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The Swiss Cheese Model of Accident Causation
Also known as Swiss Cheese Model Risk Management, this concept was introduced by Professor James Reason in 1990 at the University of Manchester. He recognized that accidents in complex systems occur through the concatenation of multiple factors, where each may be necessary but only jointly sufficient to produce the accident.
Aviation Safety Before Applying the Swiss Cheese Model
From 1970 to 1990, aviation accidents declined significantly due to technological advances and enhanced safety regulations. However, human factors were still viewed narrowly, focusing on individual errors without considering organizational context.
Understanding the Swiss Cheese Model
This model shows how accidents result from the interaction between unsafe acts by front-line operators and latent conditions within the system. It emphasizes that multiple layers of defense can be breached due to both active and latent failures.
A Basic Fact of the Swiss Cheese Model
In the mid-1990s, aviation safety began to adopt a systemic perspective, considering organizational culture and policies as key influences on safety risk controls.
Active Failures and Latent Failures
Active failures are immediate errors by front-line personnel. Latent failures are hidden organizational weaknesses that predispose active failures.
Examples of Latent Failures (HSE, 1999):
- Poor design of equipment
- Ineffective training
- Inadequate supervision
- Ineffective communications
- Unclear roles and responsibilities
Classification of Human Failures
Human failures are categorized into errors and violations. Errors can be slips/lapses or mistakes, while violations are intentional deviations from rules.
1. Human Errors
A. Skilled-Based Errors
- Slips and lapses in routine tasks
- Missing steps or applying them in the wrong order
B. Mistakes
- Rule-based: applying wrong actions to known situations
- Knowledge-based: errors in novel situations beyond rules
2. Violations
Violations are deliberate deviations from procedures. They are categorized as:
A. Routine Violations
Breaking rules becomes the norm, often unconsciously.
B. Situational Violations
Caused by environmental or operational constraints.
C. Exceptional Violations
Rare and occur in novel problem-solving situations.
Swiss Cheese Model Example
- Slice 1: Management Level – Expansion decision without adequate resources
- Slice 2: Reliable Maintenance – Missing component of maintenance
- Slice 3: Unsafe Acts – Undocumented procedures
- Slice 4: Human Failures – SOP violations, poor communication, loss of situational awareness
Do you think the accident is solely the flight crew's responsibility, or are there multiple contributing factors?
Summary
Human error goes beyond front-line personnel. Active failures are immediate errors; latent failures are hidden organizational weaknesses. Understanding both is essential for effective accident prevention.
Further Reading:
- ICAO Doc 9859 – Safety Management Manual
- Reason J (1990) – Human Error, Cambridge University Press
- HSE (1999) – Reducing Error and Influencing Behaviour, HS(G)48