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Having traveled internationally a lot over the last 20 years, I've noticed that the flights from the airline's home country are usually odd numbered, while flights to home country are usually even numbered. I have not flown enough domestically within any one country to notice any patterns though.

Are there specific rules (probably unwritten) or other things that dictate odd vs even flight numbers?

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    Outbound (whether from a home country or domestic home base) usually starts from 1 or xx1 so they will be odd, and the other way will be the corresponding even number. But I don't know if this is written down anywhere. Similarly for railways an "up" and "down" is often defined and the train numbers can also have the same pattern (e.g. odd from and even to Paris for French trains). I don't know if it's actually formally written down somewhere though for flights.
    – xngtng
    Commented Apr 10, 2022 at 19:36
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    Which airlines do you have experience with? Just browsing through currently airborne flights on an arbitrary flight tracker does not confirm your 'usual' experience. Some airlines obviously have some kind of system behind their usage of even and odd flight numbers, but not necessarily how you describe. Air France and Lufthansa are e.g. using even numbers for outbound flights and odd numbers for returning flights. Many airlines, e.g. American, Delta or United are not at all using odd and even flight numbers to distinguish between outbound and returning flights. Commented Apr 10, 2022 at 20:17
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    Lufthansa is clearly the other way around for Lufthansa. Example: FRA->BOS is 422, BOS->FRA is 423
    – Hilmar
    Commented Apr 10, 2022 at 22:13

1 Answer 1

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There is no industry standard, written or otherwise, for the way that flight numbers are allocated. Some airlines will have published or unpublished standards for how flight numbers are allocated - sometimes these will be strictly followed, whilst other times they will only be loosely followed.

As a few examples...

Australian airline Qantas uses ranges of flight numbers to designate a number of things, but the most important of these is that flight QF1 to QF399 are international flights, whilst QF400 and above are domestic flights (not including codeshare flights which start around QF2600).

Qantas DOES follow the odd/even pattern you described for international flights, with odd flight numbers being used for flights departing Australia, and even flight numbers being used for flights arriving in Australia. eg, QF7 from Sydney, Australia to Dallas, USA is an international flight (<400), departing Australia (odd).

For domestic flights, South/West bound flights have odd numbers, whilst North/East flights have even numbers. Some ranges can also designate the type of aircraft normally flying on a route. eg, QF1501 is a south-bound route that would normally be flown by a QantasLink Boeing 717!

Germany's Lufthansa Airline goes the other way when it comes to even/odd flights - even numbers designate a flight out of Germany, whilst odd designates a flight to Germany. Lufthansa flight numbers also often designate the destination of the flight. eg, flight numbers LH400-499 are flights to North America. Thus LH458 is a flight to (even) North America (4xx) - in fact it's from Munich to San Francisco.

For domestic flights, Lufthansa flights out of Frankfurt airport always appear to be even numbers - although I'm not sure if that is always the case or just the vast majority of the time.

United Airlines does not follow an even/odd flights number rule. For example, the two UA flight from San Francisco to London Heathrow today are UA901 and UA948 - one odd, the other even! United did historically follow a geographic model for international flight numbers, with UA8xx being flights to Asia Pacific/Oceania, UA9xx being flights to Europe, etc, and whilst many flights do still followed these conventions, many do not. For example, UA1 to Singapore, and UA98 to Melbourne, Australia.

Some airlines (especially those that use odd/even numbers for direction) will follow a "sequential" numbering scheme where the return flight number is one higher than the outbound flight, whilst others will not. For example, the return flight to LH458 mentioned above is indeed LH459 (SFO-MUC). Same for UA2 (SIN-SFO) and UA99 (MEL-LAX). However again this is not strictly followed by all airlines - the return flights for United's two London flights mentioned above (UA901 and UA948) are UA900 and UA949 which are sequential but in opposite directions to each other, whilst the return flight for United's UA863 (SFO-SYD) is UA870!

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  • For LH I think the flight numbers always designate the destination. 8xx is Scandinavia,11xx is south western europe, 12xx is south eastern europe, 13xx is north (western) africa. 3xxx are trains. Tho this appears to breakdown a little bit in central europe. France, Italy, Switzerland, Austria have some weird numbering. But all outbound flights from their hubs are even while return flights are odd. I don't know if they operate flights between Munich and Frankfurt tho
    – SirHawrk
    Commented Apr 11, 2022 at 6:39
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    @SirHawrk Sure there are Lufthansa flights between Munich an Frankfurt, several each day. FRA to MUC has even numbers, MUC to FRA odd - looks like Frankfurt "beats" Munich (no surprise, it's the way bigger hub).
    – Sabine
    Commented Apr 11, 2022 at 7:04
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    I'll bet some airlines started out following a pattern, but eventually had to make exceptions when they added a flight and there was no room for it where it should belong. Over time these exceptions accumulate. Airline mergers also complicate things.
    – Barmar
    Commented Apr 11, 2022 at 14:19
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    @Barmar: I bet you're right. I'd also speculate that human-interpretable flight numbers may have been more useful in a less-computerized era.
    – ruakh
    Commented Apr 11, 2022 at 18:06
  • Assuming there's some team of people setting the numbers of new routes, you may also have a situation where the first team set it up with some rules, but only wrote down some/didn't write down any and over time members of that team left and new ones came on who didn't get all the rules from the first team. Or they just decided to change the rules and grandfather in the old routes. Commented Apr 12, 2022 at 12:59

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