What does "System down" indicate in aviation operations?

The term "System down" in aviation operations indicates that a particular system is not functioning. This status signals that the system is inoperative and cannot perform its intended functions, which may affect overall operations, safety, or maintenance actions. Understanding this terminology is crucial for aviation personnel, as it ensures timely responses to system failures and helps in maintaining safety standards.

While the other options describe states related to systems, they do not capture the definitive nature of "System down." For instance, stating that a system needs repair suggests a proactive approach, while "System operational" and "System in testing phase" imply that the system is active, which contradicts the meaning of being down. Therefore, recognizing "System down" as a clear indication of non-functionality is fundamental in aviation operations.