System accident

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A system accident, or normal accident, is an "unanticipated interaction of multiple failures" in a complex system. This complexity can either be technological or organizational, and often has elements of both.[1] A system accident can be very easy to see in hindsight, but very difficult to see in foresight. Ahead of time, there are simply too many possible action pathways.

These accidents often resemble Rube Goldberg devices in the way that small errors of judgment, flaws in technology, and insignificant damages combine to form an emergent disaster. System accidents were described in 1984 by Charles Perrow, who termed them "normal accidents", as having such characteristics as interactive complexity, tight coupling, cascading failures, and opaqueness. James T. Reason extended this approach with human reliability[2] and the Swiss cheese model, now widely accepted in aviation safety and healthcare.

Once an enterprise passes a certain point in size, with many employees, specialization, backup systems, double-checking, detailed manuals, and formal communication, employees can all too easily recourse to protocol, habit, and "being right." Rather like attempting to watch a complicated movie in a language one is unfamiliar with, the narrative thread of what is going on can be lost. And other phenomena, such as groupthink, can be occurring at the same time for real-world accidents almost always have multiple causes. In particular, it is a mark of a dysfunctional organization to simply blame the last person who touched something.

In a December 2012 article in a popular magazine, Charles Perrow writes, "A normal accident is where everyone tries very hard to play safe, but unexpected interaction of two or more failures (because of interactive complexity), causes a cascade of failures (because of tight coupling)."[3]

There is an aspect of an animal devouring its own tail, in that more formality and effort to get it exactly right can make the situation worse.[4] For example, the more organizational rigmarole involved in adjusting to changing conditions, the more employees will delay in reporting the changing conditions, and the more emphasis on formality, the less likely employees and managers will engage in real communication. New rules can actually make the situation worse, both by adding a new additional layer of complexity and by reminding employees yet again that they are not to think but are just to follow the rules.

Regarding the May 1996 crash of Valujet (AirTran) in the Florida Everglades and the lack of interplay between theory and practice, William Langewiesche writes, "Such pretend realities extend even into the most self-consciously progressive large organizations, with their attempts to formalize informality, to deregulate the workplace, to share profits and responsibilities, to respect the integrity and initiative of the individual. The systems work in principle, and usually in practice as well, but the two may have little to do with each other. Paperwork floats free of the ground and obscures the murky workplaces where, in the confusion of real life, system accidents are born."[4]

In a 1999 article primarily focusing on health care, J. Daniel Beckham wrote, "It is ironic how often tightly coupled devices designed to provide safety are themselves the causes of disasters. Studies of the early warning systems set up to signal missile attacks on North America found that the failure of the safety devices themselves caused the most serious danger: false indicators of an attack that could have easily triggered a retaliation. Accidents at both Chernobyl and Three Mile Island were set off by failed safety systems."[5]

Perhaps anticipating the concept of system accident, the Apollo 13 Review Board wrote, "It was found that the accident was not the result of a chance malfunction in a statistical sense, but rather resulted from an unusual combination of mistakes, coupled with a somewhat deficient and unforgiving design."[6]

Possible system accidents[edit]

Apollo 13 space flight, 1970[edit]

For more details on this topic, see Apollo 13.

From the Apollo 13 Review Board ("Cortright Report"):

e. Although Beech did not encounter any problem in detanking during acceptance tests, it was not possible to detank oxygen tank no. 2 using normal procedures at KSC. Tests and analyses indicate that this was due to gas leakage through the displaced fill tube assembly [emphasis added].

f. The special detanking procedures at KSC subjected the tank to an extended period of heater operation and pressure cycling. These procedures had not been used before [emphasis added], and the tank had not been qualified by test for the conditions experienced. However, the procedures did not violate the specifications which governed the operation of the heaters at KSC.

g. In reviewing these procedures before the flight, officials of NASA, ER, and Beech did not recognize the possibility of damage due to overheating. Many of these officials were not aware of the extended heater operation. In any event, adequate thermostatic switches might have been expected to protect the tank[emphasis added].[6]

Three Mile Island, 1979[edit]

For more details on this topic, see Three Mile Island accident.

The 1979 Three Mile Island accident inspired Perrow's Normal Accidents book, where a nuclear accident occurs, resulting from an unanticipated interaction of multiple failures in a complex system. TMI was an example of a normal accident because it was "unexpected, incomprehensible, uncontrollable and unavoidable".[7]

Perrow concluded that the failure at Three Mile Island was a consequence of the system's immense complexity. Such modern high-risk systems, he realized, were prone to failures however well they were managed. It was inevitable that they would eventually suffer what he termed a 'normal accident'. Therefore, he suggested, we might do better to contemplate a radical redesign, or if that was not possible, to abandon such technology entirely.[8]

When systems exhibit both "high complexity" and "tight coupling", as at Three Mile Island, the risk of failure becomes high. Worse still, according to Perrow, "the addition of more safety devices — the stock response to a previous failure — might further reduce the safety margins if it adds complexity".[8]

ValuJet 592, Everglades, 1996[edit]

For more details on this topic, see ValuJet Flight 592.

Step 2. The unmarked cardboard boxes, stored for weeks on a parts rack, were taken over to SabreTech's shipping and receiving department and left on the floor in an area assigned to ValuJet property.

Step 3. Continental Airlines, a potential SabreTech customer, was planning an inspection of the facility, so a SabreTech shipping clerk was instructed to clean up the work place. He decided to send the oxygen generators to ValuJet's headquarters in Atlanta and labelled the boxes "aircraft parts". He had shipped ValuJet material to Atlanta before without formal approval. Furthermore, he misunderstood the green tags to indicate "unserviceable" or "out of service" and jumped to the conclusion that the generators were empty.

Step 4. The shipping clerk made up a load for the forward cargo hold of the five boxes plus two large main tires and a smaller nose tire. He instructed a co-worker to prepare a shipping ticket stating "oxygen canisters - empty". The co-worker wrote, "Oxy Canisters" followed by "Empty" in quotation marks. The tires were also listed.

Step 5. A day or two later the boxes were delivered to the ValuJet ramp agent for acceptance on Flight 592. The shipping ticket listing tires and oxygen canisters should have caught his attention but didn't. The canisters were then loaded against federal regulations, as ValuJet was not registered to transport hazardous materials. It is possible that, in the ramp agent's mind, the possibility of SabreTech workers sending him hazardous cargo was inconceivable[9]



  1. ^ Perrow, Charles (1984). Normal Accidents: Living with High-Risk Technologies, With a New Afterword and a Postscript on the Y2K Problem, Princeton, New Jersey: Princeton University Press, ISBN 0-691-00412-9, 1984, 1999 (first published by Basic Books 1984)
  2. ^ Reason, James (1990-10-26). Human Error. Cambridge University Press. ISBN 0-521-31419-4. 
  3. ^ GETTING TO CATASTROPHE: CONCENTRATIONS, COMPLEXITY AND COUPLING, Charles Perrow, The Montréal Review, December 2012.
  4. ^ a b Langewiesche, William (March 1998). The Lessons of Valujet 592, The Atlantic. See especially the last three paragraphs of this four-part article: “ . . . Understanding why might keep us from making the system even more complex, and therefore perhaps more dangerous, too.”
  5. ^ The Crash of ValuJet 592: Implications for Health Care, J. Daniel Beckham, Jan. '99. DOC file --> Mr. Beckham runs a health care consulting company, and this article is included on the company website.
  7. ^ Perrow, C. (1982), ‘The President’s Commission and the Normal Accident’, in Sils, D., Wolf, C. and Shelanski, V. (Eds), Accident at Three Mile Island: The Human Dimensions, Westview, Boulder, pp.173–184.
  8. ^ a b Nick Pidgeon (22 September 2011 Vol 477). "In retrospect:Normal accidents". Nature.  Check date values in: |date= (help);
  9. ^ Stimpson, Brian (October 1998). "Operating Highly Complex and Hazardous Technological Systems Without Mistakes: The Wrong Lessons from ValuJet 592". Manitoba Professional Engineer. Archived from the original (reprint) on 2007-09-27. Retrieved 2008-03-06. 

Further reading