What is an important guideline during engine startup concerning rotor movement?

During engine startup, safety precautions regarding rotor movement are crucial to prevent injuries. The guideline that states personnel should not enter the rotor arc when the rotor is spinning is particularly important because the spinning blades pose a significant risk of injury or fatality. The high velocity at which the rotor blades rotate makes it dangerous to approach or be in close proximity to the aircraft.

Understanding that rotor blades can cause severe harm, protocols are established to ensure that all personnel maintain a safe distance when the engines are started, especially before the rotor has come to a complete stop after shutdown. This guideline helps to create a safe working environment and ensures that all personnel are aware of the potential hazards associated with rotor movement.