What consequence may arise from operating the APU with insufficient fuel?

Operating the Auxiliary Power Unit (APU) with insufficient fuel can lead to significant consequences, including failure, loss of power, or various operational malfunctions. The APU is designed to run on a specific fuel supply to generate the necessary power for its operations. When fuel levels drop below the required threshold, the APU may not function properly, resulting in an inability to start or maintain operation, which can disrupt power supply to systems that rely on it.

Moreover, insufficient fuel can cause the APU to stall or shut down abruptly, which can impede critical functions like starting the main engines or supporting electrical systems. This can lead to further complications, especially during pre-flight checks or ground operations where power is essential. Therefore, maintaining an adequate fuel level is crucial for the APU to provide reliable performance and fulfill its intended functions without interruption.