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FAA Research Project on System Complexity Effects on Aircraft Safety: Testing the Identified Metrics

White Paper
This report describes a test of an algorithm for estimating the complexity of a safety argument.

Software Engineering Institute


This report describes a test of the complexity algorithm for estimating the complexity of a safety argument. The algorithm measures the complexity resulting from errors propagating from component to component, called error propagation complexity. The example used in this test was a Wheel Brake System that had an available model in the Architecture Analysis & Design Language. The method used to assess error propagation complexity for the Wheel Brake System was to obtain the architecture model of the entire Wheel Brake System, simplify the interconnections, then count the ways that errors could propagate from one element to another. The error propagation complexity formula essentially estimates the size of the safety case: assuming an average analysis time for the follow-through to determine whether a failure can propagate in an unsafe manner, the estimate of total time for safety case analysis can be created by multiplying this average time per failure propagation by the number of ways a failure can propagate, which is estimated by our formula for error propagation complexity. This report shows that the formula for error propagation complexity can be applied consistently to multiple well-defined architectures and results in reasonable answers.