Non-functional requirements such as availability, reliability, and security are often crucial in designing and implementing distributed real-time systems. As a result, such non-functional requirements should be addressed as early as possible in the system development life-cycle. The widespread interest in dependability modeling and analysis techniques at the requirements elicitation and analysis stage provides the major motivation for this research. This paper presents a novel approach to describe high-level availability requirements using the Aspect-oriented Use Case Maps (AoUCM) language. AoUCM adds aspects-oriented concepts to the Use Case Maps (UCM) language, part of the ITU-T User Requirements Notation (URN) standard. The proposed approach relies on a mapping of availability architectural tactics to reusable AoUCM models, allowing availability tactics to be encapsulated early in the software development life-cylce. Initial tool support for the resulting availability extensions, is provided by the jUCMNav tool. We demonstrate the applicability of our approach using a case study of Lawful Intercept (LI), an IP router feature.