Architecture definition and safety assessment of the flight control system for a light rotary-wing UAV

This paper deals with the architecture definition and the preliminary safety assessment of flight control systems of light rotary-wing UAVs for civil applications. The methods and tools to be used for these activities are standardised for conventional manned aircraft, while they are currently a matter of discussion among aerospace specialists in case of UAVs flying into unsegregated airspaces. In addition, certification concerns are even more problematic for light UAVs (from 20 to 150 kg), since the airworthiness permission is granted by national authorities. The lack of specific safety requirements actually entails the system engineers to analyse the existing standards for military applications and the certification guidelines used for civil airborne systems, up to derive the safety objectives for the UAV development. In this work, after a brief survey on available certification documents for defining the reference safety objectives, the most relevant functional failures of a light rotary-wing UAV are identified and analysed via Functional Hazard Assessment. Different system architectures are then compared by means of Fault-Tree Analysis, using the failure rate data of modern commercial components. The activity allows to point out that the key elements for safety are the flight control computers, the servoactuators for the flight controls and the communication link. In particular, the number of computers, the type of signal processing they implement, as well as the technological solution for servoactuators have a dramatic impact on UAV reliability, and they must be regarded as crucial choices for the development of these systems.