ICAO:Human Factors In Aircraft Maintenance & Inspection


ICAO developed this manual as a guide that should be used by not only aircraft maintenance and inspection engineers but also those in the industry in understanding the capabilities and limitations that influence performance and safety in aircraft maintenance and inspection. The manual highlights the SHEL and Reason Model repeatedly throughout the chapters, in order to drill the importance of Human Factors in aviation safety and effectiveness. This digest brings to light Human Factor issues that are important to aviation safety1.

Chapter 1: Human Factors- Aircraft Maintenance & Inspection.

This chapter addresses;

  • Contemporary Maintenance Problems.
  • The SHEL Model.
  • The Reason Model.
  • Human Error.

Contemporary Maintenance Problems.

1.1 When humans are involved in any activity human error is a certain sequel. Error in aircraft maintenance does not necessarily occur in maintenance, it may also be a design error, nevertheless it is an error that is of concern to the maintenance personnel as front- line managers of technical problems in daily operations2.

1.2 The rate of accidents and incidents involving maintenance concerns has increased. An example of this is emphasized by looking at these incidents and accidents in the past years where the annual average of these has increased to more than 100percent, while the number of flights has only increased by 55 percent.

1.3 Human Factor issues which affected aircraft maintenance inspectors was overlooked in the past, but today its clear that it does play a part in the human error that causes accidents and incidents playing a part in safety as a whole.

1.4 Today workload of aircraft maintenance technicians has increased, as they have to maintain old fleet. Its common to find aircraft's that have been operating for as long as 20-25 years. Intensive inspection is required during the maintenance of these aircraft's as aging airframes are prone to cracks and chips which have to be detected and fixed. This introduces stress into the work environment and time constraints on maintenance times due to management facing economic problems.

1.5 Apart from maintaining old fleet, the new aircraft's entering the industry have to be maintained. This requires a very skilled workforce with proper educational backgrounds, as the engineers are required to have additional knowledge as technology advances.

1.6 Safety and effectiveness of airline operations are becoming directly related to performance of those who maintain and inspect their aircraft fleet. Two models are used the SHEL and Reason in order to allow for, an organized and systematic approach in the understanding of Human Factor issues, involved in aircraft maintenance and inspection.

1.7 SHELL model.

1.10 Reason Model.

1.15 Human error rather than technical failures has the ability to affect aviation safety. Boeing produced an analysis of the three top casual factors namely3;

  • Flight crew not adhering to procedures.
  • Maintenance & Inspection errors.
  • Design defects.

1.16 Human error is not unique to aircraft engineering. In the 1960s Human error contributed to 20% of accidents in the industry, and in the 1990s it had increased to account for 80%4. Reasons for this increase in error percentage;

  • Reliability of electrical and mechanical components has increased but people have stayed the same.
  • Aircraft's have become more automated and more complex.
  • The increase of aviation system complexity brings about the potential for organizational accidents, in which latent procedural and technical failures combined with operational personnel errors and violations, they then penetrate the defenses as the,'Reason Model' suggests.

Chapter 2: Human Error in Aircraft Maintenance.

2.1 Error in aircraft maintenance takes two forms;(a)error that results after maintenance due to discrepancies that may not have occurred had the maintenance not taken place for example damaging an air duct when its used as a foothold during maintenance. (b)The second form of error is that which occurs after an unsafe condition goes undetected during maintenance. For example missing an airframe structural crack during the visual inspection.

2.2 Example of past accidents that had human error during maintenance as a contributing factor.

2.3 After analyzing 93 major world wide accidents between 1959-1983 maintenance and inspection was a cause of 12% of aviation accidents.

2.4 In some accidents error attributed to maintenance and inspection is a primary causal factor, in others its just a link in the chain of events leading to the accident.

2.5 United Kingdom Civil Aviation Authority listed leading maintenance re-occurrence discrepancies in order of occurrence 5;

  • Incorrect installation discrepancies.
  • Fitting of wrong parts.
  • Electrical wiring discrepancies.
  • Loose objects being left in the aircraft.
  • Inadequate lubrication.
  • Cowlings, access panels & fairings not secured.
  • Landing gear ground lock pins not removed before take-off.

2.6 Human Factor errors revealed from one airline revealed that during; 1989-1991, the main four categories of maintenance error was6;
omission 56 percent, incorrect installation 30percent, wrong parts 08 percent and others 06 percent.

2.7 Items often omitted during maintenance are often fastenings.

2.8 Offers an example of human error when screws were not fastened on an engine during maintenance.

2.9 Most frequent recurring maintenance errors are incorrect installations of components and lack of proper Quality Control and inspection.

2.10-2.28 Organizational perspective examples of maintenance error and recommendations and findings by the investigative bodies.

2.29 Conclusions after accident analysis in this chapter it was discovered that organizational errors within aircraft maintenance wasn't limited to one region or only one part of the organization.

2.30 The decision maker plays a big role in the organization as they are in-charge of achieving a balance on safety and on-time and cost effective transportation of passengers and cargo.

Chapter 3: Human Factors Issues Affecting Aircraft Maintenance.

Information Exchange & Communication

3.1 Communication is the most important element in Human Factors.

3.2 Maintenance information must be clear and easy to understand.

3.3 Constant communication with the aircraft manufacturer and the airline is important. This communication will aid in fixing or errors that the airline may come across, that are to do with the manufacturer.

3.4 Lack of communication within the airline itself enables serious flaws in the L-L and L-S interface.

3.5-3.8 Engineers who work on parts should also fill out work cards in-order to avoid inspection errors or maintenance errors occurring during shift change, when a different engineer has to continue a previous maintenance job. By doing this maintenance engineers will be able to know what has been worked on and what hasn't.

3.9 Maintenance and Quality Control should be carried out following the general maintenance manual.

3.10 Personnel should be encouraged to report hazardous situations or practices.

3:11-3:24 Training

This section addresses the various training methods that aircraft maintenance engineers undergo.

Facilities & Work Environment

3.25 To understand Human Factor error its important to also understand the environment aircraft maintenance technicians work in.

3.26-3.29 Lighting is the most important work parameter.

3.30 Noise plays a big part in affecting the environmental factor of the work environment that the maintenance personnel work in.

3.31 Proper training on handling hazardous substances during maintenance should be clearly taught and understood.

3.32 Proper work stations should be used during maintenance.

3.33 Noise,toxic material, work stations are a good example of how L-E (Liveware & Environment) interface flaw can occur.

Chapter 4:Teams & Organizational Issues in Aircraft Maintenance

This chapter cover these topics; Team work, Job design,Reward systems, Selection & staffing and Training.

Team work

4.1 Teamwork is very important in aircraft maintenance. However an unfortunate trend that organizations follow is splitting their technical specialists into distinct departments; this inhibits team work as well as communication.

4.2 Safety is enhanced when maintenance personnel work together as a team that communicates rather than as a collection of individuals pursuing independent courses of action.

4.3 They should be good communication and job control so that each engineer is aware of what is required of him/her.

4.4 Enabling technicians to work in teams enables a sense of unity and identity to be created, and 'ownership' in the organizational structure as a whole.

4.5 Teamwork, responsibility and especially leadership are key performance factors.

4.6 Lack of team identity can lead to indifferent worker attitudes.

4.7 To put a proper team in place requires planning. The jobs to be done should be clear to the teams. It is also important to insure that the teams have defined goals and objectives set for them.

4.8 Teams should be encouraged to voice their opinions and to participate in decision input. Continuous interaction and communication should also be encouraged among teams as it stimulates thought and innovation.

4.9 Well functioning teams improve productivity as well as offer personnel job satisfaction. However attaining both of these objectives simultaneously is not an easy task for management.

4.10 For team work design and management, certain aspects such as job design, selection and staffing, training and reward systems should be implemented.

Job Design

4.11 Proper job designs can have an important effect on work productivity.

4.12 Most important aspects in job design is productivity for self-management. Teams should be responsible for their activities, including aspects such as decision making on job scheduling and assignment. Teams should also participate in the selection of new team members.

4.13 When implementing teams during the design, culture of the organization must be considered. Teams may not necessarily work for every organization, as each organization is different.

Reward Systems

4.14 Team structure should allow or provide for independent feedback and reward. Individual team members as well as teams as a whole should be rewarded on good performance to boost moral.

Selection and Staffing

4.15 Teams should have skill diversity, this way teams are able to conduct a number of tasks.


4.16 Team members should be trained for their roles. Training should include; group decision making, development of interpersonal skills, as well as how to work well in a team environment.

4.17 The work teams should consist of individuals who express a preference for team work.

Chapter 5: Automation & Advanced Technology Systems

This chapter covers; Automation and Computerization, and Advanced job aid tools.

Automation and Computerization

5.1 Information management has benefited from automation, as information can be tracked and re-designed with computer aided design tools. This makes tracking of service bulletins and airworthiness directives easy.

5.2 Maintenance manuals can now be attained on screen displays as most aircraft manufacturers have electronic versions of their manuals available online.

Advanced Job Aid Tools

5.3 Other technology that provides automated information which may find its way into civil aircraft maintenance applications is underway, for example IMIS (Integrated Maintenance Information System).

5.4 In the past technicians would spend 25% of their time on paper work, computers have made this task easier leaving the technician with less time spent on paper work.

5.5 Once a job is complete the technician is now able to quickly log it into the system. The training, experience and technical talent at present to carry out the task of an aircraft maintenance technician is more than sufficient to successfully use these automated job aid tools.

5.6 It should be noted that with the introduction of advanced automation in aircraft maintenance, unless designed with the capabilities and limitations of the human operators in mind, they can be a source of problems that prevent rather than assist the aircraft technician to do his/her job. Due to this reason, automation devices must be designed with the principles of,'human- centered automation'7.

5.7 Some test equipment such as built-in test equipment(BITE) in aircraft's enables system malfunctions to be identified at an early stage, before a threat to the safety of the aircraft and its passengers occurs.

5.8 Most automation related to maintenance work is likely to consist of improvements in diagnostic and support systems.

5.9 This chapter offers a summary on automation and advanced job aid tools, that are currently or soon to be available to maintenance technicians. However they are some concepts under development, such as automated devices that will traverse an aircraft's external structure and inspect it for cracks, corrosion, damaged rivets and other flaws, significantly assisting the work of the inspector.

Chapter 6: Error Prevention Considerations & Strategies

6.1 No accident no matter how obvious its casual factors seem to be, ever occurs in isolation. Human error in aircraft maintenance has been remarkable managed. Lessons learned from past errors made in this sector have made their way into the maintenance system design, however there is still room for improvement.

6.2 Complexity of errors can range from a simple to complex one as discussed in Chapter 2.

6.3 In-between complex and simple errors are systematic errors which can easily be traced back to some deficiency in design of the aircraft or management of the maintenance process. The maintenance community has become adept at dealing with these errors through redesign and process change.

6.4 Designers and maintenance managers continue to get frustrated with small errors such as nuts and bolts not torqued, lock-wire not installed and so forth, however these errors are not so much the result of a system's deficiency, but more a reflection on the limitations in technology of both aircraft design and maintenance systems.

6.5 In order to take a significant step in maintenance error reduction, these 3 areas must be addressed;

1.Maintenance data should be organized in a form that will allow for study of the human performance aspect of maintenance.

2.The gap between the maintenance community and Psychology as it applies to aviation should be narrowed.

3.Methods and tools should be developed, to help aircraft designers and maintenance managers address the issue of human error in a more analytical manner.


1. INTERNATIONAL CIVIL AVIATION ORGANIZATION.(1995). Human Factors Digest No.12. Human Factors in Aircraft Maintenance and Inspection. ICAO. Montreal:Canada.

2. Tech-Log.”Is There a Maintenance Problem. AEROSPACE, June 1993.

3. Reason, J.1993. Comprehensive Error Management( CEM) in Aircraft Engineering.

4. Hollnagel, E. Human Reliability Analysis-Context & Control. Academic Press, San Diego. CA, 1993.

5. United Kingdom Civil Aviation Academy (UK CAA).”Maintenance Error”. Asia Pacific Air Safety. September 1992.

6. Graeber, R.C & D.A. Marx. Reduced Human Error in Aircraft Maintenance Operations.1993.

7. ICAO Human Factors Digest No 11- Human Factors in CNS/ATM System (Circular 249)1994.

Want to know more?

INTERNATIONAL CIVIL AVIATION ORGANIZATION.(1995). Human Factors Digest No.12. Human Factors in Aircraft Maintenance and Inspection. ICAO. Montreal:Canada.

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Melanie AttanMelanie Attan

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