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Review: Complex, Tightly-Coupled Systems Debate: Turkish 981

Jacob Wilcock and Larry Heimann November 20th, 2008

System Accident

· A system accident involves the unanticipated interaction of multiple failures in a complex system

­ Charles Perrow, "Normal Accidents"

Definition: System

Definition: System

Definition: Accident

· "An accident is a failure in a subsystem, or the system as a whole, that damages more than one unit and in doing so disrupts the ongoing or future output of the system."

Accident vs. Incident

Definition: Component Failure Accident

· "one or more component failures (part, unit or subsystem) that are linked in an anticipated sequence."

Definition: System Accident

· The unanticipated interaction of multiple failures


Note: this is the most important slide of this class!!

System Accident vs. Component Failure Accident

· What's the difference?

System Accident vs. Component Failure Accident

· What's the difference?

­ "The interaction of two or more failures is anticipated, expected or comprehensible"

· System Accidents start with component failure

"Final Accident"

"Final Accident"

· Wing falls off plane · Earthquake shatters dam

Definition "Common Mode"

· A component that services at least two other components. If it fails, the services and the other components fail.

Linear vs. Complex Interactions

· What's the difference?

Definition: Linear Interaction

· Those interactions in expected and familiar production or maintenance sequence, and those that are quite visible even if unplanned.

Definition: Complex Interaction

· Those interactions of unfamiliar sequences, or unplanned and unexpected sequences, and either not visible or not immediately comprehensible.

Characteristics of complex systems

· Proximity of parts or units that are not in a production sequence · Many common mode connections between components not in a production sequence · Unfamiliar or unintended feedback loops · Many control parameters with potential interactions · Indirect or inferential information sources · Limited understanding of some processes

Characteristics of Linear Systems

· Equipment spread out · Easily isolation of failed components · Extensive substitution of supplies and materials · Few unfamiliar or unintended feedback loops

Definition: Tight Coupling

Definition: Tight Coupling

· "No slack or buffer or give between two items. What happens in one directly affects what happens in the other."

Characteristics of a tightlycoupled system

· Time-dependent processes

· (loosely coupled systems can handle delays)

· Sequences are more rigid · Only one way to reach production goal · Little slack

· "In tightly coupled systems the buffers and redundancies much be designed in..." · "In loosely coupled systems there is a better chance that expedient, spur-ofthe-moment buffers and redundancies and substitutions can be found, even though they were not planned ahead of time.


· (1) List all relevant subsystems/components/parts · (2) Sequence Diagram: Normal operations · (3) What is the core purpose of your system? · (4) Why is your system tightly coupled? · (5) Sequence Diagram: Unexpected interactions · (6) Why were the interactions complex?

Definition: System



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