Under which conditions will the APU automatically shut down on the ground?

The automatic shutdown of the Auxiliary Power Unit (APU) on the ground is triggered by several critical conditions that ensure both safety and proper functioning of the aircraft systems. The correct understanding here includes that the APU is a crucial component providing electrical power and air for starting the engines, and it must operate within specific parameters to prevent failure or more severe consequences.

When the APU experiences any combination of failures such as underspeed, FADEC (Full Authority Digital Engine Control) critical faults, overspeed, high oil temperature, EGT (Exhaust Gas Temperature) over-temperature, low oil pressure, or a fire, it indicates that the APU is no longer operating safely. Each of these conditions could lead to severe damage to the APU, pose risks to aircraft safety, or compromise the performance of other systems reliant on the APU’s output. Therefore, the shutdown feature is designed to engage automatically under multiples circumstances to protect both the APU and the aircraft crew and passengers.

This comprehensive approach to monitoring and shutting down the APU illustrates the safety systems in place within the aircraft that react not only to singular threats but also to composite failures that could develop if multiple conditions arise simultaneously.