Human Factors
Core concepts
The SHELL model - Software, Hardware, Environment and Liveware - and the recognition that most maintenance errors arise at the interfaces between them. Human performance limits: vision (rods and cones, dark adaptation, colour vision, sight lines in poor light), hearing and noise, information processing, attention and vigilance, perception and illusions, memory (short- and long-term, and its fallibility), and motivation. Physical and physiological factors: fatigue, circadian rhythm and shift work, stress, alcohol and medication, temperature and motion. Then error: slips, lapses, mistakes and violations; Reason's Swiss cheese model; the dirty dozen (complacency, distraction, pressure, fatigue, lack of communication and so on). Communication, teamwork, leadership, assertiveness, and the role of reporting systems close the module.
Traps in questions
- Dirty dozen items are not the same as error types - complacency is a precursor, a slip is an error.
- Fatigue effects are cumulative; one good night's sleep does not clear a long-standing sleep debt.
- Dark adaptation takes around 30 minutes and is destroyed by a bright light.
- Violations are deliberate; errors are unintended.
- Vision is best in the fovea, but night vision is better with peripheral viewing, so do not look directly at a faint object.
Worked example
An engineer working a night shift after two day shifts signs off a task without checking a torque figure. Contributing factors: circadian low point around 03:00–05:00, cumulative sleep debt, time pressure, and no independent check. The error was a lapse; the system fix is a second-person verification, not blaming the individual. Reason's model shows the holes aligning: fatigue, no cross-check, and a task card with the figure in small print.
Rule of thumb
If a task depends on memory alone, it will eventually fail - use a checklist, a second check, or a physical marker.