Readers react to the June issue
The limits of certification by analysis
The June cover story “Flying digitally” by Keith Button drew attention to the popular topic of certification by analysis. Regarding CBA, two points should be made: one from the computational simulation perspective and one from the regulatory perspective.
Professor Juan Alonso believes “simulations used for certification [should be] as good as the flight test.” The critical issue is assessing the accuracy of simulation results. When computational simulation includes complex physical processes, especially coupled physics, results should be viewed more as expert opinion than physics-based simulation. Regardless of how much physics is incorporated in the simulation, there are always assumptions and approximations made not only in the physics models and their coupling but also in the common pollution of the physics by numerical solution error. The responsibility of industry management is to provide sufficiently accurate simulations for decision making, given constraints of budget and schedule. There should be a balance between these three, but the balance is always controlled by management.
The responsibility of the regulatory authority is to assess the accuracy of simulations provided by the manufacturer. However, as the story wisely points out, the European Union Aviation Safety Agency and FAA don’t “have a knowledge base to do so, nor the resources to develop that knowledge.” That’s a critical strategic problem.
Computational simulation results are delicate artifacts, especially when diverse sources of uncertainty are incorporated and rare operational scenarios are considered. Physical testing is robust, convincing, and unequivocal. We must learn from our failures: the Gulfstream G650 accident, the Boeing 737 MAX disasters, the space shuttle disasters, etc.
William Oberkampf, AIAA Fellow
Pop culture analog for Mars Sample Return
Re: “Calls grow for a safer Mars Sample Return” [June 2022], for all you readers out there concerned about contamination, NEVER watch the 2017 movie “Life” … or maybe you already have! [An organism in a soil sample from Mars turns out to have a bad side.]
Robert Allen, AIAA senior member