In which instance is it prohibited for anyone to enter or exit the rotor arc?

Entering or exiting the rotor arc is particularly dangerous when the pilot door is open while the rotor is spinning because it increases the risk of accidental injury from the rotating blades. The rotor arc is the area where the helicopter's rotor blades can strike someone, and having the door open indicates that the helicopter is likely in operation. This situation heightens the potential for the blades to make contact with an unwary person trying to enter or exit. Therefore, strict protocols and safety measures are enforced to ensure that personnel remain clear of the rotor arc during such circumstances to prevent injury or fatalities.

In other scenarios, like when the cockpit is open or during engine startup, while caution is necessary, the specific combination of an open door and spinning rotors presents the highest level of risk, underscoring the importance of situational awareness and safety protocols around rotating components.