Have you ever wondered what airline flight numbers mean? Here's how they are assigned (and why some numbers never appear)
When I started poking around on Flight Radar 24, now many years ago, what intrigued me most were [...]

When I started poking around on Flight Radar 24, now many years ago, what intrigued me most were. flight numbers assigned by companies to their connections.
In this article:
Take for example the flight AZ792 of ITA Airways, departing from Rome and bound for Tokyo-Haneda. What do those two letters represent? And based on what are the numbers chosen? It must be said, first of all. the choice of flight numbers is far from random.
What are flight numbers and based on what they are chosen
The two letters preceding the numbers are used to identify the airline: ITA-operated flights, as mentioned, have the code "AZ"; Air France's are recognizable by the letters "AF"; on routes operated by British Airways, again, the code "BA" followed by the digits, while as far as Lufthansa is concerned, the two letters are "LH", and so on.
As for the numerical part, however, the first thing to know is that it is a combination of digits between 0001 and 9999. It tends to be-but is not a rule-that companies assign a low number to the most frequent or important flights. In contrast, higher numbers are chosen for seasonal flights, or occasional flights.
Ferry flights and papal flights always have the same number
Some flights, however, have some unique and distinctly recognizable numbers. We think, for example, of the so-called "ferry flights": these are connections (non-commercial, therefore without passengers) that companies make to move an aircraft from one airport to another or to pick it up from the factory or, again, for maintenance. In the case of ITA Airways, ferry flights can be recognized by the fact that. AZ code is followed by the number 8000.

Pope Francis
Another particularly recognizable type of links are the papal flights, that is, those made by companies to transport the Holy Father (and his entire entourage). The flight number chosen by ITA is always the same, whatever the destination: AZ4000.
Some digits are never used for flight numbers
American Airlines flight numbers AA11, AA77, and United UA175 and UA93 are no longer used, and the reason is quickly stated: these are the flights hijacked by terrorists on September 11, 2001. Generally, When an aircraft is involved in a fatal accident, the flight number is permanently withdrawn: this is the case of Air France flight AF447 (crashed in the Atlantic Ocean in June 2009), and Malaysia Airlines flights MH17 and MH370 (the former, shot down by a missile; the latter, disappeared without a trace).
There are other numbers, however, that airlines never pick: the figures that correspond to an aircraft model (e.g., the 320 or 737 or 350 or 777) will never appear as a flight number. For superstition, then, carriers will never opt for the 13 or the 17.