An APU automatic shutdown occurs due to which of the following conditions?

An APU automatic shutdown can occur due to overspeed or underspeed conditions because these situations can lead to severe internal damage to the auxiliary power unit if not addressed promptly. The APU operates within a specific speed range, and maintaining this range is critical for its safe and efficient operation. If the APU exceeds its maximum speed (overspeed), it can create excessive mechanical stress, leading to potential failures. Conversely, if the APU operates below its minimum speed (underspeed), it may not generate adequate power or pressure, impacting its functionality. Therefore, the APU is designed to automatically shut down in response to these critical conditions to prevent further damage and ensure aircraft safety.