James Reason considers the aviation industry as a complex productive system. He represents it by a "layers" model. The upper layer is represented by the so called "Decision Makers" (upper management, a company's corporate body, the regulatory body). A second key element is "Line Management" (those who implement the decisions made by upper management). For upper-management decisions and line management actions, resulting in effective and productive activities by the operational personnel involved, certain preconditions have to exist (for example, equipment must be available and reliable, the operational personnel have to be skilled, knowledgeable and motivated, and environmental conditions have to be safe). “productive activities result from direct action of the operational personnel, and from indirect action of the upper layers too. The final element is "Defences" or safeguards (usually in place to prevent foreseeable injury, damage or costly interruptions of service). Due to technological progress and excellent defences, accidents are seldom originated exclusively by the errors of operational personnel - frontline operators - or as a result of major equipment failures. Instead, they result from the interactions of a series of failures, or flaws, already present in the system. Many of these failures are not immediately visible, and they have delayed consequences.
Failures can be of two types, depending on the immediacy of their consequences. Active (an error or a violation which has an immediate adverse effect. Such errors are usually made by the front-line operator) and latent (a result of a decision or an action made well before an accident, the consequences of which may lie dormant for a long time. Such failures usually originate at the decision-maker, regulator or line management level, that is, with people far removed in time and space from the event).
Latent failures, which originate from questionable decisions or incorrect actions, although not harmful if they occur in isolation, can interact to create a "window of opportunity" for a pilot, air traffic controller, or vehicle driver to commit an active failure. The front-line operators are the inheritors of a system's defects. They are the ones dealing with a situation in which technical problems, adverse conditions or their own actions will reveal the latent failures present in a system. Where front-line operators have the possibility to close their window (or defences work) the result is an incident; when they do not, it is an accident. Every attempt must be made to close
those "windows of opportunity" at upper layers, in order to provide front line operators with the safest operational conditions. What is meant by that in most cases, the accident doesn't happen because operational personnel make a mistake; it happens because someone else is aware of the hazard, does nothing and remains sitting, waiting for the mistake to be made.
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Originally Posted by Finn
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