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It's not uncommon for two distinct flights to have the same number. For example, DL10 consists of a narrow-bodied jet flying from Denver to Minneapolis–St Paul and then, two to three hours later, a wide-bodied jet flying from MSP to London Heathrow1; AA55 splits Manchester–Chicago–Orlando. This question gives another example and asks how ATC would deal with the situation where one of these flights is delayed leading to both being in the air at once.

But that begs the question of why one would give the same number to two flights in the first place. I understand that, historically, it was common for a long-haul flight to make several stops for refuelling but it's a long time since that was common and, here, we're talking about completely different aircraft and not just a refuelling stop.

What advantage is there to an airline in assigning the same number to two flights like this?

1 At least, it did when I posted the question in early October 2017. Now, a month later, it's a narrow-body from Kansas City to Minneapolis and then a Minneapolis–Heathrow flight. On some days, there's about 45 minutes between the two; on others, a little over three hours.

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  • $\begingroup$ I suggest put the word advantage in the question otherwise others may think it's a duplicate without reading the whole question $\endgroup$
    – vasin1987
    Oct 5, 2017 at 16:32
  • $\begingroup$ @vasin1987 A duplicate of what? The question I linked to asks how to cope with this practice; I'm asking why it's done at all, which seems to be quite clearly different. (Of course, if this question is a duplicate of some other, that's fine.) $\endgroup$
    – David Richerby
    Oct 5, 2017 at 16:34
  • $\begingroup$ Strikes me as an excellent question, especially since airlines seem to tend to use 1- and 2-digit flight numbers for international (or at least trans-oceanic) flights, using a 2-digit flight number for a US-domestic leg. However, if one wanted to get from Denver to Heathrow, DL10 would be the way to do it, albeit with an annoying layover at MSP. $\endgroup$
    – FreeMan
    Oct 5, 2017 at 16:48
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    $\begingroup$ Related if not duplicate: Can two different planes share the same flight number? and Is a flight number unique for the schedule, origin and destination? $\endgroup$
    – Farhan ♦
    Oct 5, 2017 at 16:51
  • $\begingroup$ @FreeMan Quite a lot of flights between Los Angeles and the east coast have two-digit flight numbers. There are plenty of non-stop flights from LHR to DEN but DL10 with its layover is the only options with Delta. $\endgroup$
    – David Richerby
    Oct 5, 2017 at 17:06

3 Answers 3

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As far as I know, this practice is mostly just limited to very large airlines in the U.S. The reason for this is that the airlines are so large that they'd literally run out of flight numbers in some of the allocated ranges. Usually the 9000 range is reserved for special flights (e.g. repositioning flights, or, as discussed in the answer to the linked question, when a different callsign is needed,) but other ranges are also reserved for things like codeshares onto the flights of partner airlines and for flights operated by regional carriers instead of by mainline.

For example, this post from this flyertalk thread lists the following ranges used by Delta observed by a frequent flier on that airline:

GoJet: 6000s
Endeavor: 3000s
Shuttle America: upper 2000s to lower 3000s.
Compass: 5000s
SkyWest: 4000s, 7000s
Mainline: 0001-2999

From personal experience, I've noticed that partner airline codeshares on Delta are usually up in the 8000 range (maybe also part of the 7000 range?) Delta has a lot of partner airline codeshares, so quite a lot of the flight numbers go to this. Delta mainline operates a lot of flights. According to Wikipedia, Delta mainline and its regional carriers operated 5,400 flights per day as of October 2016. Having only 3,000 flight numbers available to mainline means that they'd almost certainly not have enough if they didn't reuse some of the numbers.

In total, according to Delta, their route network, include partner airline flights, totals over 15,000 flights per day. There are only 10,000 possible flight numbers and, when the reserved 9000 range is removed from consideration, only 9,000 possible flight numbers (or 8,999 if "Delta Flight 0" isn't allocated, which I expect it isn't.) As more and more of Delta's partner flights are given codeshares to Delta flight numbers, it would be very easy to exhaust the entire range of possible flight numbers.

An additional reason (possibly more relevant historically than today) is that it allows the airline to have a "direct" service to a city it doesn't actually fly to non-stop. Nowadays, people commonly use the term "direct flight" to mean "non-stop flight," but that's not the technical use of the term in airline-speak. The term "direct flight" really just means "we have a flight that gets you from A to B, but it might stop in C and D along the way."

Back in the day, this made more sense, as it wasn't as common for aircraft to have enough range to connect cities non-stop. So, a flight from New York to Seoul, for example, would typically stop in Anchorage to refuel before crossing the Pacific. Going even farther back, the first trans-Pacific flight (from San Francisco to Manila) had stops in Honolulu, Midway, Wake Island, and Guam. Since almost all of the passengers were going from San Francisco to Manila (not to, say, Wake Island or Midway, which were previously uninhabited,) it made sense to have a single flight number rather than using a different one for each segment. Nowadays, however, due to the much longer range of aircraft, there's not as much need for 'direct' flights with stops.

First Trans-Pac flight Pan-Am's SFO-MNL route via HNL, Midway, Wake, and Guam

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  • $\begingroup$ There's also the pattern where back-to-back flights AAA-BBB and BBB-AAA, on the same plane, share a number. I associate this with going out and back from a hub. In the cases I'm familiar with, American (hub at PHL) runs PHL-ATL-PHL flights and Delta (hub at ATL) runs ATL-PHL-ATL flights. $\endgroup$
    – Michael Lugo
    Jan 8, 2019 at 19:11
  • $\begingroup$ @MichaelLugo: That's arguably "just" the same flight - a single directed path forming a cycle (in graph-theory terms). $\endgroup$
    – einpoklum
    Jan 10, 2021 at 11:36
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I think the reason for situations as described is the limited number of "good" flight numbers, compared with the number of flights -- especially when codeshared flights are considered.

Generally, airline flight numbers are from one to four digits. There are cases where a letter gets appended, although this seems mostly reserved for special cases, and is not a universal practice. So in theory, that would be 9999 possible flight numbers, which seems like it should be enough to more than cover any airline's schedule (a few thousand flights max) in a day. However...

Some ranges of flight numbers get reserved for things like charter flights, maintenance ferries and other aircraft repositioning, and "stubbed out" flights. The latter occur when, for instance, flight 123 is scheduled to operate from AAA to BBB to CCC, but the AAA-BBB flight is running so late that the airline decides to use another aircraft & crew (if they weren't already) and operate the BBB-CCC leg on time. That latter leg needs a new flight number, since you can't have two "flight 123" in the air at once. So some range of flight numbers gets reserved for use in situations like that, and ATC sees flight 9123 (or 9001) operating the BBB-CCC leg that day.

Beyond this, codeshare flights get a flight number. Often, when airline X sells a ticket to their customer so that he'll fly on a flight operated by airline Y, the flight will have both an airline Y flight number (which is what ATC will use) and also an airline X flight number (which may be printed on the ticket & may show on the monitors in the airport). So while the passenger's itinerary may say Airline X flight 12 AAA-BBB and Airline X flight 7422 BBB-CCC, the reality may be that the second flight is really Airline Y flight 1422. But to make things look like you're flying entirely on Airline X, that carrier uses the 7xxx range of flight numbers for their codeshare flight. Considering how large the codeshare networks can be, that could easily run to several thousand flight numbers reserved for this purpose. (As reirab's excellent answer clearly demonstrates.)

So they start to run out of numbers, and the desirable numbers (i.e. low & recognizable, like 10 or 12 or 200) end up being reused. Thus the frequent flyer knows that he always catches flight 76 at 9:00 Monday morning from AAA to BBB, which is easier than flight 5976 today and flight 3921 last week and some other long identifier next week. He doesn't care that after BBB that same flight number is used an hour later, perhaps with another aircraft and crew, to go on to CCC. He's just taking the same "flight 12" that he's taken for the last several weeks or months.

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  • $\begingroup$ If you allow leading zeros eg BAW007 is considered valid and of course different than BAW7, then you have more than 9999. 11110 if I calculated correctly :D $\endgroup$
    – Stelios Adamantidis
    Dec 13, 2017 at 15:29
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    $\begingroup$ @SteliosAdamantidis I've never seen or heard of that convention. Flight numbers are almost always used at ATC callsigns, and those may PAD a single-digit flight number with leading zeroes in many cases, but they certainly don't differentiate flight 7 from flight 07 from flight James Bond from flight 0007. So I don't think anybody is actually able to expand the range of usable callsigns that way. Would work fine in computer code, but not so well for human factors. $\endgroup$
    – Ralph J ♦
    Dec 13, 2017 at 15:33
  • $\begingroup$ Sorry I don't understand you, you say it can't happen or it doesn't differentiate them? There was the ill-fated KAL007 (that some believe it was in a James Bond mission) so it can happen. As if it doesn't differentiate, it does, though it's a good practice not doing it for avoiding call-sign confusion $\endgroup$
    – Stelios Adamantidis
    Dec 13, 2017 at 15:42
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    $\begingroup$ @SteliosAdamantidis Flights may be referred to as KAL 7 or KAL 007, but it's two ways of referring to the same flight. Nobody uses leading zeroes to differentiate two separate flights, at least as far as I've ever seen. Not saying that a different ATC system couldn't use the convention where flight 1 is different from flight 01 and flight 001 and etc, but AFAIK from a couple decades of observation, that isn't the way it's done in US (or worldwide) ATC today. $\endgroup$
    – Ralph J ♦
    Dec 13, 2017 at 15:47
  • $\begingroup$ If you're splitting an AAA-BBB-CCC flight (let's call it flight 1234) into AAA-BBB and BBB-CCC parts because the AAA-BBB part is running really late, that seems like a good use case for appending letters to distinguish the off-nominal situation (for instance, designating the BBB-CCC part as flight 1234A). $\endgroup$
    – Vikki
    Apr 1, 2020 at 19:58
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flight number is colliquial for the IATA flight code used in booking systems.

As FreeMan has observed correctly, shorter flight numbers identify longer or otherwise more premium (i.e. business class only) flights.

The same number only refers to the route and may not only change the aircraft, but also the airline operating the flight (leg).

The examples you have provided are each one flight with different legs.

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    $\begingroup$ Thanks for posting but I don't see how this answers the question at all. In what sense is it "one flight" when you go from Denver to Detroit in one plane, getting off that plane, sitting in Detroit for a couple of hours and then getting on a different plane to fly to London? Why is DEN-DTW-LHR sometimes considered as a single route but most often (even by the same airline) considered as two separate routes. Specifically, as the question says, "What advantage is there to an airline in assigning the same number to two flights like this?" $\endgroup$
    – David Richerby
    Nov 13, 2017 at 13:38

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