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?''
The answer to that question is often unclear - partly because of how systems are initially designed. When deciding on the configuration of aircraft, engineers typically design for the optimal condition: a scenario in which all components are working perfectly. However, de Weck notes that much of a plane's lifetime is spent in a partially failed state. What if, he reasoned, aircraft and other complex systems could be designed from the outset to operate not in the optimal scenario, but for suboptimal conditions?
De Weck and his colleagues at MIT and the Draper Laboratory have created a design approach that tailors planes to fly in the face of likely failures. The method, which the authors call a ''multistate design approach,'' determines the likelihood of various failures over an airplane's lifetime. Through simulations, the researchers changed a plane's geometry - for example, making its tail higher, or its rudder smaller - and then observed its performance under various failure scenarios. De Weck says engineers may use the approach to design safer, longer-lasting aerial vehicles. The group will publish a paper describing its approach in the Journal of Aircraft.
''If you admit ahead of time that the system will spend most of its life in a degraded state, you make different design decisions,'' de Weck says. ''You can end up with airplanes that look quite different, because you're really emphasising robustness over optimality.''