What signals the pilot to perform a gear check?

The pilot is signaled to perform a gear check when the gear warning horn activates. This warning mechanism is designed to alert the pilot to a potential issue with the landing gear configuration before landing. Typically, the gear warning horn activates in situations where the landing gear is not down and locked when the aircraft is at a low altitude or if the flaps are extended beyond a certain position while the gear is still retracted. This activation serves as an important safety feature, prompting the pilot to verify the landing gear status and ensure it is properly set for landing.

The other options may indicate different aspects of aircraft operation but do not specifically signal a gear check. For example, a green light illumination usually indicates that the landing gear is in the down and locked position, which does not require a check. A red warning light generally signifies a malfunction or unsafe condition but isn't solely related to the landing gear configuration. Lastly, while flaps indicating down can suggest that the aircraft is configured for landing, they do not directly trigger a gear check. Thus, the activation of the gear warning horn is the most reliable and specific cue for the pilot to verify the gear status before landing.