Aviation Human Factors for EASA Part-66: What Module 9 Expects

Why Human Factors Matters for Part-66 Engineers

Industry statistics consistently show that human error is a contributing factor in roughly 70 to 80 percent of aviation accidents and incidents. For maintenance, the numbers are just as striking: maintenance errors are a factor in roughly 12 to 15 percent of all aviation accidents, and the vast majority of those errors trace back to human performance issues rather than technical failures. That is why EASA embeds human factors into the Part-66 licensing requirements — Module 9 (Human Factors, 34 questions in 45 minutes, 75% pass mark) is a basic module required for every licence category — and why Part-145 organisations must train their personnel in error prevention.

The purpose of human factors training is not just to test your knowledge — it is to make you a safer, more effective engineer. When you understand how fatigue, stress, communication breakdowns, and complacency affect performance, you can recognise these hazards in yourself and your colleagues and take corrective action before they lead to mistakes. EASA considers this professional knowledge, not an optional extra: it is part of what it means to be a certifying engineer.

The Dirty Dozen: The Core of Maintenance Human Factors

The single most important human factors framework in aviation maintenance is the Dirty Dozen — twelve common error conditions identified in the 1990s and now used worldwide, including in EASA-aligned training programmes. Every Part-66 candidate should know them inside and out:

1. Lack of Communication — Failing to share critical information between shifts, teams, or departments. Example: a day-shift engineer does not tell the night-shift engineer that a component was left disconnected.
2. Complacency — Becoming overconfident because you have done the same task hundreds of times. Complacency leads to skipping steps in a procedure.
3. Lack of Knowledge — Not understanding the task, the system, or the applicable maintenance data before starting work.
4. Distraction — Interruptions during critical maintenance tasks. A phone call, a colleague asking a question, or a loud noise can cause you to miss a step.
5. Lack of Teamwork — Poor collaboration or friction between team members that prevents effective task execution.
6. Fatigue — Physical or mental exhaustion that impairs judgement, reaction time, and attention to detail. Includes both acute fatigue (from long shifts) and chronic fatigue (from poor sleep patterns over time).
7. Lack of Resources — Missing tools, outdated manuals, insufficient personnel, or inadequate time to complete a task properly.
8. Pressure — Real or perceived pressure to complete a task quickly, often from management, schedule deadlines, or self-imposed expectations.
9. Lack of Assertiveness — Hesitating to speak up when you see something wrong, especially when a more senior engineer is involved.
10. Stress — Personal or work-related stress that reduces cognitive capacity and increases error rates.
11. Lack of Awareness — Not paying attention to your surroundings, the status of the aircraft, or the stage of the maintenance task.
12. Norms — Unwritten rules or habits that deviate from approved procedures. "Everyone skips that step" is a norm that leads to errors.

You will encounter scenario questions in training and in the module exams that present a situation and ask you to identify which Dirty Dozen factor is at play. Being able to distinguish between similar factors — for example, knowing the difference between "lack of knowledge" (you truly do not know) and "lack of awareness" (you know but are not paying attention) — is critical.

The SHELL Model: Understanding the Human-Machine Interface

Another important framework is the SHELL model (Software, Hardware, Environment, Liveware, Liveware). Developed by Elwyn Edwards in 1972 and refined by Frank Hawkins, it helps analyse the interfaces between humans and the various elements of the aviation system.

• Software — Procedures, checklists, manuals, computer programs, and other non-physical elements that guide maintenance work.
• Hardware — The physical tools, equipment, aircraft, and components used in maintenance.
• Environment — The physical and organisational environment: temperature, noise, lighting, hangar conditions, and also organisational culture and management policies.
• Liveware (L) — The human element: the engineer themselves, including skills, knowledge, physical condition, and psychological state.
• Liveware (others) — Other people in the system: colleagues, supervisors, flight crews, and inspectors.

The Module 9 exam expects you to understand how mismatches at these interfaces lead to errors. For example, a mismatch between Liveware and Hardware might be a tool that is poorly designed for the task, while a Liveware-Software mismatch might be a checklist that uses confusing language. You should be able to look at a maintenance scenario and identify which SHELL interface is most at risk.

Fatigue: A Major Topic in Module 9

Fatigue is such a significant factor in maintenance errors that it receives dedicated attention in the Module 9 syllabus and in Part-145 training programmes. You should understand:

• Types of fatigue: Physical fatigue (muscle tiredness from manual work) versus mental fatigue (cognitive exhaustion from prolonged concentration). Also distinguish acute fatigue (short-term, from a single long shift) from chronic fatigue (long-term, from sustained poor sleep or overwork).
• Causes of fatigue: Shift work, overtime, sleep disorders, poor nutrition, and personal life factors. Be able to identify which factors are most likely contributing in a given scenario.
• Effects of fatigue: Reduced attention span, slower reaction times, impaired decision-making, increased error rates, and a tendency to take shortcuts. Know how these effects manifest in maintenance tasks specifically.
• Fatigue countermeasures: Strategic napping (for example, a 10–20 minute nap before driving home after a night shift), proper sleep hygiene, regular exercise, and organisational policies that limit shift lengths and overtime.

Shift Turnover: Where Errors Are Born

The handover between shifts is one of the highest-risk moments in aircraft maintenance. When information is lost or assumed — an incomplete task, an open panel, a de-energised circuit, a test not yet run — the next shift can unknowingly continue with a false picture of the aircraft condition. This is a classic breakdown of the first Dirty Dozen factor: lack of communication.

Best practices include face-to-face turnovers, written handover notes, walking the aircraft together when practical, reviewing the work package and logbook as a team, and confirming configuration explicitly: which systems are still in work, which tests are pending, and which cautions apply. EASA-aligned training emphasises structured turnovers and briefings, treating the handover as a formal communication event rather than a casual chat. Knowing how to structure a good turnover — and how to recognise a bad one — is exactly the kind of knowledge that appears in scenario questions and in daily professional practice.

Stress and Its Impact on Maintenance Performance

Stress is another heavily covered topic. The Yerkes-Dodson Law is a concept you should know: performance increases with physiological or mental arousal up to a point, after which it declines. In other words, a moderate amount of stress keeps you alert and focused, but too much stress — or too little — reduces effectiveness.

Understand the different types of stress: acute stress (short-term, from specific events like an unexpected inspection finding), chronic stress (long-term, from ongoing work or personal pressures), and the physiological effects of stress (increased heart rate, sweating, tunnel vision, reduced cognitive capacity). You should also know stress management strategies like proper planning, delegation, deep breathing, and seeking support from colleagues.

Communication and Teamwork in Maintenance

Effective communication is a pillar of aviation safety. Key concepts include:

• Barriers to communication: Language differences, jargon, noise, hierarchy, and assumptions. A junior engineer might not speak up when they see an error because they assume the senior engineer knows better — a lack of assertiveness combined with a communication barrier.
• Shift handover communication: The transfer of information between shifts is one of the most vulnerable moments in maintenance. Best practices: face-to-face turnovers, written summaries, and standardised handover processes.
• Human factors in the organisation: EASA expects maintenance organisations to foster a positive safety culture — just culture, error reporting, and open communication — as part of the Part-145 quality system and the SMS principles endorsed by ICAO.

How to Study Human Factors

Human factors knowledge is examined in Module 9, required for every Part-66 category. Here are targeted study strategies:

• Memorise the Dirty Dozen and SHELL model — These are foundational. Create mnemonics to remember all twelve factors.
• Use real-world scenarios — The module exams are scenario-based. Instead of memorising definitions, practise applying the concepts to realistic maintenance situations. Skylicence Europe question bank includes human factors scenarios that present a situation and ask you to identify the relevant factor or model.
• Understand the relationships between factors — Fatigue and stress often interact. Complacency and norms are related. Being able to explain how different factors combine to create error chains is exactly what scenario questions test.
• Review the recommended references — The Part-66 Module 9 syllabus references the ICAO Human Factors Digests, the UK CAA CAP 716 (Aviation Maintenance Human Factors) and CAP 718, and the AMC material supporting Part-66 — all cover the core topics.
• Study with colleagues — Human factors concepts are intuitive and benefit from discussion. Talking through scenarios with other apprentices or licensed engineers helps cement the material.

Sample Scenario Question

Here is the type of scenario you might encounter in training or on the Module 9 exam:

"An engineer is performing a scheduled maintenance check on a piston-engine aircraft. Halfway through, their supervisor calls them to another aircraft for a quick troubleshooting issue. When they return to the check two hours later, they cannot remember whether they already checked the magneto timing. They decide to move on anyway, assuming it was fine."

Which Dirty Dozen factors are involved? The correct answers are Distraction (the interruption) and Complacency or Lack of Awareness (deciding to move on without verifying). Understanding which factors apply — and why — is the key to mastering these questions.

Why Human Factors Knowledge Sets You Apart

Beyond licensing, understanding human factors is what separates a technically competent engineer from a truly professional one. Engineers who internalise the Dirty Dozen and the SHELL model make fewer mistakes, catch more potential issues before they become problems, and communicate more effectively with their teams. Interviewers at major airlines, MROs, and maintenance organisations specifically look for candidates who demonstrate human factors awareness during hiring assessments and on the job.

Human factors is not the hardest technical subject on the Part-66 path — but it is one of the most important for your long-term career. Take it seriously, study the frameworks, and apply what you learn every day on the job.

Practise human factors questions with Skylicence Europe →

Read our Human Factors FAQ for more common questions →