A tourist takes a photo as the Acropolis’ Propylaea are seen in the background, in Athens, Greece, on June 28, 2024.Elias Marcou | ReutersFlights between the U.S. and Europe have not been this cheap in three years, when many countries were just lifting Covid-19 era rules.Fares are low even for the traditionally slow late-fall and winter months outside of major holidays.”It is brutal to fill seats during these times of year,” said Brett Snyder, who writes the Cranky Flier travel industry site.According to flight-tracking company Hopper, “good deal” fares across the Atlantic to Europe are averaging $578 in November, down from $619 a year earlier.It is the lowest deal fare for this month since 2021, when they were going for $479 and much of international travel was in a slump because of the pandemic, Hopper data shows.In January, after the year-end holidays, 2025 fares are even lower: $558 compared to $578 for the same month in 2024, though higher than $488 in January 2022, according to Hopper.U.S. domestic airfare, on the other hand, is more expensive compared with last year in every month from November through March.Many airlines from financially troubled Spirit Airlines to profitable Southwest Airlines have cut flights or trimmed growth plans into next year, which has helped keep U.S. fares firm. Aircraft scarcity is also limiting airlines from adding many flights.There are also some periods of weaker demand overall, executives at the largest U.S. carriers, Delta Air Lines, United Airlines and American Airlines have said, calling out the week before and after the U.S. presidential election on Tuesday.How airlines got hereCarriers raced to add seats between the U.S. and Europe to cater to post-pandemic travel demand.That buildup was not just during the peak months. Executives noted that they are seeing more shoulder-season demand to Europe as travelers look to escape scorching summer temperatures and crowds. As a result, they have also added flights outside of peak periods.Airline capacity between the U.S. and Europe in the fourth quarter is marginally lower than last year, but it is higher than in 2019 and nearly double the amount in the same period of 2021, according to Cirium.”I expect airfare [to Europe] to be low into next year,” said Hayley Berg, Hopper’s lead economist.Now, on the heels of two big years for European travel, many customers are fresh off their big trips to popular destinations such as Spain and Italy, which means fewer people to fill seats in the offseason.”It’s not as though there is so much low-hanging fruit and where airlines could print money hand-over-fist like last year,” said Scott Keyes, founder of travel app Going, which was previously known as Scott’s Cheap Flights.Airlines traditionally discount flights in the offseason, but they are even cheaper this year.”That’s the tell,” Keyes said. “When they’re having to go out and discount, they’re having to juice the demand.”So that travelers do not get bored with European vacation mainstays when next year’s peak warm-weather travel season rolls around, airlines are trying new things. United Airlines has noted many customers have already taken trips to major European cities and the airline plans to expand its schedule next year to more off-the-beaten-path destinations such as Greenland and Mongolia.”We’re also able to do just as well financially outside of our partner hubs,” United’s Chief Commercial Officer Andrew Nocella said on an earnings call last month. “So we look across the globe, we look for new destinations, we look for hot destinations and destinations, most importantly, we can make money in.”Don’t miss these insights from CNBC PRO