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Computer-Related Incidents with Commercial Aircraft

Perrow's `System Accidents' - An Aviation Example

Charles Perrow's book Normal Accidents:Living With High-Risk Technologies (New York: Basic Books, 1984) is a standard amongst those of us recently concerned with accidents in complex systems. Perrow's thesis is that some accidents cannot be put down to failures of individual parts of the system, but to unfortunate and complex interactions of many traditional parts (I say `traditional', because the interface between parts throught which the interactions take place can itself properly be considered a `part' of the system). Two characteristics that Perrow singles out are interactive complexity and tight coupling. The first concept refers to how complex the interfaces between traditional parts are; the second to how easily events in one part of the system can propagate their effects through to remoter parts of the system. Perrow says "if interactive complexity and tight coupling - system charateristics - inevitably will produce an accident, I believe we are justified in calling it a normal accident, or a system accident." (p5). (He doesn't quite mean that - it's not those characteristics alone which produce accidents, since they're both state predicates; an accident is an event, and you normally cannot produce an event from a state alone on the level of quantum mechanics. Events cause other events, given a certain state.) He focuses on six system components: design, equipment, procedures, operators, supplies and materials, and environment; the DEPOSE components, and analyses various systems and accidents in terms of these.

Perrow is a pioneer in this area, and whether you agree with him in detail or not (and I have some reservations), it's pretty much required that one understand his work. A recent essay on an aviation accident by William Langewiesche, who writes responsibly and well, and whose prose is a joy to read, attempts to apply Perrow's ideas (and those of later sociologists Scott Sagan and Diane Vaughan), as well as describing in terms which one rarely reads exactly what it is like being around a major accident. I must confess to being speechless, sad, a little frightened, and feeling much too close to things after reading his account. Even though The Lessons of ValuJet 592 deals with an accident that is not computer-related, as far as anyone knows, the immediacy which Langewiesche brings to his descriptions, and the consideration to his thoughts, illuminates the horror and tragedy in our subject in a way I am unlikely to forget. Which is why I include it here.


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