Flight number record

OO 3991

SkyWest Airlines · Seattle to San Francisco

Route: SEA → SFO
Departs: 17:45
Flights: 292
Period: July 2025 to June 2026

Historical statistics for July 2025 to June 2026, not live status. For today’s OO 3991, check with SkyWest Airlines. SkyWest Airlines flies under contract for larger airlines, so a ticket for this flight may show a different airline code and number.

OO 3991 sits in the middle of the pack with a C+. About seven flights in ten arrive on time by the DOT's definition, 8 points worse than the national rate of 77.6%. The late ones land about 50 minutes behind schedule. That rests on 292 flights, with a median arrival right on schedule.

The national on-time rate over the same period is 77.6%, so OO 3991 runs 8 points behind the country as a whole. Across a thousand flights that is about 83 extra late arrivals.

On time

69.3%

Arrivals under 15 minutes late, of 290 completed flights

Avg delay

50 min

Across delayed arrivals only, not all flights

Cancelled

0.68%

National rate is 1.81%

1 hour+ late

7.6%

National rate is 8.1%

These figures cover 292 flights between July 2025 and June 2026. On-time means an arrival less than 15 minutes after the scheduled gate time, the same threshold the Department of Transportation uses in its own reports. Cancelled and diverted flights are excluded from the on-time rate and counted separately, which is why the cancellation column matters on its own.

Schedule

When OO 3991 flies

OO 3991 flies SEA to SFO without a single fixed slot. Its most common scheduled departure was 17:45, used on 47 of 292 flights, with 16:25 (39 flights) and 16:40 (39 flights) next. Carriers retime flights through the season, so the rates here blend slightly different positions in the aircraft's day.

Departs 17:4547 flights16.1%
Departs 16:2539 flights13.4%
Departs 16:4039 flights13.4%

Scale runs 0% to 100%. Rows with fewer than 30 flights show a dash.

Share of scheduled departures at each local time. Times used on fewer than one flight in ten are not listed.

A 17:45 departure sits in the 5pm–8pm window. Across every US flight scheduled in that window the on-time rate is 68.2%, and OO 3991 runs level with that at 69.3%. By that time of day most aircraft have flown several legs already, and whatever went wrong earlier is still in the schedule.

Against the route

OO 3991 next to SEA to SFO

Every carrier on SEA to SFO combined arrived on time 65.6% of the time across 6,764 flights, so OO 3991 is 4 points ahead of it. SkyWest flew 1,081 flights on the same city pair under all its flight numbers, including this one, and those ran 75.4% on time. Across its whole network SkyWest Airlines managed 79.0%, which this flight trails by 10 points.

OO 3991 against its route and carrier
Compared with	Flights	On time	Avg delay	Cancelled	Grade
OO 3991thin sample	292	69.3%	50 min	0.68%	C+
All SkyWest Airlines on SEA–SFO	1,081	75.4%	53 min	1.02%	B
All carriers on SEA–SFO	6,764	65.6%	59 min	1.20%	C
SkyWest Airlines, whole network	852,935	79.0%	88 min	1.77%	B

Across the year

OO 3991 quarter by quarter

OO 3991 did not run the same all year. Its best quarter was Jul–Sep 2025 at 79.0% on time and its weakest Apr–Jun 2026 at 48.4%, a swing of 31 points on roughly 73 flights a quarter. The twelve-month rate averages over that, so if you are flying in a weak quarter the quarter's own figure is the better guide.

Jul–Sep 202581 flights79.0%
Oct–Dec 202572 flights78.9%
Jan–Mar 202648 flights78.7%
Apr–Jun 202691 flights48.4%

Scale runs 40% to 85%. Rows with fewer than 30 flights show a dash.

By weekday, OO 3991 did best on Saturday at 80.0% on time and worst on Sunday at 60.5%. Each day rests on about 42 flights, so a gap of 20 points is worth noticing but could still narrow in another year.

Delay causes

Why OO 3991 runs late

One category carries most of the weight here. Carrier accounts for 45% of the 4,488 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.

Carrier
45%
National Airspace System
45%
Extreme weather
10%
Security
0%

The largest share of delay minutes, 45%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. Unlike weather, that is a management outcome, and it is the category where the gap between two carriers on the same schedule is genuinely earned. It accounts for 2,026 of the 4,488 delay minutes reported here. The split is drawn from 4,488 delay minutes reported on OO 3991.

Cancellations are rare at 0.68%, below the national 1.81% and low enough to earn the plus on the grade. Roughly one flight in 100 is called off; the risk here is arriving late, not not arriving.

Connections and routes

What a delay on this flight costs

7.6% of arrivals land an hour or more late, close to the national 8.1%. That is the figure that decides a connection, and it says a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last about 50 minutes, which a normal layover absorbs.

The 679 miles between these two airports put this in the middle of the US network, around two and a half to three hours of block time. That is enough cruise to absorb a ten-minute late pushback and not enough to absorb thirty. It is also long enough that the aircraft usually flies three or four legs a day rather than six, which slows how fast delay compounds.

OO 3991 is not only a SEA to SFO flight. It also flew SFO to SEA (292 flights, 70.7% on time) often enough to be scored on its own. Airlines attach one number to several legs of an aircraft's day, and each leg keeps its own record here, so check which city pair is on your booking before reading a rate.

One caution: this is a thin sample. 292 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.

Other routes flown as OO 3991
Also flown as	Flights	On time	Avg delay	Cancelled	Grade
SFO to SEAthin sample	292	70.7%	47 min	0.68%	C+

What to do with this

Booking OO 3991

For a booking, read OO 3991 as 69.3% on time with 0.68% cancelled over 292 flights. Those are rates from the past twelve months, not a forecast for any particular day.

Where to go next

A flight number here means SkyWest Airlines’s own number as it reports to the DOT. How the figures are computed is on the methodology page.

Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.

Key figures