In early 2019, the Seattle metro area got a second commercial airport: Paine Field in Everett, Washington. While Paine Field is better known as the home of Boeing's main assembly plant for wide-body jets, a privately owned two-gate terminal has enabled commercial flights to resume at the airport for the first time since World War II.

Two airlines operate at Paine Field: Alaska Air (NYSE:ALK) and United Airlines (NASDAQ:UAL). However, United recently confirmed that it will suspend service there in early October. That could potentially enable Alaska to grow its footprint at Paine Field over the next year or two.

A United Airlines regional jet parked on the ground.

Image source: United Airlines.

A reliever airport for Seattle's northern suburbs

Paine Field is located a little more than 20 miles north of downtown Seattle. By contrast, Seattle-Tacoma International Airport (Sea-Tac) -- the city's main airport -- is almost 15 miles south of downtown.

The Puget Sound region's growth has led to frequent traffic jams on highways in and around Seattle. That makes flying out of Paine Field a big time-saver for many of the 800,000-plus residents of Snohomish County (of which Everett is the county seat).

Nevertheless, airlines haven't found it easy to succeed at Paine Field. In late 2018, Southwest Airlines canceled its plans to operate five daily flights there, leaving Alaska and United as the only tenants when the new commercial terminal opened. And Alaska reshuffled its Paine Field route map and flight frequencies multiple times even before the COVID-19 pandemic upended the airline industry.

United gives up while Alaska tries new things

Initially, United Airlines flew from Paine Field to San Francisco and Denver. The airline dropped its San Francisco service in February 2020, though, opting to route all Paine Field traffic through Denver, which is a bigger connecting hub.

It appears that United Airlines doesn't see a path to profitability for the Denver route, either, leading to the recent decision to pull out of Paine Field. This move is particularly striking in light of United's aggressive growth plans. However, most travelers appear to prefer a longer drive to Sea-Tac -- which has numerous nonstop flight options -- over flying from Paine Field to Denver and then connecting to another destination.

Alaska Airlines' point-to-point strategy avoids this problem. That (along with its status as Seattle's hometown airline) has made it somewhat more successful at Paine Field. Last month, Alaska announced that it expects to return to its full schedule of 18 daily departures from Paine Field by the spring of 2022, if not earlier, noting that the airport "remains very popular with our guests."

An Alaska Airlines jet flying over clouds.

Image source: Alaska Airlines.

That said, Alaska Airlines has had to adapt its strategy at Paine Field to match demand. This fall, it plans to operate 13 daily departures to nine destinations, mainly warm-weather leisure spots and key markets in the Pacific Northwest. Nearly half of these flights will serve destinations that weren't on Alaska's original Paine Field route map from 2019. Conversely, Alaska Airlines won't fly from Paine Field to Portland or Los Angeles this fall. Those two cities accounted for eight of the airline's 18 daily Paine Field departures two years ago.

An opportunity to expand

Paine Field's two-gate terminal can support just 24 daily departures. United's decision to exit the market will put six of those departure "slots" up for grabs.

Alaska Airlines will probably pursue those slots to provide room for future growth. Indeed, without extra slots, the airline would have to cut back on the leisure routes and regional routes where it has added flights recently to restore meaningful service in top business markets like Portland, San Francisco, San Jose, and Los Angeles.

Other airlines are unlikely to seek those slots, particularly because Paine Field's gate constraints limit its attractiveness to budget carriers looking for growth opportunities. Thus, Alaska Airlines could have a monopoly at Paine Field by year-end, complementing its leading position at Sea-Tac.

With a maximum of 24 daily departures permitted at Paine Field, Alaska's operations there will never be the biggest driver of its earnings. However, Paine Field service could become a profitable, deep-moat niche for Alaska Airlines -- and that's incredibly valuable in the ultra-competitive airline industry.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.