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Does anyone know if there is a minimum time that an airline has to leave before reusing the same callsign?

I have noticed that airlines reuse callsigns, they usually leave a day or two between reusing them. I'm just wondering is there any regulation on the minimum amount of time that must pass before they can reuse the same call sign again?

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    $\begingroup$ Usually the call sign for a flight is the airline plus the flight number. Many, many flights run the same number daily, which means that the call sign is reused daily. Do you have some reference that they "usually leave a day or two between reusing them"? $\endgroup$
    – FreeMan
    Commented Feb 10, 2020 at 17:53
  • $\begingroup$ Related: How are duplicate call signs dealt with? $\endgroup$
    – Bianfable
    Commented Feb 10, 2020 at 18:17
  • $\begingroup$ @FreeMan - thanks for that. No hard numbers, just something I have noticed looking at ADS-B networks. I'm developing an application which monitors flight cycles and want to know if there is any regulation regarding the legal amount of time that has to pass before a particular call sign can be reused? $\endgroup$
    – Simon Mullaney
    Commented Feb 11, 2020 at 9:19
  • $\begingroup$ @Freeman what do you mean by flights reuse a flight number? As I explained in the comments on the answer, whilst one flight number can cover a multi segment journey, there are rules and conventions on that. And that it is really rare to see multiple allocations of the same flight number. $\endgroup$
    – user1937198
    Commented Nov 11, 2020 at 3:07
  • $\begingroup$ @Freeman it's really rare to see the same flight number used with more than 8 segments and even 4 is unusual. So I'd really surprised if it was common for the same call sign to pop up more than 4 times a day. $\endgroup$
    – user1937198
    Commented Nov 11, 2020 at 3:09

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Airlines work to make sure there is no duplicate callsigns in the same area at the same time. But there is no regulation for time between the reuse of a callsign.

They achieve this by either a callsign being used only once per day or with enough time between flights that there is no chance for both flights in the air at the same time.

The issue comes if two aircraft are in the air with the same callsign. The air traffic control systems will not be able to tell an ATS message (e.g. DEP, EST, CNG) for one aircraft from an ATS message from the other aircraft with the same callsign. Plus many other issues.

So if an airline needs to use the same callsign twice, like if the first flight is really delayed, they tend to append a letter to the end of the callsign, for example UAE212 becomes UAE212C.

Not asked but for completeness; there is also a risk if the callsigns are close but not the same, for example United 222 and United 232, there could be confusion between the aircraft over the radio. In these situations the air traffic controller can change the callsign to make sure there is no confusion. This could be while they are in the same airspace or for the entire flight.

The risk of callsign confusion has been documented many times in many incidents around the world. e.g. May 10, 2004 near Julian, California Callsign Confusion

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  • $\begingroup$ Generally speaking for scheduling airlines will only use the same flight number once per day. Its extremely unusual to see the same flight number scheduled at an airport more than once per day, though occasionally it happens during DST changes. The most common reason for flight number reuse on the same day is delays. (same day being the 24 hours 0:00 to 23:59 local time). $\endgroup$
    – user1937198
    Commented Nov 10, 2020 at 10:35
  • $\begingroup$ There are examples where the same flight number with stops is operated with multiple aircraft, and while not friendly to customers, it’s at least theoretically possible for the second aircraft to depart before the delayed first aircraft lands. $\endgroup$
    – StephenS
    Commented Nov 10, 2020 at 15:44
  • $\begingroup$ @user1937198 The same flight number at the same airport on the same day would be quite rare, but the same flight number can operate several times during the day, with the aircraft flying from A to B, then B to C, then C to D, and perhaps on to more cities beyond that, all with the same callsign. In domestic US operations, that's not at all uncommon. $\endgroup$
    – Ralph J ♦
    Commented Nov 11, 2020 at 1:09
  • $\begingroup$ @Ralph J In scheduling parlance, that's the flight number being used once, calling at A then B then C then D. This does not need disambiguation unless delays cause a later segment to depart before an earlier one. Disambiguation would only be scheduled if the next days flight departed A before the previous day's flight arrives at D. Since these flights tend to be short haul and during the day that's pretty rare. $\endgroup$
    – user1937198
    Commented Nov 11, 2020 at 2:44
  • $\begingroup$ @StephanS that still counts as a single flight number for scheduling purposes. Flight numbers are more closely tied to fare structures than how physical planes actually operate. In the case of delays either a planed or an unplanned airplane switch could result in an unplanned operational suffix being needed. $\endgroup$
    – user1937198
    Commented Nov 11, 2020 at 2:50

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