The design and implementation of distributed real-time systems is often dominated by non-functional considerations like timing, distribution and fault tolerance. As a result, it is increasingly recognized that non-functional requirements should be considered at the earliest stages of system development life cycle. The ability to model non-functional properties (such as timing constraints, availability, performance and security) at the system requirement level not only facilitates the task of moving towards real-time design, but ultimately supports the early detection of errors through automated validation and verification. This paper introduces a novel approach to describe availability features in Use Case Maps (UCM) specifications. The proposed approach relies on a mapping of availability architectural tactics to UCM components. We illustrate the applicability of our approach using the ISSU (In Service Software Upgrade) feature on IP routers.