What is a common visual indication of a system that requires immediate crew attention?

A red warning light serves as a critical visual cue indicating a system that requires immediate crew attention. This type of indicator is designed to alert the flight crew to serious issues that could pose a direct threat to the safety of the aircraft or its operation. The use of red for warning lights is standardized in aviation, signaling a condition that demands prompt action or investigation.

In contrast, green lights typically indicate normal operation or that a system is functioning properly, and yellow caution lights serve as a warning of potential issues that may require monitoring but do not necessitate immediate action. Blue information lights are used to convey non-critical information, making them less urgent than warning lights. Therefore, when it comes to identifying a situation that could jeopardize safety and requires swift intervention, the presence of a red warning light is the most significant visual indication for the crew.