What causes the APU to shut down when the STOP/START switch light is selected?

The APU, or Auxiliary Power Unit, is an essential component in ensuring that electrical power is available when the main engines are not running. When the STOP/START switch light is selected, a shutdown occurs typically due to a signal that indicates an overspeed condition. The APU ECU, or Electronic Control Unit, monitors various parameters related to the APU's operation, including its speed.

If the APU exceeds its designated operational speed—known as overspeed—this poses a safety risk and could lead to catastrophic failure. The ECU is designed to automatically sense this condition and initiate a shutdown to prevent damage to the APU, ensuring the integrity of the unit and the safety of the aircraft.

Understanding that the ECU plays a vital role in monitoring APU performance helps clarify why overspeed is critical in this context. The other options like battery power loss, fuel flow interruption, or overtemperature signals can also cause shutdowns, but in the specific case referenced regarding the STOP/START switch light, overspeed is the key condition that triggers the immediate cessation of operation to promote safety and reliability.