US Airlines Ran Later in September — Southwest Slid 10 Points But Almost Never Cancelled

US airlines had a rougher September than the year before, and nobody felt it more than Southwest. According to Cirium's monthly rankings, every North American carrier it tracks was less punctual and cancelled more flights than in September 2025. Southwest's on-time arrival rate dropped from 81.46% to 71.14% — a fall of more than ten percentage points, the steepest decline of any airline on the list. It slid from sixth place to last among the nine carriers ranked.

The September picture wasn't all bad: all nine airlines improved on their August numbers, so operations are trending the right way within the month. WestJet topped the North American table at 85.31%, followed by Delta at 82.52% and Alaska at 80.73%. United managed 78.65%, American 77.11%, with Frontier, JetBlue and Air Canada clustered in the mid-70s ahead of Southwest.

Here's the twist: Southwest was still the airline least likely to leave you stranded. Its completion factor of 99.62% means just 0.38% of flights were cancelled — roughly one in 263, and less than half the cancellation rate at United, Delta or American. If your priority is simply getting there on the day you planned, Southwest quietly did that better than anyone. If you need to be somewhere on time, its record is now the weakest in the group.

The schedule squeeze looks like a big part of Southwest's problem. The airline has been cutting ground time between flights and adding overnight redeyes to squeeze more flying from each aircraft — cheaper when it works, but with less slack when anything goes wrong. Crowded cabins slow boarding too, an issue that's grown since the airline dropped its free checked bags policy. Southwest's own COO, Andrew Watterson, flagged on the July earnings call that packed flights were taking longer to turn, especially in the final ten minutes before departure.

Delta recovered well from a messy summer. On-time arrivals climbed from 79.15% in August to 82.52%, and cancellations halved to 0.92%. But its edge over United has thinned to under four points, and United actually cancelled slightly less often. American improved too — cancellations fell from 2.28% to 1.01% — yet Delta's lead over American barely moved versus last September, suggesting American's schedule padding and its reshaped Dallas–Fort Worth connection banks haven't moved the needle yet.

In the airport rankings, only two US hubs cracked the global large-airport top twenty: Delta's Salt Lake City (13th, 85.56% of departures on time) and Minneapolis–St. Paul (16th, 85.13%). Harbin in China led the category at 96.74%, with Santiago and Lima close behind. Portland placed 12th among medium airports and Kona 12th among small ones.

For travellers, the practical read is this: book Delta or Alaska if punctuality matters most, Southwest if avoiding a outright cancellation is the bigger worry — and build a buffer into any tight connection this autumn, because everyone is running later than last year.