The use of Unmanned Aerial Systems can be leveraged in many application domains ranging from agriculture to industry, opening up a wealth of new possibilities. However, UAS obviously raise important safety concerns and the use of the techniques, processes and standards developed for the aeronautic industry is not a feasible solution for most UAS. There is a need to bring in novel and pragmatic solutions to develop provably safe UAS in a time and cost-affordable manner. This paper reports on the development of a smart parachute solution for UAS, one of the core safety requirements which can be complemented by other safety components in an incremental manner. The requirements elicitation phase, and the design of the termination system has been carried out using CPAL, a lightweight model-based design environment for embedded systems.