After canceling more than 1,800 flights in the last two days (with another 150 at least expected today), Delta finally says it expects to resume normal operations later this afternoon. That’s probably a relief to Delta staff and any passengers with flights planned for the back half of the week. But for the thousands and thousands of passengers left in the lurch so far, relief has proven slow to come. Slower, in fact, than it would have been in the past. Why?

It’s not all just down to capacity, as the Wall Street Journal reports; instead, it has as much to do with agreements between the ever-fewer remaining airlines as it does with the actual number of planes in the sky.

Admittedly, the math is simply not on Delta’s side here. If every aircraft in Delta’s fleet were theoretically to operate at the same time, they would be carrying more than 124,000 concurrent passengers. Now do the math that these 809 aircraft among them perform about 6,000 scheduled flights per day and, well, you can see how cascading backups get very ugly, very fast.

Scrubbing over 2,000 flights across three days might only be a small fraction of the 6,000 daily scheduled flights that Delta operates, but it still leaves potentially hundreds of thousands of passengers in the wrong places, clamoring to get whatever seats they can on whatever planes will get them to the cities they need to be going to.

That’s where the industry comes in, and the big problem: there aren’t as many planes in the air both willing and able to get those passengers moving. It’s not that competitors aren’t flying; it’s that the modern structure of business limits your airline’s ability to rebook you on one of them.

That process, where gate agents can snag you travel on a competitor when they have to cancel their own flight, is called interlining. And as the WSJ points out, it’s just a lot harder to do these days.

All those airline alliances you see — the Star Alliance, Oneworld, and the rest — help the airlines inside of them, it’s true. But while those contracts mean that while Delta and Air France code shares, for example, might become prevalent, you’re going to have a much harder time using your Delta ticket to get rebooked on a non-SkyTeam flight than you used to.

Major airlines kept some deals in place outside of the international alliances, the WSJ says, to help keep the flow of passengers moving. But last year, Delta and American, two of the nation’s three remaining large legacy carriers, ended their interlining agreement when they couldn’t agree on compensation. (Delta and United do still have a deal.) And many of the non-legacy, lower-cost carriers don’t have any interlining agreements at all.

There’s also just less wiggle room, the WSJ adds: fewer than 20% of seats, on average, are empty when a plane takes off. If you have 10,000 people to rebook but each flight only has a couple of empty seats to put them in, well, there’s that pesky math again.

Passengers are, of course, free to walk up to the competition (or pull it up on their phones) and book their own last-minute alternate travel when their reserved flight gets canceled. Those fares are generally the most expensive an airline ever charges, so the airline will be more than happy to have you… but customers often don’t want to or simply can’t afford to eat that cost.

Carriers’ Pacts Leave Delta Passengers in Lurch [Wall Street Journal]

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