What is a common indicator of a successful fire extinguishing action in an APU?

A successful fire extinguishing action in an Auxiliary Power Unit (APU) is commonly indicated by the ceasing of the rotation of the APU. This is because when a fire occurs, one of the immediate protective actions is to shut down the APU to prevent further fuel being drawn into the combustion area. If the APU stops rotating, it suggests that the fire suppression system has effectively limited its operation, indicating that the threat of fire has been mitigated.

The cessation of the APU's rotation also implies that the engine's fuel and air supply have been effectively cut off, which is crucial in extinguishing the fire. Once an APU is stopped, it reduces the risk of further combustion from residual fuel within the system. Therefore, seeing the APU stop is a clear, tangible sign that the fire fighting system has been engaged effectively.

This is an important part of aircraft safety protocols, as the proper handling of an APU fire ensures that crew and aircraft remain secure.