When should pilots have the anti-collision light system operating?

Pilots should have the anti-collision light system operating during all types of operations, both day and night, because this enhances safety by increasing the visibility of the aircraft to other pilots and ground personnel. The primary purpose of the anti-collision lights is to prevent mid-air collisions and accidents on the ground; therefore, keeping them on at all times ensures that the aircraft is easily seen, regardless of the visibility conditions. This practice is particularly vital as it helps create an awareness of aircraft presence in busy airspace or on crowded runways and taxiways.

While the other options suggest scenarios where the lights could be operational, they are less comprehensive than the requirement that encompasses all operational situations. For instance, having the lights on only when an engine is in operation or just during night flights would limit their effectiveness, as there are many phases of flight and operations during the day where visibility is essential. Additionally, requiring the lights to be on only when the pilot is in the cockpit does not account for ground operations or scenarios where personnel may be moving around the aircraft. Hence, the broad stipulation of having the lights on at all times is vital for maximizing safety in aviation operations.