I'm using QNX at work. It's an interesting OS, but why use it instead of Linux? Well, the argument is that QNX is suitable for safety critical systems, while Linux... maybe? While looking for vendors who offer certifiable Linux distributions, I came across these rants from Green Hills CEO, Dan O'Dowd. He makes a few supertroll statements, like "Windows and Solaris have achieved EAL 4. But to date, Linux has only achieved EAL 2." and "Linux development and support are being outsourced to China, Russia, and other countries from which commercial defense[sic] software would never be purchased." and "Linux has been deployed in few, if any, military systems." Green Hills produce the INTEGRITY real-time OS, which is used in flight control systems like Boeing's B1-B and B-52 bombers, Lockheed Martin's F-16, F-35, and F-22 fighter jets.
I've seen (non-US) defence products that run Linux. In a hypothetical world without EAL and DO-178B, would run Linux on an aircraft control system? Would you fly in such an aircraft?