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The Science behind Flight Numbers


What do you call a flight with eight legs?

a. SpiderRouting b. OctoFlight c. Totally Confusing Technically, both “a” and “b” are correct—spiders have eight legs, and octopi have eight tentacles.  But also correct answer, at least in this context, is “c,” totally confusing! I know many of you have wondered what Network Planning is thinking, stringing flight legs together with no apparent regard to geography.  But we really do have a method to this apparent lapse in routing logic.  It’s really very simple.  

We’re running out of flight numbers!

Like any airline, Southwest has to live within a very strict set of flight number ranges and rules.  To start with, the numero-uno industry-wide rule is that no flight number can contain more than four digits, meaning we only have up to flight number 9999 to work with.  (No airline can use 5-digit flight numbers!   While this has been debated in the industry for years, the level of effort to make the change from four to five digits would be HUGE, and even the level of technology change to add alpha characters to published flight numbers would be gargantuan…although it would be fun to “name” flights—“Now boarding, Southwest Airlines flight FRED to Los Angeles.”)  

We make the rules.

The majority of our current flight numbering rules are more internal to Southwest in nature, dictated primarily by our operating and scheduling systems, as well as by history.  A few examples:
  • Scheduled, WN-operated flights must (for now) be numbered between 1 and 6999.  Numbers 7000-9999 are reserved for use by Maintenance, Dispatch, and Charters, in specific ranges for each.
  • DAL-HOU/HOU-DAL numbers are absolutely sacred during the week, with flight 1 operating the 0700 DAL-HOU and flight 2 as the 0630 HOU-DAL.  You can even see “holes” in the flight number sequencing reflecting our frequency decrease in that market, with those gaps reflecting flight numbers that were assigned to flights that have since been eliminated.   And even if the DAL-HOU/HOU-DAL legs are on multi-legged flights—the DAL-HOU/HOU-DAL flight number “wins!”
  • Flight 711 must be assigned to a SAT-LAS flight, just because.  🙂


There are many flight numbers that are simply off-limits.  Some are pure superstition—you won’t see flight numbers 13, 1313, or 666. Others are operational/safety prohibitions, like 737 or any flight number ending in 000 (1000, 2000, etc.) which could cause confusion between the cockpit and ATC.  And some flight numbers literally cancel each other out because they are operating through the same airport at the same time—Southwest has a very complex automated algorithm (called MUFN—Maintain Unique Flight Number) to prevent flights arriving and/or departing at the same airport from sounding too similar to other flights at similar times, confusing flight crews or air traffic controllers.  

We’re not the only ones!

Southwest isn’t alone in doing this—most other airlines have new ways to save flight numbers, because they’re running out of numbers as well.  For example, most of the others used to assign odd flight numbers to Westbound or Southbound flights, with even flight numbers assigned to Eastbound or Northbound flights.  But that’s changed . Some, like AA and DL, are scheduling many out-and-backs, where a flight leaves one of their hubs for somewhere then comes back to the same hub.  For example, over at AA their flight 1958 operates DCA-PBI-DCA, while Delta’s flight 1990 operates ATL-SAV-ATL.  United has a different flight numbering strategy, in which they string together flights together but change the aircraft types on each leg (a practice called “change-of-gauge” but really amounts to a connection from the Customer standpoint).  United 1644 operates BDL-IAD-IAH-MCO, yet BDL-IAD operates as a 737-800, IAD-IAH operates as a 757-200, and IAH-MCO flies with a757-300.  And yes, they sell BDL-IAH as a onestop through flight!  

Why eight legs?

Unfortunately, neither of those flight-numbering methods work for Southwest, due to technology issues in numerous Southwest systems.  But because of the linear nature of our network, we came up with a different solution to minimize flight number usage.  At Southwest, any flight can have up to eight legs, as long as they don’t touch the same city more than once, and each flight has to do (more or less) the same things at the same thing at the same times each day of the week.  Not all of our flights have that many legs—actually this summer only three have eight legs, and 17 others have seven legs. But our flight-numbering scheme creates some very “interesting” flight routings, like our flight WN3 which operates ICT-DAL-HOU-MDW-OMA-DEN-ABQ-LAS-BDL.  Trust me….we don’t expect anyone including our enemies or ex’s to fly that itinerary from ICT to BDL, and to prevent that we suppress sale of anything more than a three-stop direct.  And while we do suppress most of the more obvious and bizarre onestop and twostop directs that just fly in the face of geography (like WN4562 LAS-OAK-PHX), sometimes when there are no better alternatives we’ll sell some seemingly odd ones, like WN835 GSP-BWI-BHM or WN125 ATL-BWI-CHS.A change is coming.  

Eventually this will change. 

We’re working to implement technology changes to give us more lee-way on flight numbering.  But given Southwest’s vast, deep, and linear network, we’re always going to be a maverick (which should surprise nobody!).  So next time you’re looking at a flight number’s routing and you think “ARE THEY CRAZY?” you can answer …… YES.  Yes, we are … crazy like a fox, just like Southwest has always been!  Have a good week everyone!