What happens to the APU if it exceeds its operational limits?

When the Auxiliary Power Unit (APU) exceeds its operational limits, it is designed to shut down automatically to protect itself and the aircraft systems. This safeguard ensures that any potential damage caused by overheating, over-speeding, or other critical conditions is avoided. Automatic shutdown prevents further complications that could lead to hazardous situations while also safeguarding the integrity of the APU system and the aircraft.

In contrast, a standby mode would not provide sufficient protection under extreme conditions as it implies that the APU is still somewhat operational. Continuing to operate normally while exceeding limits would risk severe damage and is not a safe operational protocol. Requiring manual intervention to restart may occur after a shutdown, but it is the automatic shutdown that serves as the first line of defense against operational exceedance, making it a vital characteristic of the APU's safety features.