Under what circumstance must a pilot submit a written report during an emergency?

A pilot is required to submit a written report during an emergency upon request from Air Traffic Control (ATC). This requirement is in place because ATC has the responsibility to ensure the safety of air traffic and to gather information on incidents that could affect air navigation or indicate a broader safety issue.

In situations where an emergency occurs, ATC might need detailed information about the circumstances surrounding the event, such as the nature of the emergency, actions taken by the flight crew, and any specific issues that may have arisen during the flight. This information can be crucial for incident investigation and improving future safety measures.

The other scenarios described do not have the same regulatory basis for a report. While the aircraft crew or home base might request information or debriefings, those do not carry the same mandatory nature as a request from ATC, which is specifically tied to maintaining safety in the airspace and effective communication among flight crews and air traffic services. Submitting a report once back at home base or only after landing could lead to critical information being lost or delayed, which is why the immediate response to ATC's request is emphasized.