Carrier route record
ASSTL→SEA
Alaska Airlines · St. Louis to Seattle
- Flights
- 781
- Period
- September 2025 to August 2026
- Share of route
- 88%
- Carriers
- 2
Historical statistics for September 2025 to August 2026, not live status. For today’s flights, check with Alaska Airlines.
Alaska on STL to SEA lands a B, 5 points better than the national rate of 77.3%. About one flight in six arrives 15 minutes late or worse, and those flights are late by about 50 minutes on average. That rests on 781 flights, with a median arrival 7 minutes early.
Set against the rest of the country, Alaska's STL to SEA record is 5 points better than the US average of 77.3%. That gap is worth roughly 52 fewer late arrivals for every thousand flights.
Key figures
On time
82.5%
Arrivals under 15 minutes late, of 772 completed flights
Avg delay
50 min
Across delayed arrivals only, not all flights
Cancelled
1.02%
National rate is 1.89%
Whole route
78.6%
Every carrier on STL to SEA, 884 flights
These figures cover 781 flights between September 2025 and August 2026. The median arrival is 7 minutes early, which is the part an average hides: most flights here are fine, and the on-time rate is decided by a tail of badly late ones. On-time means under 15 minutes late, the DOT's own threshold.
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Carrier by carrier
Alaska against every carrier on STL to SEA
Alaska flew 781 of the 884 flights on STL to SEA, most of the route. Its 82.5% on time is 4 points better than the route as a whole, which runs 78.6%. Ranked by on-time rate it is first of two carriers on the route, where the range runs from Alaska at 82.5% to Southwest at 49.5%. One of those carriers flew fewer than 365 flights here, so treat their positions as rough.
Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Alaska Airlines (this page) | 781 | 82.5% | 50 min | 1.02% | B |
| Southwest Airlinesthin sample | 103 | 49.5% | 35 min | 0.00% | F+ |
Flight by flight
Alaska flight numbers on STL to SEA
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Departure window
When Alaska runs best on this route
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 25 points, against half a point between the best and worst carrier.
- 5am–8am322 flights90.0%
- 8am–11am—
- 11am–2pm—
- 2pm–5pm308 flights82.8%
- 5pm–8pm82 flights64.6%
- 8pm–5am69 flights67.6%
Scale runs 55% to 95%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 64.6% on time, against 90.0% for the 5am–8am slot. By evening the aircraft flying these rotations have absorbed a full day of small delays and there is no slack left to recover in. The 5am–8am window is the busiest, with 322 flights scheduled in it.
Across the year
Alaska on STL to SEA through the year
December is the weak month at 54.7% on time, against 94.8% in April. Winter delays cluster: a single storm system can account for most of a month's shortfall, so an average December hides both very good weeks and genuinely unflyable days. July was the busiest month here, with 92 flights.
Across the week, Friday is the strongest day at 87.3% on time and Saturday the weakest at 74.5%, a spread of 13 points.
Arrival by arrival
How late Alaska on STL to SEA runs when it is late
Alaska on STL to SEA often beats the schedule: 223 of 772 completed flights, 29%, arrived 15 minutes or more early. The 135 late arrivals split 57 under half an hour, 55 between 30 and 60 minutes, and 23 an hour or more.
- 15+ min early223 flights28.9%
- Within 15 min414 flights53.6%
- 15–29 min late57 flights7.4%
- 30–59 min late55 flights7.1%
- 1–2 hours late15 flights1.9%
- 2+ hours late8 flights1.0%
Scale runs 0% to 100%.
Share of 772 completed flights by arrival against schedule.
35 of 130 late departures made up the time and arrived on time, 27% of them. In the other direction, 40 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at SEA.
The worst days
Worst days for Alaska on STL to SEA
The latest arrival of the period was on 16 April 2026, 8 hours 42 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). On three of the five worst days the largest share was filed as a late-arriving aircraft.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 16 April 2026 | 8 hours 42 minutes | Carrier |
| 19 December 2025 | 5 hours 26 minutes | Late-arriving aircraft |
| 14 May 2026 | 3 hours 49 minutes | Late-arriving aircraft |
| 18 June 2026 | 3 hours 28 minutes | Late-arriving aircraft |
| 2 December 2025 | 3 hours | National Airspace System |
Delay causes
Why Alaska runs late on this route
Carriers report a reason for every delay minute they cause, in five DOT categories. The split below is by minutes, not by number of flights, so a handful of very long weather delays weighs as much as many short ones. It covers 6,751 reported delay minutes.
- 34%Late-arriving aircraft
- 31%National Airspace System
- 30%Carrier
- 5%Extreme weather
- 1%Security
The practical consequence: the 5am–8am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 90.0% on time here. The split is drawn from 6,751 reported delay minutes.
No single cause dominates. late-arriving aircraft at 34%, national airspace system at 31% and carrier at 30% split the 6,751 reported delay minutes between them. An even spread like this usually means the operation is running close to its designed capacity with no one obvious weak point, and it is the hardest pattern for a traveller to route around.
Cancellations sit at 1.02% against a national 1.89%, which is ordinary. Diversions add another 0.13%.
Connections and stage length
What a delay here costs
Badly late arrivals are rare here: only 3.0% land an hour or more behind schedule, against 8.3% nationally. Connections built on this flight are about as safe as US domestic connections get, though a cancelled flight still ignores every layover you give it.
At 1,709 miles this is transcontinental flying, and long routes get a quiet structural advantage. Scheduled block time on a flight this length carries more padding in absolute minutes, so a twenty-minute late departure often still becomes an on-time arrival. Read the on-time rate here as slightly flattering next to a short-haul route with the same operational quality.
The return leg is a different operation and performs like one: it runs 85.8% on time across 780 flights, 3 points above this direction. Departure banks, aircraft rotations and the congestion at each end all differ, so scoring the two directions together would average away a real 3 points of difference.
One caution: this is a thin sample. 781 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.
What to do with this
Booking Alaska on STL to SEA
Put together: treat all of this as provisional. 781 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 82.5% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 772 completed flights over 12 months sit behind that.
Related
Where to go next
- STL to SEA, every carrier
- Alaska on SEA to STL, the return leg
- Southwest Airlines on STL to SEA49.5%
- Alaska Airlines network-wide record
- St. Louis Lambert International (STL) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.05 million flight records
Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.