• United last flew to JFK in October 2015 and the current CEO called the carrier's departure a mistake.
• The Chicago-based airline is starting out with two round trips from Los Angeles and San Francisco to JFK.
• United, like other carriers, are taking advantage of a lull in traffic because of Covid-19 to explore once-congested airports.

United Airlines on Sunday flew to New York's John F. Kennedy International Airport for the first time in more than five years as the carrier takes advantage of a lull in air travel to snag space at the once-congested airport.

United's JFK service kicked off with a 7:30 a.m. PT flight from Los Angeles International Airport and a 9:30 a.m. PT flight from its San Francisco International Airport hub. Both were operated with a Boeing 767-300.

The JFK-San Francisco flight departed at about 5:30 p.m. ET and the Los Angeles-bound flight left just after 7 p.m. ET. Both westbound flights were full and the eastbound flights had about 85% of the 167 seats taken, a spokesman said.

The airline will operate five flights a week from JFK to Los Angeles and five San Francisco flights a week, doubling that in May.

Sandra Vazquez, who took the JFK-San Francisco flight after visiting her son on Long Island, said she thought it "was a mistake" on her ticket when she saw JFK on her reservation, recalling that she told her husband to "make sure it's right."

United's New York-area service has been concentrated at its Newark Liberty International Airport hub and at New York's LaGuardia Airport. Carriers have pulled back service to the Northeast during the Covid-19 pandemic with business and international travel still at paltry levels, though domestic leisure demand has ticked up nationally.

Airlines' scheduled service in New York state is down 56% in April compared with the same month of 2019, more than any other state, according to Airlines for America, an industry group that represents most large U.S. carriers. The national average is a 32% decline. That makes it easier for airlines to add service.

United CEO Scott Kirby, who took the reins last May, has said leaving JFK in October 2015 was a mistake and has expressed a desire to return to the New York City airport because moving transcontinental flights to Newark allowed competitor American Airlines to win some lucrative corporate clients.

"We want to expand [JFK service] to other hubs, too," Ankit Gupta, the airline's vice president of network and schedule planning, told CNBC, citing Houston's George Bush Intercontinental Airport and to Chicago O'Hare as options.

CNBC first reported in September that United was planning to return to JFK.

Other carriers are using low air traffic to move into airports that were harder to enter previously because of congestion. Southwest Airlines, for example, last year added new service from United's hubs O'Hare and Houston Intercontinental.