During which situations is it prohibited to enter the rotor arc?

Entering the rotor arc is particularly dangerous due to the potential for serious injury from the aircraft's rotating blades. The correct answer highlights that it is prohibited during engine start because this is when the rotors are actively spinning up and pose immediate risk.

At this phase, the aircraft is preparing to enter a flight state, and the rotor blades can reach high speeds very quickly, drastically increasing the likelihood of an accident. The danger is not just from the blades themselves but also from the downdraft created by the rotors, which can affect personnel and equipment near the aircraft.

While there are other scenarios like maintenance checks and taxiing out where caution is necessary and specific protocols should be followed, the engine start phase stands out as a critical time when entering the rotor arc is entirely prohibited. This focus on safety during ignition procedures underscores the importance of maintaining a clear, controlled environment when the aircraft is preparing for flight.