What is a critical condition for APU shutdown in flight?

Overspeed is a critical condition for APU shutdown in flight because it indicates that the APU is operating at a speed that exceeds its designed limits. This can lead to mechanical failure, potential damage to the APU components, and a risk to overall system safety. An overspeed situation often arises from a malfunction of the control system or a disruption in the APU's normal operation, which can cause the engine to spin uncontrollably fast. To mitigate risks, automatic protective systems are in place to shut down the APU promptly in the event of an overspeed condition, ensuring that any potential hazard is minimized.

In contrast, while engine starting malfunctions, low fuel pressure, and high altitude can affect APU operation, they do not always warrant an immediate shutdown of the unit during flight in the same critical and time-sensitive manner as an overspeed condition. Proper management and monitoring of these other conditions do take place, but they typically allow for more troubleshooting or corrective actions before a complete shutdown is necessary.