The present article details the research steps and experimental results obtained during the development of a safety critical mini unmanned aerial vehicles (UAV). The research effort is driven by the need of developing a low cost UAV platform which demonstrates several capabilities required for future integration of such systems into the national airspace (NAS). One key aspect is the sense-and-avoid (SAA) capability, which is solved via an electro optical sensor suite, coupled with the necessary onboard image processing and estimation tasks. The other aspect is the reliability of the overall flight control system, which is solved with a distributed redundant flight control architecture using smart actuators and relying on both hardware and analytical redundancy. The present article gives an overview of the system characteristics and the architectural considerations during the development of the platform, which is partially based on legacy software and hardware components using commercial of the shelf (COTS) hobby components. The experimental results are obtained with a high-fidelity hardware-in-the-loop (HIL) environment of the Unmanned Aerial System (UAS) where system faults are injected and effectively handled with reconfiguration after detecting them.