC 1-11 windscreen accident, Chapter 4. In: Maurino, D., Reason, J.T., Johnston, N., Lee, R. (Eds) (1995) Beyond Aviation Human Factors. Aldershot: Avebury Aviation. c) Reason, J.T. (1997) Managing the Risks of Organisational Accidents. Aldershot: Ashgate. d) Reason, J.T. (1991) Human Error. Cambridge: Cambridge University Press. e) NTSB. Aircraft Accident Report--Aloha Airlines, Flight 243, Boeing 737-200, N73711, near Maui, Hawaii, April 28, 1988. NTSB 89/03. f) AAIB (1992) Report on the accident to BAC 1-11, G-BJRT over Didcot, Oxfordshire on 10 June 1990. Aircraft Accident Report 1/92. g) AAIB (1995) Report on the incident to Airbus A320-212, at London Gatwick Airport, on 26 August 1993. Aircraft Incident Report 2/95. h) AAIB (1996) Report on the incident to a Boeing 737-400, G-OBMM near Daventry on 25 February 1995. Aircraft Accident Report 3/96. 3 Murphy’s Law 3.1 There is a tendency among human beings towards complacency. The belief that an accident will never happen to “me” or to “my Company” can be a major problem when attempting to convince individuals or organisations of the need to look at human factors issues, recognise risks and to implement improvements, rather than merely to pay ‘lip-service’ to human factors. “Murphy’s Law” can be regarded as the notion: “If something can go wrong, it will.” 3.2 If everyone could be persuaded to acknowledge Murphy’s Law, this might help overcome the “it will never happen to me” belief that many people hold. It is not true that accidents only happen to people who are irresponsible or ‘sloppy’. The incidents and accidents described in paragraph 2. show that errors can be made by experienced, well-respected individuals and accidents can occur in organisations previously thought to be “safe”. 22 January 2002 Chapter 1 Page 8 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 Chapter 2 Human Performance and Limitations The intention of this chapter is to provide an overview of those key physical and mental human performance characteristics which are likely to affect an aircraft maintenance engineer in his working environment, such as his vision, hearing, information processing, attention and perception, memory, judgement and decision making. 1 Human Performance as Part of the Maintenance Engineering System 1.1 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 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). 1.2 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. 1.3 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 his body and mental processes function and how performance limitations can influence his effectiveness at work. 22 January 2002 Chapter 2 Page 1 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 2 Vision 2.1 The Basic Function of the Eye In order to understand vision, it is useful first to know a little about the anatomy of the eye (see Figure 4). The basic structure of the eye is similar to a simple camera with an aperture (the iris), a lens, and a light sensitive surface (the retina). Light enters the eye through the cornea, then passes through the iris and the lens and falls on the retina. Here the light stimulates the light-sensitive cells on the retina (rods and cones) and these pass small electrical impulses by way of the optic nerve to the visual cortex in the brain. Here, the electrical impulses are interpreted and an image is perceived. Figure 4 The human eye 2.2 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. 2.3 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. 22 January 2002 Chapter 2 Page 2 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 2.4 The Lens After passing through the pupil, the light passes through the lens. Its shape is changed by the muscles (cillary 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 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). 2.5 The Retina 2.5.1 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. 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 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. 2.5.2 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). 2.6 Factors Affecting Clarity of Sight 2.6.1 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. 2.6.2 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. 22 January 2002 Chapter 2 Page 3 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 2.6.3 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. 2.6.4 Various factors can affect and limit the visual acuity of the eye. These include: Physical factors such as: physical imperfections in one or both eyes (short sightedness, long sightedness), age. The influence of ingested foreign substances such as: drugs, medication, alcohol, cigarettes. Environmental factors such as: amount of light available, clarity of the air (e.g. dust, mist, rain, etc.). Factors associated with object being viewed such as: size and contours of the object, contrast of the object with its surroundings, relative motion of the object, distance of the object from the viewer, the angle of the object from the viewer. 2.6.5 Each of these factors will now be examined in some detail. 2.7 Physical Factors 2.7.1 Long sight - known as Hypermetropia - is caused by a shorter than normal eyeball which means that the image is formed behind the retina (Figure 5). If the cornea and the lens cannot use their combined focusing ability to compensate for this, blurred vision will result when looking at close objects. Figure 5 A convex lens will overcome long sightedness by bending light inwards before it reaches the cornea. 22 January 2002 Chapter 2 Page 4 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 2.7.2 Short sight - known as Myopia - is where the eyeball is longer than normal, causing the image to be formed in front of the retina (Figure 6). If the accommodation of the lens cannot counteract this then distant objects are blurred. Figure 6 A concave lens will overcome short sightedness by bending light outwards before it reaches the cornea. 2.7.3 Other visual problems include: cataracts - clouding of the lens usually associated with ageing; astigmatism - a misshapen cornea causing objects to appear irregularly shaped; glaucoma - a build up in pressure of the fluid within the eye which can cause damage to the optic nerve and even blindness; migraine - severe headaches that can cause visual disturbances. 2.7.4 Finally as a person grows older, the lens becomes less flexible meaning that it is unable to accommodate sufficiently. This is known as presbyopia and is a form of long sightedness. Consequently, after the age of 40, spectacles may be required for near vision, especially in poor light conditions. Fatigue can also temporarily affect accommodation, causing blurred vision for close work. 2.8 Foreign Substances Vision can be adversely affected by the use of certain drugs and medications, alcohol, and smoking cigarettes. With smoking, carbon monoxide which builds up in the bloodstream allows less oxygen to be carried in the blood to the eyes. This is known as hypoxia and can impair rapidly the sensitivity of the rods. Alcohol can have similar effects, even hours after the last drink. 2.9 Environmental Factors 2.9.1 Vision can be improved by increasing the lighting level, but only up to a point, as the law of diminishing returns operates. Also, increased illumination could result in increased glare. Older people are more affected by the glare of reflected light than younger people. Moving from an extremely bright environment to a dimmer one has the effect of vision being severely reduced until the eyes get used to less light being available. This is because the eyes have become light adapted. If an engineer works in a very dark environment for a long time, his eyes gradually become dark adapted allowing better visual acuity. This can take about 7 minutes for the cones and 30 minutes for the rods. As a consequence, moving between a bright hanger (or the inside of an aircraft) to a dark apron area at night can mean that the maintenance engineer must wait for his eyes to adjust (adapt). In low light conditions, it is easier to focus if you look slightly to one side of an object. This allows the image to fall outside the fovea and onto the part of the retina which has many rods. 22 January 2002 Chapter 2 Page 5 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 2.9.2 Any airborne particles such as dust, rain or mist can interfere with the transmission of light through the air, distorting what is seen. This can be even worse when spectacles are worn, as they are susceptible to getting dirty, wet, misted up or scratched. Engineers who wear contact lenses (especially hard or gas-permeable types) should take into account the advice from their optician associated with the maximum wear time - usually 8 to 12 hours - and consider the effects which extended wear may have on the eyes, such as drying out and irritation. This is particularly important if they are working in an environment which is excessively dry or dusty, as airborne particles may also affect contact lens wear. Goggles should be worn where necessary. 2.10 The Nature of the Object Being Viewed Many factors associated with the object being viewed can also influence vision. We use information from the objects we are looking at to help distinguish what we are seeing. These are known as visual cues. Visual cues often refer to the comparison of objects of known size to unknown objects. An example of this is that we associate small objects with being further away. Similarly, if an object does not stand out well from its background (i.e. it has poor contrast with its surroundings), it is harder to distinguish its edges and hence its shape. Movement and relative motion of an object, as well as distance and angle of the object from the viewer, can all increase visual demands. 2.11 Colour Vision 2.11.1 Although not directly affecting visual acuity, inability to see particular colours can be a problem for the aircraft maintenance engineer. Amongst other things, good colour vision for maintenance engineers is important for: Recognising components; Distinguishing between wires; Using various diagnostic tools; Recognising various lights on the airfield (e.g. warning lights). 2.11.2 Colour defective vision is usually hereditary, although may also occur as a temporary condition after a serious illness. Colour-defective vision (normally referred to incorrectly as colour blindness) affects about 8% of men but only 0.5% of women. The most common type is difficulty in distinguishing between red and green. More rarely, it is possible to confuse blues and yellows. 2.11.3 There are degrees of colour defective vision, some people suffering more than others. Individuals may be able to distinguish between red and green in a well-lit situation but not in low light conditions. Colour defective people typically see the colours they have problems with as shades of neutral grey. 2.11.4 Ageing also causes changes in colour vision. This is a result of progressive yellowing of the lens, resulting in a reduction in colour discrimination in the blue-yellow range. Colour defective vision and its implications can be a complex area and care should be taken not to stop an engineer from performing certain tasks merely because he suffers from some degree of colour deficient vision. It may be that the type and degree of colour deficiency is not relevant in their particular job. However, if absolutely accurate colour discrimination is critical for a job, it is important that appropriate testing and screening be put in place. 22 January 2002 Chapter 2 Page 6 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 2.12 Vision and the Aircraft Maintenance Engineer 2.12.1 It is important 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 that their vision is deteriorating. 2.12.2 In the UK, the CAA have produced guidance1 which states: “A reasonable standard of eyesight is needed for any aircraft engineer to perform his 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 small or miniature components. The use of 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.” 2.12.3 Often, airline companies or airports will set the eyesight standards for reasons other than aircraft maintenance safety, e.g. for insurance purposes, or for driving on the airfield. 2.12.4 Ultimately, what is important is for the individual to recognise when his vision is adversely affected, either temporarily or permanently, and to consider carefully the possible consequences should they continue to work if the task requires good vision. Further Reading: a) Campbell, R.D. and Bagshaw, M. (1999) Human Performance and Limitations in Aviation (2nd edition). Oxford: Blackwell Scientific, Section 3.2. b) Hawkins, F.H. (1993) Human Factors in Flight (2nd edition). Aldershot: Ashgate - Chapter 5. c) Thom, T. (1999) The Air Pilot’s Manual Volume 6: Human Factors and Pilot Performance (3rd edition). Shrewsbury: Airlife Publishing - Chapter 2. d) Green, R.G., Muir, H., James, M., Gradwell, D. and Green, R.L. (1996) Human Factors for Pilots (2nd edition). Aldershot: Ashgate - Sections 1a9 and 1b2. 1. CAA (1999) CAP455: Airworthiness Notices. AWN47. UK Civil Aviation Authority, paragraph 3.4. 22 January 2002 Chapter 2 Page 7 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 3 Hearing 3.1 The Basic Function of the Ear 3.1.1 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. 3.1.2 As can be seen in Figure 7, the ear has three divisions: outer ear, middle ear and inner ear. These act to receive vibrations from the air and turn these signals into nerve impulses that the brain can recognise as sounds. Ossicles Outer ear Eustachian Tube Figure 7 The human ear 3.2 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. 3.3 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 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. 22 January 2002 Chapter 2 Page 8 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 3.4 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. 3.5 Performance and Limitations of the Ear 3.5.1 The performance of the ear is associated with the range of sounds that can be heard - both in terms of the pitch (frequency) and the volume of the sound. The audible frequency range that a young person can hear is typically between 20 and 20,000 cycles per second (or Hertz), with greatest sensitivity at about 3000 Hz. 3.5.2 Volume (or intensity) of sound is measured in decibels (dB). Table 1 shows intensity levels for various sounds and activities. Table 1 Typical sound levels for various 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 Propellor aircraft at 300m 100 Jet aircraft at 300m 110 Standing near a propellor aircraft 120 Threshold of pain 140 Immediate hearing damage results 150 3.6 Impact of Noise on Performance 3.6.1 Noise can have various negative effects in the workplace. It can: be annoying (e.g. sudden sounds, constant loud sound, etc.); interfere with verbal communication between individuals in the workplace; cause accidents by masking warning signals or messages; be fatiguing and affect concentration, decision making, etc.; damage workers’ hearing (either temporarily or permanently). 22 January 2002 Chapter 2 Page 9 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 3.6.2 Intermittent and sudden noise are generally considered to be more disruptive than continuous noise at the same level. In addition, high frequency noise generally has a more adverse affect on performance than lower frequency. Noise tends to increase errors and variability, rather than directly affect work rate. This subject is discussed further in Chapter 5. 3.7 Hearing Impairment 3.7.1 Hearing loss can result from exposure to even relatively short duration noise. The degree of impairment is influenced mainly by the intensity of the noise. Such damage is known as Noise Induced Hearing Loss (NIHL). The hearing loss can be temporary - lasting from a few seconds to a few days - or permanent. Temporary hearing loss may be caused by relatively short exposure to very loud sound, as the hair-like cells on the basilar membrane take time to ‘recover’. With additional exposure, the amount or recovery gradually decreases and hearing loss becomes permanent. Thus, regular exposure to high levels of noise over a long period may permanently damage the hair- like cells in the cochlea, leading to irreversible hearing impairment. 3.7.2 The UK ‘Noise at Work’ regulations1 (1989) impose requirements upon employers. They stipulate three levels of noise at which an employer must act: a) 85 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) 90 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). 3.7.3 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 90 decibels. 3.7.4 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. 1. Stranks, J. (2000) Handbook of Health and Safety Practice (5th edition). Pearson Education Ltd. 22 January 2002 Chapter 2 Page 10 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 3.8 Hearing Protection 3.8.1 Hearing protection is available, to a certain extent, by using ear plugs or ear defenders. Noise levels can be reduced (attenuated) by up to 20 decibels using ear plugs and 40 decibels using ear muffs. However, using ear protection will tend to adversely interfere with verbal communication. Despite this, it must be used consistently and as instructed to be effective. 3.8.2 It is good practice to reduce noise levels at source, or move noise away from workers. Often this is not a practical option in the aviation maintenance environment. Hearing protection should always be used for noise, of any duration, above 115 dB. Referring again to Table 1, this means that the aviation maintenance engineer will almost always need to use some form of hearing protection when in reasonably close proximity (about 200 - 300m) to aircraft whose engines are running. 3.9 Presbycusis Hearing deteriorates naturally as one grows older. This is known as presbycusis. This affects ability to hear high pitch sounds first, and may occur gradually from the 30’s onwards. When this natural decline is exacerbated by Noise Induced Hearing Loss, it can obviously occur rather sooner. 3.10 Hearing and the Aircraft Maintenance Engineer 3.10.1 The UK CAA1 makes the following recommendations regarding hearing: “The ability to hear an average conversational voice in a quiet room at a distance of 2 metres (6 feet) from the examiner is recommended as a routine test. Failure of this test would require an audiogram to be carried out to provide an objective assessment. If necessary, a hearing aid may be worn but consideration should be given to the practicalities of wearing the aid during routine tasks demanded of the individual.” 3.10.2 It is very important that the aircraft maintenance engineer understands the limited ability of the ears to protect themselves from damage due to excessive noise. Even though engineers should be given appropriate hearing protection and trained in its use, it is up to individuals to ensure that they actually put this to good use. It is a misconception that the ears get used to constant noise: if this noise is too loud, it will damage the ears gradually and insidiously. Noise in the workplace is discussed further in Chapter 5. Further Reading: a) Campbell, R.D. and Bagshaw, M. (1999) Human Performance and Limitations in Aviation (2nd edition). Oxford: Blackwell Scientific, Section 3.3. b) Thom, T. (1999) The Air Pilot’s Manual Volume 6: Human Factors and Pilot Performance (3rd edition). Shrewsbury: Airlife Publishing - Chapter 3. c) Green, R.G., Muir, H., James, M., Gradwell, D. and Green, R.L. (1996) Human Factors for Pilots (2nd edition). Aldershot: Ashgate - Sections 1a8 and 1b1. 1. CAA (1999) CAP455: Airworthiness Notices. AWN47. UK Civil Aviation Authority - paragraph 3.5. 22 January 2002 Chapter 2 Page 11 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 4 Information Processing The previous sections have described the basic functions and limitations of two 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. 4.1 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 8. Smells Sounds Sights Sensations Tastes STIMULI Receptors and Sensory Stores Attention Mechanism Perception CENTRAL Short Term DECISION Long Term Memory Memory MAKER Motor Programmes FEEDBACK Actions Figure 8 A functional model of human information processing 22 January 2002 Chapter 2 Page 12 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 4.2 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. 4.3 Attention and Perception 4.3.1 Having detected information, our mental resources are concentrated on specific elements - this is attention. Attention can be thought of as the concentration of mental effort on sensory or mental events. Source: Solso, 19951 4.3.2 Although attention can move very quickly from one item to another, it can only deal with one item at a time. Attention can take the form of: selective attention, divided attention, focused attention sustained attention. 4.3.3 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. 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. 4.3.4 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. 4.3.5 Focused attention is merely the skill of focussing one’s attention upon a single source and avoiding distraction. 4.3.6 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 research has been carried out in connection with monitoring radar displays, but there is also associated research which has concentrated upon inspection tasks.2 4.3.7 Attention is influenced by arousal level and stress. This can improve attention or damage it depending on the circumstances. This is covered in more detail in Chapter 4, Sections 2, 3 and 4. 1. Solso, R.L. (1995) Cognitive Psychology (4th edition.). Boston: Allyn and Bacon. 2. Search for “Inspection” on the Human Factors in Aviation Maintenance and Inspection (HFAMI) website http:// hfskyway.faa.gov 22 January 2002 Chapter 2 Page 13 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 4.3.8 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. Perception can be defined as the process of assembling sensations into a useable mental representation of the world. Perception creates faces, melodies, works of art, illusions, etc. out of the raw material of sensation. Source: Coon, 1983.1 Examples of the perceptual process: the image formed on the retina is inverted and two dimensional, yet we see the world the right way up and in three dimensions; if the head is turned, the eyes detect a constantly changing pattern of images, yet we perceive things around us to have a set location, rather than move chaotically. 4.4 Decision Making 4.4.1 Having recognised coherent information from the stimuli reaching our senses, a course of action has to be decided upon. In other words decision making occurs. 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 judgement. 4.4.2 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. 4.4.3 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’. 4.4.4 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. 1. Coon, D. (1983) Introduction to Psychology (3rd edition). St. Paul, Minesota: West Publishing Co. 22 January 2002 Chapter 2 Page 14 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 4.5 Memory 4.5.1 Memory is critical to our ability to act consistently and to learn new things. Without memory, we could not capture a ‘stream’ of information reaching our senses, or draw on past experience and apply this knowledge when making decisions. Memory can be considered to be the storage and retention of information, experiences and knowledge, as well as the ability to retrieve this information. 4.5.2 Memory depends on three processes: registration - the input of information into memory; storage - the retention of information; retrieval - the recovery of stored information. 4.5.3 It is possible to distinguish between three forms of memory: a) ultra short-term memory (or sensory storage); b) short term memory (often referred to as working memory) c) long term memory. 4.5.4 Ultra short-term memory has already been described when examining the role of sensory stores. It has a duration of up to 2 seconds (depending on the sense) and is used as a buffer, giving us time to attend to sensory input. 4.5.5 Short term memory receives a proportion of the information received into sensory stores, and allows us to store information long enough to use it (hence the idea of ‘working memory’). It can store only a relatively small amount of information at one time, i.e. 5 to 9 (often referred to as 7 ±2) items of information, for a short duration, typically 10 to 20 seconds. As the following example shows, capacity of short term memory can be enhanced by splitting information in to ‘chunks’ (a group of related items). A telephone number, e.g. 01222555234, can be stored as 11 discrete digits, in which case it is unlikely to be remembered. Alternatively, it can be stored in chunks of related information, e.g. in the UK, 01222 may be stored as one chunk, 555 as another, and 234 as another, using only 3 chunks and therefore, more likely to be remembered. In mainland Europe, the same telephone number would probably be stored as 01 22 25 55 23 4, using 6 chunks. The size of the chunk will be determined by the individual’s familiarity with the information (based on prior experience and context), thus in this example, a person from the UK might recognise 0208 as the code for London, but a person from mainland Europe might not. 4.5.6 The duration of short term memory can be extended through rehearsal (mental repetition of the information) or encoding the information in some meaningful manner (e.g. associating it with something as in the example above). 22 January 2002 Chapter 2 Page 15 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 4.5.7 The capacity of long-term memory appears to be unlimited. It is used to store information that is not currently being used, including: knowledge of the physical world and objects within it and how these behave; personal experiences; beliefs about people, social norms, values, etc.; motor programmes, problem solving skills and plans for achieving various activities; abilities, such as language comprehension. 4.5.8 Information in long-term memory can be divided into two types: (i) semantic and (ii) episodic. Semantic memory refers to our store of general, factual knowledge about the world, such as concepts, rules, one’s own language, etc. It is information that is not tied to where and when the knowledge was originally acquired. Episodic memory refers to memory of specific events, such as our past experiences (including people, events and objects). We can usually place these things within a certain context. It is believed that episodic memory is heavily influenced by a person’s expectations of what should have happened, thus two people’s recollection of the same event can differ. 4.6 Motor Programmes If a task is performed often enough, it may eventually become automatic and the required skills and actions are stored in long term memory. These are known as motor programmes and are ingrained routines that have been established through practice. The use of a motor programme reduces the load on the central decision maker. An often quoted example is that of driving a car: at first, each individual action such as gear changing is demanding, but eventually the separate actions are combined into a motor programme and can be performed with little or no awareness. These motor programmes allow us to carry out simultaneous activities, such as having a conversation whilst driving. 4.7 Situation Awareness 4.7.1 Although not shown explicitly in Figure 8, the process of attention, perception and judgement should result in awareness of the current situation. Situation awareness is the synthesis of an accurate and up-to-date 'mental model' of one's environment and state, and the ability to use this to make predictions of possible future states. 4.7.2 Situation awareness has traditionally been used in the context of the flight deck to describe the pilot’s awareness of what is going on around him, e.g. where he is geographically, his orientation in space, what mode the aircraft is in, etc. In the maintenance engineering context, it refers to1: the perception of important elements, e.g. seeing loose bolts or missing parts, hearing information passed verbally; the comprehension of their meaning, e.g. why is it like this? Is this how it should be? 1. Endsley, M.R. (1988) Design and Evaluation for Situation Awareness Enhancement. In: Proceedings of the Human Factors Society 32nd Annual Meeting, pp. 97-101. 22 January 2002 Chapter 2 Page 16 CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 the projection of their status into the future, e.g. future effects on safety, schedule, airworthiness. An example is an engineer seeing (or perceiving) blue streaks on the fuselage. His comprehension may be that the lavatory fill cap could be missing or the drainline leaking. If his situation awareness is good, he may appreciate that such a leak could allow blue water to freeze, leading to airframe or engine damage. 4.7.3 As with decision making, feedback improves situation awareness by informing us of the accuracy of our mental models and their predictive power. The ability to project system status backward, to determine what events may have led to an observed system state, is also very important in aircraft maintenance engineering, as it allows effective fault finding and diagnostic behaviour. 4.7.4 Situation awareness for the aircraft maintenance engineer can be summarised as: the status of the system the engineer is working on; the relationship between the reported defect and the intended rectification; the possible effect on this work on other systems; the effect of this work on that being done by others and the effect of their work on this work. This suggests that in aircraft maintenance engineering, the entire te