After a fatal accident, most airlines quietly stop using the associated flight number. The four flight numbers involved in the Sept. 11 attacks—United Flights 93 and 175 and American Airlines Flights 11 and 77—are all permanently retired.

Yes, it is standard industry practice for airlines to retire a flight number following a fatal or high-profile crash, though it is not a formal requirement by international aviation authorities. This is done primarily out of respect for the victims and their families, as well as to avoid the "stigma" or psychological unease passengers might feel booking a flight with a tragic history. For example, the flight numbers from the September 11 attacks (AA11, AA77, UA175, UA93) and Malaysia Airlines MH370 were permanently retired. Occasionally, an airline might keep a number if the incident was minor or non-fatal, but for significant disasters, the number is usually replaced by a new one on the same route. In 2026, this process is almost instantaneous; as soon as a major accident is confirmed, the airline's global distribution system (GDS) is updated to ensure the specific flight number is no longer searchable for future bookings, replacing it with a fresh numeric designation to maintain the scheduled service.