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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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