Which system is responsible for alerting the crew of APU malfunctions during flight?

The aircraft's warning system is crucial for alerting the crew of any malfunctions or anomalies, including issues with the Auxiliary Power Unit (APU), during flight. This system is designed to monitor various parameters and conditions within the aircraft. It provides immediate feedback to the crew through visual and auditory alerts, enabling them to identify problems quickly and take necessary actions to ensure safety.

In the context of the APU, the warning system can detect situations such as temperature fluctuations, pressure changes, or operational failures, and promptly communicates these to the crew. This capability is vital because timely alerts can prevent further complications and enhance overall flight safety.

While the communication, navigation, and control systems serve important functions, they are not primarily designed to alert the crew about APU malfunctions. The communication system focuses on transferring information between the aircraft and external entities, the navigation system is responsible for guiding the aircraft on its flight path, and the control system manages the aircraft's flight controls. Therefore, it is the warning system that specifically addresses the need for immediate malfunctions alerts, making it the correct choice.