Activating which system is essential after an engine shutdown to prevent cockpit contamination?

Activating the firewall shutoff system is critical following an engine shutdown to prevent cockpit contamination. This system is designed to seal off the engine's airflow, effectively isolating it from the rest of the aircraft, including the cockpit. When an engine is shut down, there can be residual fuel leakage, oil fumes, or exhaust gases which, if not contained, can infiltrate the cockpit area, posing a risk to the crew's health and safety.

By engaging the firewall shutoff, it limits the opportunity for these harmful substances to enter the cockpit environment. This action is crucial in maintaining a safe atmosphere, especially in the event of an engine malfunction or a fuel leak that could cause further complications if not promptly addressed.

The other systems mentioned, while important for various operational aspects of the aircraft, do not specifically address the contamination risk in the cockpit following an engine shutdown.