What should be done right after activating fire suppression for an APU fire?

After activating the fire suppression system for an Auxiliary Power Unit (APU) fire, the appropriate next step is to shut down the APU. This action is critical because an active fire poses a significant safety risk, and stopping the APU helps to eliminate a potential ignition source, ensuring the fire does not escalate. Shutting down the APU also allows safety personnel to assess the situation without the complications that could arise from running equipment that might be contributing to the fire.

Following the immediate shutdown, a thorough inspection for damage would be warranted, but that comes after ensuring that the APU is no longer operating and the risk is mitigated. This sequence of actions is vital for maintaining safety protocols and protecting personnel and equipment.