There may or may not be a historical reason but for now the answer is almost certainly no more complicated than "there has to be SOME time boundary, if it's too short then there aren't enough valid routings and if it's too long then you get a bunch of other issues around logistics of too many people in airports complaining about their long layovers, someone picked this number that was 'good enough' and that's it"

It may be due to historical business flying fare discrimination practices (as business flyers tend to be less price sensitive than leisure flyers). Business flyers often book multi-city trips to visit multiple clients and in most cases they will likely need a layover of more than 4 hours to visit a client (thus creating a fare break). Cheaper fares can also have fare combination restrictions (such as requiring A-B-A routing, aka, they can only be booked on a one-way or roundtrip ticket). This also serves as a form of fare discrimation for less price sensitive business flyers on multi-city trips. Back in 2016, AA tried to push these restrictions up into higher class fares and got some serious blowback (and ultimately went back to only having them on the cheapest fares) -- https://www.flyertalk.com/forum/amer...s-cheaper.html

yeah that definitely is a factor in the practical upper bound for picking this sort of number

Regardless of price discrimination, it's not appropriate to ticket a transfer as an intended "destination" because the airline is under no obligation to deliver the pax to that point - so even absent financial considerations DL would want to discourage this.

interesting.
So then, if one were to book a fare with a longer than 4 hour layover by constructing 2 legs (like multi-city in the search), and there were IRROPS on the first flight, can the second be rebooked without having to pay a fare difference?

The real annoyance, for me, is when this forces a tight connection. My "ideal" connection at a hub w/lounge is 2-3 hours, but lounge or no lounge I get antsy at anything under 60-90 minutes (depending on the airport - 90 is reserved for places like LGA, with known congestion issues, being in the mix). Five hours is a long connection, but if the choice is between 50 minutes or 5 hours...

Wrong terminology. You are not booking a single fare when layover is longer than 4 hours for US48. It's a broken fare (a separate fare for each leg, instead of single fare based on O&D) booked on one ticket. If there were IRROPs on first flight, yes, they would reaccomodate you on the second when the two fares are booked on one ticket. Schedule changes would also potentially permit free changes to one or both fares when booked on one ticket. Although it may be cheaper to purchase fares on separate tickets instead of a single one due to end-on-end fare combination restrictions on cheaper fares. If you use Google Flights multi-city search, it will alert you if it would be cheaper to purchase the fares on separate tickets instead of a single one.

Savings are usually not all that significant. The below would save $39 if flights were booked separately as an L fare + K fare vs. 2 K fares on one ticket. The L fare on DTW-ATL requires A-B-A end-on-end combinations and can't be booked as part of an A-B-C routing.

https://cimg2.ibsrv.net/gimg/www.fly...817e90b41d.png


https://cimg2.ibsrv.net/gimg/www.fly...1d4e6050c7.png

have you considered how much time you would save over a year if you booked 50 minute connections vs. 3-hour connections? You would have to be extremely unlucky with misconnects to come out behind booking the shorter connections. I connected 59 times last year between ATL, DTW, MSP, LGA, SLC and LAX. If I added 2 hours on to each of those that's 118 hours extra sitting in airports. In that same year, I had exacty 2 IROPS issues at connecting airports that required rebooking (e.g. I had more IROPs but I excluded the incidents at the origin since the length of the connection didn't matter in those cases) and in neither of those cases did I absorb an overnight delay (total actual delay time was 6 hours).

I usually like this kind of analysis, but in my life, time spent at an airport isn't wasted time :). Maybe better question, is what are you doing with your extra time saved :D

Disagree on the fare savings not being insignificant. Granted this is just my own anecdotes, but if I want to fly A to D (where B & C are Delta hubs, and A & D are spokes), booking separate ticket A-B + B-D is usually considerably more expensive than single ticket A-B-C-D. When it’s A-B-C-D, both start&end are at spokes. When it’s separate A-B and B-D, I am effectively buying two tickets each with a start or end at a DL hub, and like all airlines DL charges relatively more if you start/end in a hub. Booking A-B-D as one ticket would be ideal, but the 4-hour rule prevents that.

It’s also a giant pain because I have to explain to my travel coordinator why separate tickets were booked and justify why they cost more. If the cost difference is too much, I can’t justify it. Even though A-B-D (or technically A-B, B-D) is usually overall quicker than A-B-C-D, and less risky for misconnects with only one connection needed vs two.

(1) I am, in fact, quite unlucky in this respect (especially at larger airports). I think I went like six trips this summer without not having a significant delay or canx or other IRROPS-y fun. I have joked that I seem to attract flight chaos.
(2) I don't like running at the airport, and again, at e.g. ATL if I suddenly end up with my flight being moved from E/F to B and they have this stupid reduced-frequency thing on the Plane Train (both happened last time I was there), that's suddenly a 30-minute hike. The far corners of JFK are similar (it was even worse when they still had T2). DTW isn't as bad, MSP is pretty bad, and LGA is a somewhat awkward mess. Basically, I want 30-40 minutes of "leg time" and you have to slash 15 minutes off of the departure time because of gate closing rules.
(3) A blown connection can easily obliterate multiple sets of time savings there if there's no space on the replacement flight. Now, mind you that also means I can play IRROPS games...but the thesis of the above is basically killed.
(4) Time at the airport isn't totally lost. I can work in the lounge. And getting home at, say, 9 PM vs midnight isn't a big deal, so some of those losses are ephemeral.
(5) Finally, if it's a tight connection between two somewhat lengthy flights with no meal service (e.g. ORF-DTW/DTW-MCO), that means I'm now going quite a while without being able to (practically speaking) stop for a meal and 45 minutes in the lounge becomes necessary vs hoping the line is short at McDonald's.

Basically, I'd prefer to be able to make my own judgment call on which risks I'm willing to take or not, and there are quite a few risks on a short connection where I'm willing to burn the time to avoid them.

Edit to add: I'm just curious as to what your connections tended to be "as booked"? "If I added two hours..." feels like a bit of a fallacy insofar as some connections will natively be on the longer side (just because that's how the flights line up).

And Delta gets to decide (and always has) the limits both on the short side (MCT) and long side. Although I admit, I did talk them into letting me book under MCT at SLC once.

If not four hours, how long? Six? Eight? 24? A week? A month? A year?

one guy’s opinion — I was never a “road warrior” in the classic sense, although I had more than a few months of weekly travel pretty much every year from 1982 to 2012, and with the exception of 2020 have logged 50 to 80 flights a year since — is that four hours was a reasonable number for its time

since ~2000: airline consolidation, the overwhelming dominance of hub-and-spoke route systems (with the attendant crushes on terminal infrastructure at those hubs), and the airlines’ obsession with maximizing aircraft operating time (with the attendant schedule ramifications that are the basis for this thread) have strained the system to where — from a passenger satisfaction perspective — six hours would make a lot more sense

I'm the same way. My parents who normally pick me up (theyre retired) have a standing outlook. Till i connect to Wifi in air, they don't assume I'm getting home. The one plus, i can normally book cheaper AUS fare and use the IRROP to goto SAT instead.

While I agree 6 hours is obviously better than 4, I think it also depends a lot on how frequently the route is operated. For instance, why are international flights given a 24 hour connection window, or Alaska 6 hours? My guess is because those routes are only flown once or twice a day from a given hub. But if then a small domestic station is also only flown twice a day, why isn’t it also given those same longer windows?

I would be happy with 6 hours, if also longer windows were provided for those small destinations that may only have 3 or less flights a day.