Many airlines (at least here in the U.S.) have flight numbers that use different aircraft for different legs of the flight. For example, Delta 158 uses a 747-400 from Incheon International in Seoul to Detroit, then an A320 from Detroit to Boston a couple of hours later. This got me to wondering what happens if an earlier leg is delayed such that it's arriving around the time that the next leg is departing the same airport? In particular:

Will both flights use the same radio call sign? That is, will they both be "Delta 158" (ignoring the "Heavy" difference in this particular case?)

If they do use the same call sign, how do controllers know which one is talking, especially if they end up on the same frequency at some point?

It seems like this sort of situation would happen somewhat frequently, at least here in the U.S. where it's common for a given flight number to include multiple legs and for those legs to operate on different aircraft.

This question is related, but doesn't actually address this particular question: Why are some ATC call signs different than the flight number?

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    $\begingroup$ Your question and the answers given are great - I had never considered the dilemma of Flight numbers. I used to own a Navion with 2FM in Alaska as the whole N number. Twice when I flew to the lower 48, I had a conflict. ATC simply informed us to use our WHOLE number. For me, I continued to use 2FM but the other aircraft had to bark out their entire N number. - I learned something here - great question!!! $\endgroup$
    – jwzumwalt
    Commented Dec 13, 2017 at 4:41

1 Answer 1


The scenario you're asking about is common. Let's say that your Delta 158 from South Korea to Detroit is running several hours late, and the decision is made to operate the A320 DTW-BOS on time. ATC doesn't accept having two aircraft flying with the same callsign at once due to exactly the sort of confusion you suggest, so something has to change.

What will happen is that the second flight -- i.e. the one that isn't already airborne, will get a new ATC callsign. Sometimes you'll hear something like "Delta 158A" as the callsign of the flight, or they may use something entirely different like "Delta 9158" as the ATC callsign for that flight. Each company will have their own method of resolving the conflict; some use letters, others don't. Some can go to "9000 + original flight number," and others will simply use something like 9001 for the first "stubbed out" flight that day, 9002 for the next, and so on.

But, the basic answer is that the ATC callsign of the flight changes.

Which does NOT mean, incidentally, that the flight number as far as the passengers are concerned has changed. It's possible to be "Delta 9158" or "Delta 9001" or "Delta 158A" or whatever for ATC and still be "Flight 158" when talking on the PA to the passengers. The fact that the ATC callsign changed is transparent to the rest of the operation, since reservations, ticket counter, baggage handling, and so forth isn't upset when 158 ICN-DTW happens to overlap (in time) 158 DTW-BOS. Only ATC worries about that, so the fix is only visible to them (and the pilots & dispatchers).

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    $\begingroup$ Are the pilots allows to just make up their own callsign? Like "oh crap, 158 is taken, no problem... Chicago tower, Delta 069..." $\endgroup$
    – Keegan
    Commented Apr 25, 2015 at 21:15
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    $\begingroup$ Sadly, no. They are assigned by Dispatch, who knows ALL of todays callsigns. Because just because ATC doesn't have Delta 069 in the system NOW doesn't mean that they won't before you land, and the cascade of "oh that's being used" events would get ugly fast for a big operation like Delta! $\endgroup$
    – Ralph J
    Commented Apr 25, 2015 at 21:55
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    $\begingroup$ @SpongeBob If the pilot could just make up their own call sign, it would be much more fun to use, say, Air Force One. :) Imagine the President's pilots' surprised when they call clearance delivery and are informed the call sign is in use. "Uhh... ok... I guess we'll have to use Air Force 1A..." $\endgroup$
    – reirab
    Commented Apr 26, 2015 at 5:50
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    $\begingroup$ @reirab, wouldn't that be Air Force 9001? $\endgroup$
    – sleblanc
    Commented Apr 26, 2015 at 10:36
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    $\begingroup$ Who is ultimately responsible for the second callsign? Does the airline suggest it or ATC? If ATC how would they know each airline's policy with these overlap flights eg. use '9' prefix on flight number vs. 'add alpha character to flight number'? Australia used to use aircraft registrations as call-signs until a few years ago, which avoided the problem, but to match international standards, they now use flight numbers too, although the overlapping flight number issue is less prevalent there because the 'main' airlines stick strictly to their flight number ranges for domestic .vs intnl flights. $\endgroup$
    – Pete855217
    Commented Oct 6, 2017 at 4:29

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