How reliable is Southwest? Great at dawn, rarely cancels, slips late

Southwest's first departures of the day are among the most punctual flights any US airline operates. Between 5am and 8am it ran 93.4% on time from September 2025 to August 2026; the best figure in that window among the 14 reporting carriers was Southwest Airlines's 93.4%. By the evening the same airline was running 62.7% on time between 5pm and 8pm and 62.5% after 8pm.

That slope is the whole Southwest story in one number: 30.7 points from dawn to early evening, against 21.3 for all US flights and 20.1 for the median airline. The steepest slope of any carrier was 30.7 points. Across the year Southwest ranked 8 of 14 on time at 75.7%.

On the measures that decide whether a trip actually happens, it did far better.

Four measures that disagree

Southwest cancelled 0.92% of its scheduled flights, well under half the national 1.89% and among the lowest rates of any airline. Only 6.7% of its flights ran an hour or more late, against 8.3% nationally, and when it was late the delay averaged 53 minutes, one of the shortest figures in the table. 27.6% of its late flights crossed the hour, against 36.6% nationally.

So a Southwest flight is late often and badly late rarely. Many small delays, few long ones, and few cancellations. The middle-tier carriers guide sets that against Alaska and JetBlue.

{
 "flights": 1391794,
 "completed": 1375352,
 "onTimePct": 75.7,
 "veryLatePct": 6.7,
 "avgDelayMin": 53.3,
 "medianArrDelay": -3,
 "cancelPct": 0.92,
 "divertPct": 0.27,
 "causes": {
  "carrier": 0.2819,
  "weather": 0.0228,
  "nas": 0.1813,
  "security": 0.0031,
  "lateAircraft": 0.5109
 },
 "byHourBlock": [93.4, 86.4, 76.8, 67.5, 62.7, 62.5 ],
 "byMonth": [84.8, 81.7, 74.4, 77.6, 72.4, 66, 69.2, 70.1, 83, 77.4, 79.8, 75.4 ],
 "byDow": [73.8, 80.8, 80.7, 73.4, 73.4, 78, 71 ]
}

This is the file my pipeline writes for Southwest, the largest airline in the data at 1,391,794 scheduled flights; every Southwest figure in this guide is recomputed from it each month.

Why the day falls so far

Southwest does not run hubs with connecting banks. It runs aircraft in long chains across the map: one aircraft might fly Baltimore to Nashville to Dallas to Phoenix to Las Vegas in a day, with short turns between legs. Late-arriving aircraft account for 51% of Southwest's delay minutes, against 40% nationally, among the highest shares of any carrier. Every delay on the first leg is carried into the second, and there is little slack in a short turn to absorb it.

The first flight of the day has no inbound leg to inherit from, which is why Southwest's dawn figure is so good. By the fifth leg, the aircraft carries the day's accumulated delay from four other cities. The first flight of the morning guide explains why that window is special; for Southwest it matters more than for almost anyone.

The same structure explains the low cancellation and long-delay figures. With hundreds of aircraft of one type at big stations, a broken aircraft can be swapped quickly, and a late aircraft can be replaced rather than waited for. Delays stay short because a spare is rarely far away.

The stations

Southwest's biggest station is Denver, and it is one of its weakest. Denver as a whole ran 76.6% on time, and Southwest's departures there ran well below that, falling hardest in summer afternoons. Las Vegas, Phoenix and Dallas Love Field also trail Southwest's network figure. These are big, hot, storm-exposed stations where most aircraft arrive from somewhere else in the afternoon.

The best Southwest stations are small California airports and Hawaii. Long Beach, Burbank and Orange County run well above its network figure, and so does Honolulu, where most Southwest flying is short inter-island hops. The Kona to Honolulu route and Maui to Honolulu are among the most punctual Southwest routes in the country.

San Francisco is the outlier at the other end, and the reason is arrivals. Southwest aircraft flying into SFO in the morning are likely held for the low-cloud arrival restrictions, then leave late on their next leg. The Las Vegas to San Francisco route is Southwest's worst busy route; the route as a whole ran 58.0% on time, with Southwest well below the other carriers on it. Part of that is one stretch of the year: from April to July 2026, flights into San Francisco fell to roughly 35 to 50% on time on several routes. I took it for an aggregation bug at first, but it is in the raw month files, and the data does not say why.

Best and worst routes

Southwest's strongest busy routes are short and western: Reno to Las Vegas, which ran 81.4% as a route, Burbank to Las Vegas and San Jose to Las Vegas. Short sectors between uncongested airports leave little to go wrong.

The weakest leave Las Vegas for the California coast in the afternoon. The Las Vegas to San Diego route ran 62.4% on time, against 73.7% for the return. The flights back to San Diego are likely to be late legs of an aircraft's day, and San Diego's single runway has no slack at the evening peak.

The calendar and the week

Southwest's best months came in the middle of winter. Its best month was January, at 84.8%, and its worst June, at 66.0%. Its summer ran well below its winter, and below the national summer. Southwest's schedule is heavy at Denver, Las Vegas, Phoenix, Dallas, Houston, Austin and Orlando, all of which see afternoon thunderstorms in summer, and its quick turns spread each storm across the network by evening. One winter and one summer are one sample each; the Denver June page shows what a summer afternoon there does to the whole airport.

Tuesday was Southwest's best day at 80.8% and Sunday its worst at 71.0%. Monday ran 73.8%, Thursday 73.4% and Friday 73.4%, the days leisure and weekly business traffic peak. Fuller aircraft take longer to board, and on a schedule built on short turns that is likely enough to show.

Why so few Southwest flight numbers have pages

Delta has flight pages for most of its schedule. Southwest has six. It reuses flight numbers across different city pairs on different days, so only six Southwest numbers fly one route often enough to earn a page. The exceptions are on short Texas routes. WN 1636, the 6am from Houston Hobby to Dallas Love Field, is one of the most punctual flights in Southwest's schedule. WN 46 flies the same route at 5:30pm and runs far behind it, which is Southwest's time-of-day slope in a single pair of flights.

How I would fly Southwest

Take the first wave. A Southwest departure before 8am is as reliable as anything in US aviation. After 5pm it is below the national average, and at 62.5% after 8pm I would not put a connection behind its last departures.

Use Southwest when the trip must happen on the day. Its low cancellation rate and short delays make it the safer choice for a wedding or a cruise departure, as long as the flight is early.

Avoid afternoon departures from Denver, Las Vegas and Phoenix in summer. Those stations carry the network's worst hours.

Prefer the small California airports. For Los Angeles, Long Beach and Burbank beat LAX on Southwest; for the Bay Area, San Jose and Oakland beat San Francisco. The best time to fly tool shows the hour on any route.

Southwest's record in detail: Southwest Airlines · Southwest at Denver · Southwest at Las Vegas · Southwest at Baltimore · Southwest at Midway · Middle-tier carriers