What status symbol indicates a fault or condition making the aircraft unsafe to operate?

The status symbol that indicates a fault or condition rendering the aircraft unsafe to operate is the grounding X, commonly referred to as the red X. This symbol signifies that there is a critical issue that requires immediate attention, typically necessitating repairs or maintenance before the aircraft can be safely flown again. The red X effectively communicates to maintenance personnel and flight crews that there is a significant concern that prevents the aircraft from meeting safety standards, thereby ensuring that only airworthy aircraft are approved for operation.

In contrast, the circled X denotes a discrepancy that is acknowledged but does not necessarily imply that the aircraft is unsafe; it may indicate that the condition is being monitored or that further action is needed but does not prohibit flight. The horizontal dash is often used to indicate that a particular system or component is inoperative but does not affect the overall safety of the aircraft. Lastly, the diagonal line may indicate a temporary or non-critical issue but does not explicitly denote that the aircraft is unsafe to fly. Thus, the grounding X is the unequivocal signal of an aircraft being unfit for flight due to serious operational concerns.