What condition will cause the landing gear warning signal to sound?

The condition that will cause the landing gear warning signal to sound is when one power lever is retarded to idle and the landing gear is not locked down. This configuration is critical because it alerts the flight engineer or pilot to a potential safety hazard. The system is designed to trigger a warning when the aircraft is in a lower power state while also indicating that the landing gear is not properly deployed for a landing.

In normal operating procedures, if power is significantly reduced (like when a throttle is at idle), it typically signals that the aircraft is preparing to land or is in a low-speed situation. However, if the landing gear is not locked in the down and locked position, it poses a risk of a gear-up landing, which can lead to damage and hazards during touchdown.

Thus, the combination of having one power lever at idle and the landing gear being unextended ensures that the crew is alerted to this critical situation, allowing for corrective action to be taken.