Which term describes an aircraft's status when it is overdue, lost, or in emergency?

The term that describes an aircraft's status when it is overdue, lost, or in emergency is "distress aircraft." This designation indicates that the aircraft is experiencing a situation where safety is compromised, and urgent assistance is needed. The use of the term 'distress' typically signals that the aircraft is faced with serious operational challenges that require immediate attention from air traffic control or search and rescue operations.

In aviation, when an aircraft is labeled as being in distress, it often triggers specific protocols and priority responses to ensure the safety of those onboard. This includes coordinating search efforts and providing any assistance necessary to resolve the emergency.

The other options do not convey the same urgency or critical nature associated with being overdue, lost, or in emergency situations. For instance, a routine flight refers to standard operations without any issues, while an alert aircraft may simply be a situation that requires monitoring or heightened attention without implying an immediate crisis. Non-operational aircraft indicates that the aircraft is not in flight or actively being used, which does not relate to its emergency status.