Army Aviation vehicles are complex systems of systems and require many resources to operate and sustain, especially in a combat environment where aircraft availability and readiness are essential to the successful completion of battlefield missions. The Communications-Electronics Lifecycle Management Command (C-E LCMC) Software Engineering Center (SEC) is responsible for providing diagnostic products to support these aircraft in the field and is facing the challenge to produce more products with similar or fewer resources, brought on by the current business environment and operational tempo (OPTEMPO). This presentation discusses how the C-E LCMC SEC is meeting the challenge through the adoption of software product line engineering practices, especially that of Architecture Definition, for the Advanced Multiplex Test System (AMTS) product line. A key factor in the success of this product line is the software architecture. This presentation focuses on that architecture and its evolution from supporting a single system to an architecture that supports a product line of diagnostic tools. These tools are currently used to support diagnostics/maintenance of the AH-64A Apache Attack Helicopter with other product line products in development for the Armed Reconnaissance (ARH-70A), Kiowa Warrior Scout/Attack (OH-58D), Chinook Cargo (CH-47F), and Black Hawk Utility (UH-60M) helicopters. The architecture will continue to evolve to support new features, including tele-maintenance, condition-based maintenance, and service-oriented diagnostics/maintenance. The development of a flexible software product line architecture will be used to facilitate production of avionics maintenance software products that improve avionics field maintenance practices, reduce sustainment costs, and increase aircraft readiness.