What is the primary precaution to take when starting and running an aircraft engine?

The primary precaution to take when starting and running an aircraft engine is ensuring the propeller area is clear of debris. This is critical because propellers can create a significant wind blast and can be dangerous if any loose objects are caught in this airflow. Additionally, if there are any personnel or items too close to the propeller, they could be injured or damaged when the engine is started.

While ensuring a safe distance for personnel, checking fuel levels, and having a mechanic present are all important safety measures in their own right, the immediate and most critical risk during engine start-up involves the propeller area. Therefore, confirming that this area is free from obstruction is paramount to ensure the safety of everyone involved in the operation.