Swiss Cheese Model


Swiss cheese model 
of accident causation is a model used in risk analysis and risk management and as the principle behind layered security.
Reason has highlighted the concept of ‘defences’ against human error within an organisation, and has coined the notion of ‘defences in depth’. 
Examples of defences are duplicate inspections, pilot pre-flight functional checks, etc., which help prevent to ‘trap’ human errors, reducing the likelihood of negative consequences. 
When defences are weakened and breached that human errors can result in incidents or accidents. 
These defences have been portrayed   diagramatically,  as several slices of Swiss cheese.
Some failures are latent, meaning that they have been made at some point in the past and lay dormant. This may be introduced at the time an aircraft was designed or may be associated with a management decision. 
Errors made by front line personnel, such as aircraft maintenance engineers, are ‘active’ failures. 
The more holes in a system’s defences, the more likely it is that errors result in incidents or accidents, but it is only in certain circumstances, when all holes ‘line up’, that these occur. Usually, if an error has breached the engineering defences, it reaches the flight operations defences (e.g. in flight warning) and is detected and handled at this stage. However, occasionally in aviation, an error can breach all the defences (e.g. a pilot ignores an in flight warning, believing it to be a false alarm) and a catastrophic situation ensues
Holes and slices - In the Swiss cheese model, an organisation's defenses against failure are modeled as a series of barriers, represented as slices of cheese. The holes in the slices represent weaknesses in individual parts of the system and are continually varying in size and position across the slices. The system produces failures when a hole in each slice momentarily aligns, permitting (in Reason's words) "a trajectory of accident opportunity", so that a hazard passes through holes in all of the slices, leading to a failure

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