While pre-flighting, you hear a horn in the nose wheel well. If the APU is not running, what might this indicate?

Hearing a horn in the nose wheel well during pre-flight checks indicates a warning system activation. In this scenario, if the APU (Auxiliary Power Unit) is not running, the horn is likely signaling a duct overheat condition in the avionics ventilation system. This system is responsible for maintaining proper temperatures and airflow around critical avionics components.

The significance of this indication is crucial for operational safety. When the APU is not supplying power, the aircraft relies on its primary systems, and any overheat condition could lead to potential failures of essential electronic components, which would affect the aircraft's functionality and safety. Therefore, recognizing this alarm is vital for ensuring that the avionics system remains operational and that the aircraft is safe for flight.

Other systems mentioned in the available options, such as the cargo heat system or cabin ventilation systems, would typically have their specific warning indications and are less likely to produce a signal directly from the nose wheel well. Thus, the presence of the horn in this context is specifically tied to concerns about the avionics environment, leading to the conclusion that duct overheat in the avionics ventilation system is the most accurate interpretation.