Short flights out-and-back from a hub can have the same number on the two legs. An example is AA 1992, which today departs PHL 3:06 PM, arrives ATL 5:17 PM, departs ATL 5:57 PM, arrives back at PHL 8:05 PM. (All times US Eastern.)

My mother recently had an itinerary where both her flights were this flight - the PHL-ATL leg on a Sunday and the ATL-PHL leg on the following Saturday.

(It looks like as of Tuesday AA 1992 becomes a DFW-SEA-DFW flight, but it's the same sort of thing.)

I figure this isn't a problem as long as the two legs are flown by the same plane. But what if the PHL-ATL flight were delayed and American decided to fly the ATL-PHL flight as scheduled with a different plane (say because lots of people booked on that flight had connections at PHL?) That seems like it could create trouble.

I recently flew SLC-CLT and CLT-SLC, and both flights were DL2070.

I've also seen this on Delta on ATL-PHL. I had guessed it was limited to relatively short flights - because why waste a number on a short flight, and because there's less room for irregularity where you end up with both flights in the air at the same time - but it looks like at least SLC-CLT on Delta and DFW-SEA on American do it.

If they had to switch to a new plane, wouldn't they also need to refile the flight plan? From what I understand that would be the case, so either a new number would be assigned or the details would be updated in the system. I'm not a pilot, I know a few instrument rated pilots but they're private, not commercial so this may not be the case.

To be fair, this problem is semi-solved by European airlines by having callsigns like BAW44W.

Which uses the last 2 numbers of the flight number, and an alphabetical character. BA use this on a lot of transatlantic type flights where delays etc can be an issue.

> But what if the PHL-ATL flight were delayed and American decided to fly the ATL-PHL flight as scheduled with a different plane

Unless the PHL-ATL flight is more than 40 minutes late, I don't think you'd necessarily have two planes moving through air traffic control with the same code at the same time. While the PHL-ATL flight lands and goes to its terminal, the ATL-PHL plane can be boarded, cargo can be loaded, and such. If PHL-ATL is less than 40 minutes late, you wouldn't even cause a delay for the ATL-PHL flight.

> I'm certain each flight has its own completely unique ID buried somewhere in a database

There is. It's called the Globally Unique Flight Identifier (GUFI), and it's essentially a UUIDv4[1].

[1]: https://docs.fixm.aero/#/general-guidance/flight_identificat...

This line made me chuckle:

> The `Aircraft Identification` is NOT an identifier of an aircraft. It is an identifier of a flight.

Naming things are hard you know ...

>> I'm certain each flight has its own completely unique ID buried somewhere in a database

> There is. It's called the Globally Unique Flight Identifier (GUFI), and it's essentially a UUIDv4[1].

Great, step one cleared.

Now have all the legacy systems everywhere that use 4-digit flight numbers been updated or replaced?

My understanding is that air traffic control is extremely computerised even though direct control is still done over analogue VHF voice radio. The flight numbers are essentially only human-facing (both passenger flight numbers, and ATC callsigns). When flights are handed off between different stations or different flight information regions, their computer representations must conform to the Flight Information Exchange Model (that's the FIXM in the website I linked above).

It's a very top-down initiative by ICAO and all the large aviation authorities—quite unlike the rest of the software development industry.

Frankly speaking ATC is the only thing that really matters in the context of flight safety—ticketing, terminal operations, etc don't (and don't need to) deal with FIXM.

Like the mechanical displays at various airports across the world pictured in the article?

Overall seems like a project of huge size and somewhat unknown reach. Not my domain, but I would imagine all kinds of ground support systems to have the four letter flight numbers baked deep into their assumptions. And not only direct support like fuel, cleaning, work shift scheduling, but also logistics systems for cargo, military systems, myriad of ticketing systems etc.

So we’re already too late by about 20 years to fix the issue properly.

Undoubtedly we’ll push the issue forward for another 10 years before realizing that the next best time to start was today :/

4-digit flight numbers are for humans, not computers

But humans, who interface with computers, are known to cause trouble.

Humans write the code that makes those computers do things, though. And humans don't always read the standards, docs, or manuals before doing so.

Please tell me it's pronounced "goofy."

It actually is. I interned at a civil aviation authority where I had to implement a GUFI generator, and they all pronounced it 'goofy'.

I thought that airlines that fly the same origin to destination daily at the same time always use the same flight number for that flight each day.

At least that's what I feel like I normally see.

Not always but often.

Yeah, always was being generous. Often is probably an accurate qualifier.

Tangentially, train 'headcodes' in the UK are not unique for various historical and convenience reasons. They're supposed to be unique within a given 'area' and period of time, but it's not very well specified what either of those are.

It rarely causes problems, but very occasionally does.

This is true in the US as well. For example, all northbound trains on Amtrak's Coast Starlight route are train 14 and all southbound trains on the same route are train 11. There's currently a train 14 west of Los Angeles that originated this morning. Meanwhile train 14 that originated in Los Angeles yesterday is a couple hours from pulling in to Eugene, Oregon. Likewise there's a train 11 that just departed Seattle and another train 11 that's just about to pull in to San Jose.

I assume that in theory any given train #/station/date is unique, but given Amtrak's notorious delays, I'm sure that theory has been disproven at least once.

There are usually flights from hub to spoke, and spoke to hub, and each one will have the same number. This is within a 24 hour period. 3934 is one such flight between PHX and SEA.

Flight numbers get changed when airlines re-do their scheduling. It's still a fairly manual process as sometimes the numbers are symbolic.

Either way, when they talk about re-using flight numbers on the same day, what they mean is they create multi-stop flights to use the same number for each segment instead.

So you'll see flights like, hypothetically...

> AA 1325 ORD-LGA

> AA 1351 LGA-YUL

> AA 1352 YUL-LGA

> AA 1326 LGA-YUL

Getting converted into a single flight number.

> AA 1325 ORD-LGA-YUL-LGA-ORD

Basically converting this to a flight from Chicago to Chicago via New York and Montreal nets them a 4:1 reduction in numbers used.

You can still book each segment separately, and if they generally book the same plane, say some Envoy Air regional jet, for all four segments, there's little risk that irregular operations will lead to two planes in the air at the same time with the same number. Worst case they can give that one segment a new flight number for that day.

They've been doing this a lot in the last few years, especially on regional flights, and especially in the northeast where there's a ton of short hops.

[edit] If they want a bunch of their flight numbers back they can stop codesharing and switch back to relying on interline ticketing and sell the operating carriers flight number. I can't really think of anything good that's come of codesharing to individual passengers.

> Flight 1555 from ORD to BWI departs at 4PM today, and flight 1555 from ORD to BWI departs at 4:20PM tomorrow.

Why do you find that remarkable? That's just the same flight departing at slightly different times on two different days.

Flight # of Theseus?

Let me try to explain: On routes with high frequency flights, airlines will generally issue a different flight number for each unique departure time to a particular destination. E.g. YY500 departing at 9am to LAX on Mon/Wed/Fri and YY502 departing at 9:05am to LAX on Tue/Thu/Sat/Sun. It's unusual to have the same flight number YY500 apply to either the 9am or 9:05am flight. But if there is no collision (i.e. 9am and 9:05 doesn't operate on the same days) it seems a sensible optimization to reuse the flight numbers. It's just not very common practice in the airlines I regularly fly with (NZ, QF, JQ etc) so can understand why it seems remarkable. Air New Zealand for example only 'reuses' flight numbers for their flagship (e.g. NZ1, NZ2, NZ3, etc) flights whereas their NZ5xx NZ6xx series flights for high frequency domestic flights generally are unique in respect to departure times and rarely do they reuse a flight number for a different departure time.

Like I said, 1 day of separation doesn't seem like anything remarkable, but it begs the question: do the numbers get reused for the same flight on shorter intervals than a day? If so, when? Even more interesting, why?

And, technically, the two flights 1555 in my example aren't the same - one departed at 4:00, and the other at 4:20. Both flights were on-time, so this wasn't a scheduling issue delaying one or the other. 20 minutes isn't a big difference, but it is still a difference. How big does the difference have to be to make Southwest assign two different codes?

Even more philosophically asinine, what do we even mean when we say two flights are "the same?" If Southwest thinks those two flights are the same, then clearly departure time only matters within a certain fudge factor. So that's one part of the picture. Do the make and model of plane need to be the same? Does it need to be the very same physical airplane?

Are any of these questions important to my everyday life? No! Are they interesting? Absolutely!

I've seen situations in the past where the same flight number referred to multiple flights between endpoints that had no apparent relationship to each other. I've also seen a single flight number refer to a flight from A to B to C, but used a different aircraft to get from A to B than from B to C. I can't find any examples at the moment though so maybe the airlines have stopped that particular insanity.

Also, airlines regularly make minor tweaks to their schedules. It's possible that what you saw was one of those tweaks taking effect.

Flight codes are meant to signify the particular starting point, stops, and destinations that will be made along the way.

You can change the aircraft and the flight code stays the same. Many flight codes happen on a regular schedule, like daily. But the same flight path and pattern at a different time on the same day will get a different flight code.

Some patterns have many flights per day along the same path — maybe ten or more. But they each get a different flight code.

All of this is within a single airline or codeshare, of course. Two different airlines can use the same four digit set of numbers for totally different flights, because the full flight code includes both the airline designator and the four digit number.