This paper introduces the Functional Reasoning Design Language (FRDL), a formal modeling language designed to help engineers more effectively represent behavioral interactions in complex systems. Traditional system models used in functional hazard assessments often fail to capture the intricate behaviors and interactions that can lead to system failures. FRDL addresses this limitation by enabling engineers to trace how failures in one part of a system can propagate and impact other components. The researchers applied FRDL to an autonomous rover case study and found that it improved the identification of hazard causes and effects compared to conventional methods. By providing a clearer representation of system behavior and failure pathways, FRDL supports more comprehensive and insightful functional hazard assessments. It enables engineers to reason more effectively about failure propagation, facilitating earlier identification of potential risks during the design process. Additionally, its graphical representation enables a model-based approach to functional hazard assessments, improving both traceability and communication. Overall, FRDL shows strong potential for improving safety in complex engineered systems such as aircraft, autonomous vehicles, and medical devices.