Section 9.8

Human Error

Error models and theories, types of error in maintenance tasks, the Dirty Dozen, implications of errors, avoiding and managing error.

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Main ideas

James Reason's model distinguishes error types: a slip (right intention, wrong action — e.g. torquing the wrong bolt), a lapse (forgetting a step), and a mistake (wrong plan followed correctly — e.g. misdiagnosing a fault).
The Swiss Cheese Model shows that accidents rarely have one cause — they happen when holes (weaknesses) in successive defensive layers (design, procedures, training, supervision, inspection) momentarily line up.
The "Dirty Dozen" (Gordon Dupont / Transport Canada) are twelve recurring precursors to maintenance error: 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.
Errors are best managed with error-tolerant design (defences that catch an error before it causes harm — independent inspection, duplicate inspection for critical tasks) rather than by demanding zero mistakes from people.
Implications of an uncaught maintenance error range from a minor rework to a hull-loss accident — which is why critical tasks (flight controls, engine controls) require independent duplicate inspection by a second, qualified person.
⚠ Exam trap: a violation is a DELIBERATE deviation from a known rule, unlike a slip or lapse (unintended); and error management works by avoiding, trapping and mitigating — not by demanding zero mistakes.

Solved examples

A technician intends to torque bolt A but, distracted, torques the visually identical bolt B instead. Which error type is this, per Reason's model?
A slip — the intention (torque the correct bolt) was correct, but the action was misapplied due to distraction, not a knowledge or planning failure.

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