Alaska Airlines had requested a temporary grounding of its operations from Seattle-Tacoma International Airport (SEA), resulting in delays to departing flights from its main hub.

Automation problems at Alaska Airlines and Horizon Air

According to the Air Traffic Control System Command Center (ATCSCC) of the Federal Aviation Administration (FAA), Alaska Airlines requested a ground closure for departing flights from SEA on September 22 at 2:45 UTC, or 19:45 local time (UTC-7).

As a result, the total delay, the maximum delay and the average delay increased from 0 to 645, 49 and 46 minutes respectively, with a medium probability of extension.

The FAA's explanatory document for ATCSCC, the National Airspace System (NAS) Status Summary System, states that a moderate probability means that the chance that the ground stop will be extended is 30 to 60 percent.

Alaska Airlines' ground stop at SEA airport initially had an effective duration of two hours and lasted until 9:45 p.m. local time.

Extension and lifting of the ground stop

At 3:38 UTC, or 8:38 p.m. local time, the FAA updated the ground stop order, which still only affected flights operated by Alaska Airlines and its regional affiliates, and estimated new total, maximum and average delays at 1,453, 124 and 66 minutes, respectively.

The probability of an extension remains medium, the FAA said in its statement. The actual time was extended to 10:45 p.m.

But more than an hour later, at 21:53, FAA has lifted the ground stop order. According to Flightradar24, delays for departing flights at 01:08 were on average 45 minutes, with 94% of departing flights not leaving SEA at the scheduled time.

In addition, the delay statistics from the flight tracking service showed that the delay rate is increasing.

Alaska Airlines' largest hub

Data from aviation analytics firm Cirium showed that Alaska Airlines and its regional affiliates, including Horizon Air and SkyWest Airlines, had 45 flights scheduled from SEA between 7:45 p.m. and 9:45 p.m. In total, there were 6,650 departing seats during that period, an average of 147.7.

For example, Alaska Airlines had six flights scheduled to depart the airport at 9:45 p.m. on September 22, but they were nonetheless affected by the ground stop order. According to Flightradar24, all six took off at least an hour later:

Four departures of other airlines were scheduled for the same time slot at 19:45: Delta Air Lines (two flights), JetBlue and Southwest Airlines.

None of them had left SEA on time, with Delta's flight DL567 to Hartsfield-Jackson Atlanta International Airport (ATL) with a delay of one hour and two minutes.

Data from Cirium showed that SEA was by far Alaska Airlines' largest airport in terms of the number of weekly departing flights in September.

Of the 8,525 weekly departures scheduled for September, 2,414 or 28.3% were scheduled to depart SEA. The second busiest airport in the airline's network was Portland International Airport (PDX) with 735 weekly departures or 8.6%.