Which condition requires the use of the inertial separator?

The correct condition that requires the use of the inertial separator is when operating in icing or foreign object debris (FOD) conditions. The inertial separator is designed to protect the engine from unfiltered air that may contain ice or debris. When flying in environments where these hazards are prevalent, such as severe icing conditions or areas with loose dirt, rocks, or other particulate matter, the use of the inertial separator becomes crucial.

By engaging the inertial separator, the airflow is diverted, allowing only the cleanest air to reach the engine's intake while minimizing the risk of ice ingestion or debris entering the engine. This is particularly important for maintaining engine performance and reliability, ensuring safe operation during these hazardous conditions.

In contrast, the other conditions listed do not specifically necessitate the inertial separator's deployment for operational safety.