When will the APU automatically shut down in the air?

The APU, or Auxiliary Power Unit, is designed with safety features to protect itself and the aircraft systems it supports. One critical scenario that warrants an automatic shutdown of the APU in flight is the occurrence of a FADEC (Full Authority Digital Engine Control) critical fault combined with overspeed conditions.

When the APU experiences a FADEC critical fault, it indicates a serious malfunction that can compromise the engine's operational integrity. Overspeed refers to the APU exceeding its designed rotational speed, which can lead to mechanical failure or catastrophic damage. The integrated safety systems monitor these parameters closely, and the combination of a critical fault with overspeed situations would trigger an automatic shutdown to prevent damage to the APU and ensure the safety of the flight.

The other scenarios presented, such as underspeed, low oil pressure, and high oil temperature, may prompt warnings or require crew intervention, but they do not typically result in an automatic shutdown while in the air like the combination of FADEC critical fault and overspeed would.