Human Error
Error models and types
Reason's Swiss cheese model: hazards are blocked by successive defensive layers (design, procedures, training, supervision, inspection); each layer has holes, and an accident needs the holes to line up. Holes are active failures (unsafe acts by front-line people) and latent failures (decisions by designers, managers and planners that lie dormant). Latent failures are often the more important.
Error types: an error is an unintended action; a violation is a deliberate deviation from a rule. Skill-based slips (right intention, wrong execution, e.g. wrong switch) and lapses (memory failure, e.g. a forgotten step) contrast with mistakes, which are intention errors: rule-based (wrong rule applied) and knowledge-based (wrong because knowledge is inadequate). Violations may be routine, situational or exceptional.
Maintenance errors commonly involve wrong part, wrong torque, omitted step, wrong location, tool left in, panel not secured or cross-connection. The Dirty Dozen and MEDA help investigators find precursors rather than scapegoats.
Avoiding and managing errors: design out the possibility (foolproofing, different connectors, captive fasteners); reduce it (procedures, training, checklists, independent inspection, task lighting); and contain it (functional checks, duplicate inspection, reporting). A just culture - reporting without fear of punishment for honest error - is what makes the data available at all.
Traps
Exam questions often label a deliberate shortcut as an error, and mistakes (rule- or knowledge-based) are frequently confused with slips and lapses.
Worked example: a technician fits a wrong but similar fastener because two bins look alike - a slip triggered by a design weakness. The fix is colour-coded, differently sized bins, not be more careful.