When should a safety alert be issued to aircraft under control?

Issuing a safety alert is critical when an aircraft is in unsafe proximity to terrain or other obstacles. This situation poses an immediate risk to the safety of the flight; thus, it is the priority for air traffic controllers to make pilots aware of any hazards that could jeopardize the safe operation of their aircraft.

When an aircraft is identified as being too close to terrain or obstacles, a timely safety alert allows the pilot to take necessary actions to avoid potential collisions. This could involve altering altitude, changing course, or taking other corrective measures.

While the other options describe situations that may require attention or potential intervention, they do not all present an immediate danger as direct as being too close to terrain or obstacles. For instance, if an aircraft is lost or deviates from its flight path, these situations can often still be managed without an immediate collision risk, and procedures can be enacted to address them. Similarly, a low fuel situation might warrant a different type of communication and prioritization, but it does not inherently signal an immediate threat like proximity to terrain does.