Part of the problem is huge blocks of numbers are wasted numbering codeshares and other duplicate or miscellaneous flights.

in the case of DL, I believe most codeshare numbers are in the 3000 series and up (save some AM flights which I think are in the 1000 series)

does DL even operate 2000+ mainline flights on a daily or almost-daily basis?

Mainline? Don’t know. Wikipedia cites “Delta Corporate Stats and Facts” from 2015: “The airline, along with its subsidiaries and regional affiliates, including Delta Connection, operates over 5,400 flights daily”.

9,999 flight numbers should easily cover that. As stated, perhaps the codeshare numbers are the issue.

I think AM, AF, AZ, any codeshare flight from JFK T1 uses a 10xx codeshare number. (AM used to depart T1) Of course right now DL can't codeshare on AM , but that should change back soon.

It was part of scheme where DL used 4xx and 2xx to try and denote which terminal flight was from. Of course that quickly went away. A Terminal gate change would end up confusing passengers/staff.

ATC doesn't always use the same callsigns as appear in GDS systems or are used on FIDS, RIDS, GIDS, apps, etc. When I worked in SOC for UA, we had lots of transcons that were renumbered on day of departure to avoid confusion with other transcons. We would mostly use 8xxx numbers for those flights. So for example, To keep "United 555" from being confused with "Delta 555", it would be filed as say UA8105. This is transparent to basically everyone but ATC, dispatch and the pilots. Example here of why this is important:
As I said way back, generally on round robins, a letter or something is appended to the end of the callsign if it gets stubbed (meaning both segments are in the air at the same time or in the wrong order). There's tons of hacks that go on like this behind the scene to make it easier.

As others have mentioned, adding a character would be a significant work effort (something akin to Y2K or Y2K38) and have minimal benefit.

I am not a frequent flyer. However I have flown Delta a number of times. I made a reservation for December ( so far one time change--both flights) When I made my initial booking the outgoing ( JFK-FLL) flight and the return flight had the same flight numbers. Now, with both flights leaving earlier--it is a different flight number, BUT both the incoming and outgoing are AGAIN the same number.

I've never came across something like this--is this normal? ( last Delta flight was to Vegas in Sept 2019 AND numbers were different)

Thanks in advance.

I have seen it somewhat frequently with DL. There are not that many available flight numbers (so much of the space is reserved for codeshares, Connection, etc.) so they try to conserve. It's not the norm though.

Often it is when the frame is doing a turn, so it's a way to compress flight numbers without having the same callsign in the air multiple times (although obviously they can do that if needed).

existing thread