Assessment of non-functional reliability and safety requirements in the early development phases helps to prevent conceptually wrong decisions and, as a consequence, significantly reduces overall development costs. The application of model-based system analysis techniques demonstrates promising results for complex avionics systems, especially software-intensive Unmanned Aerial Vehicles (UAV). Such systems are commonly designed to accomplish a specific mission consisting of multiple mission phases. The concept of phased mission systems enables the specification of individual requirements for different phases. For instance, the reliability requirements or system specifications are different for UAV flights over an agricultural field and a highway. Therefore, modern analytical methods have to distinguish between different mission phases and enable the analysis of phased missions. In this paper, we propose a new model-based method that allows system engineers to assess a conceptional design specification of the UAV concerning the fulfillment of phase-specific requirements. The proposed approach exploits modern probabilistic model checking techniques for the quantification of several dependability metrics. The method supports the systematic analysis of system specifications that contain both structural and behavioral system properties. A case study demonstrates the feasibility of the proposed method.