Final answer:
The removal of flight director bars generally occurs when the pilot disengages the Flight Director switch, the autopilot is not in use, or a mode switch occurs that does not require the bars for guidance. The provided reference, discussing radiation detectors and medical treatments, is not relevant to flight director bars.
Step-by-step explanation:
The question of when the flight director bars are removed is specific to the operation of an aircraft's flight management system. The flight director bars are a part of the Flight Director (FD) system, which provides pilots with guidance to maintain the desired flight path. There isn't clear information related to the removal of flight director bars in the reference provided, as it discusses aviation security regarding patients who have recently undergone certain medical treatments that involve radioactive materials. These patients can trigger radiation detectors at airports and as a result, might require special permission to fly. However, typically in aviation, the flight director bars are removed from the display when the pilot disengages the Flight Director switch or when the autopilot is not in use, and their guidance is not required or when switching to a different mode that does not use the bars for visual guidance.