Complex systems inhabit a “gray world” of partial failures, MIT’s Olivier de Weck says: While a system may continue to operate as a whole, bits and pieces inevitably degrade. Over time, these small failures can add up to a single catastrophic failure, incapacitating the system.

“Think about your car,” says de Weck, an associate professor of aeronautics and astronautics and engineering systems. “Most of the things are working, but maybe your right rearview mirror is cracked, and maybe one of the cylinders in your engine isn’t working well, and your left taillight is out. The reality is that many, many real-world systems have partial failures.”

This is no less the case for aircraft. De Weck says it’s not uncommon that, from time to time, a plane’s sensors may short-circuit, or its rudders may fail to respond: “And then the question is, in that partially failed state, how will the system perform?
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Aircraft engineered with failure in mind may last longer - MIT News Office