What causes the yellow DUCT TEMP warning?

The yellow DUCT TEMP warning is triggered specifically when the temperature within the environmental duct or defog duct exceeds 300 degrees Fahrenheit. This high temperature indicates a potential issue that could affect the performance of the air conditioning or defogging systems, which are critical for maintaining visibility and comfort within the aircraft cockpit or cabin. Monitoring these temperatures is essential for safety, as extreme heat could lead to system malfunctions or failures.

The other options address different scenarios that do not directly result in the yellow DUCT TEMP warning. For example, duct blockage or high average ambient temperatures may present operational issues, but they do not specifically indicate that the duct temperature itself has reached a critical threshold. Excessive humidity, while relevant to aircraft environmental conditions, is also not directly related to the temperature concerns that trigger this specific warning.