Part-66 Module 9: human error, fatigue and the Dirty Dozen

Since Regulation (EU) 2023/989, Module 9 is a multiple-choice paper of 28 questions with no essay. It covers how people perform, why they make errors and how organisations manage them.

Models you must know

• SHELL: Liveware (the person) in the centre, with interfaces to Software (procedures, manuals), Hardware (tools, equipment), Environment and other Liveware.
• Swiss cheese (James Reason): an accident happens when holes in several defences line up. Holes come from active failures (immediate) and latent conditions (hidden weaknesses, often organisational).
• Dirty Dozen: lack of communication, complacency, lack of knowledge, distraction, lack of teamwork, fatigue, lack of resources, pressure, lack of assertiveness, stress, lack of awareness and norms.
• Murphy's law: if something can go wrong, it will, so designs should make errors impossible or obvious.

Human performance

Cones give colour and sharp central vision; rods work in low light and detect movement. Full dark adaptation takes about 30 minutes. Hearing protection is needed above about 85 dB(A) averaged over a day. Short-term memory holds about seven items for around 20 seconds, so always work from current approved data, not memory. Expectation can make an inspector see what they expect rather than what is there.

Factors affecting performance

Alertness is lowest between about 03:00 and 05:00. Seventeen hours awake impairs performance roughly like 0.05% blood alcohol. The body removes about one unit of alcohol per hour, and nothing speeds this up. Stress improves performance up to a point and then degrades it (the inverted U). Time pressure, workload extremes and complacency on repetitive tasks all increase errors.

Human error

• Slip: right plan, action goes wrong through inattention.
• Lapse: memory failure, such as a forgotten step.
• Mistake: the plan itself is wrong (rule-based or knowledge-based).
• Violation: a deliberate departure from procedures; routine violations become norms.

Omissions are the most common maintenance error. Defences include task cards and stage sheets, independent inspections of critical tasks, functional checks and not working on identical systems at the same time.

Communication, culture and hazards

Hand over shifts face to face with written records. Record work as it is done. A just culture encourages honest reporting while not tolerating recklessness; mandatory occurrences are reported within 72 hours under Regulation (EU) No 376/2014. Control hazards by elimination, substitution, engineering, administrative measures and finally PPE.