Using Systems Theory to Understand Accidents

Safety approaches based on systems theory consider accidents as arising from the interactions among system components and usually do not specify single causal variables or factors [112]. Whereas industrial (occupational) safety models and event chain models focus on unsafe acts or conditions, classic system safety models instead look at what went wrong with the system s operation or organization to allow the accident to take place. 

This systems approach treats safety as an emergent property that arises when the system components interact within an environment. Emergent properties like safety are controlled or enforced by a set of constraints (control laws) related to the behavior of the system components. For example, the spacecraft descent engines must remain on until the spacecraft reaches the surface of the planet and the car deck doors on the ferry must be closed before leaving port. Accidents result from interactions among components that violate these constraints—in other words, from a lack of appropriate constraints on the interactions. Component interaction accidents, as well as component failure accidents, can be explained using these concepts. 

While events reflect the ejfects of dysfunctional interactions and inadequate enforcement of safety constraints, the inadequate control itself is only indirectly reflected by the events—the events are the result of the inadequate control. The control structure itself must be examined to determine why it was inadequate to maintain the constraints on safe behavior and why the events occurred. 

Systems theory provides a much better foundation for safety engineering than the classic analytic reduction approach underlying event-based models of accidents. It provides a way forward to much more powerful and effective safety and risk analysis and management procedures that handle the inadequacies and needed extensions to current practice described in chapter 2. 

Combining a systems-theoretic approach to safety with system engineering processes will allow designing safety into the system as it is being developed or reengineered. System engineering provides an appropriate vehicle for this process 

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