Even as a wave of mergers has cut the number of major carriers to four and significantly reduced competition, lower-cost airlines continue to play a role in moderating ticket costs.
While such airlines offer a no-frills passenger experience and charge plenty of fees for such luxuries as additional bags or extra legroom, they are able to stimulate new demand from occasional fliers with relatively cheap prices and even take passengers from the major carriers.
This dynamic is not new: In 1993, researchers at the Department of Transportation called the same trend the “Southwest effect,” named for Southwest Airlines, which grew rapidly thanks to basic, low-cost flights. A recent study by a University of Virginia professor and a consultant at the Campbell-Hill Aviation Group calculated that average one-way fares are $45 lower when Southwest serves a market with nonstop flights. Researchers have shown other low-cost carriers also push down fares.
“It’s impossible to underestimate just how important the effect of low-cost carriers are on a given route,” said William McGee, the aviation adviser for Consumers Union.
Carriers like United and American do not compete with carriers like Frontier and Spirit on every type of passenger. Lucrative corporate accounts are owned by the big carriers, and business travelers avoid the cheaper airlines, often choosing to pay premium prices at the last moment to get seats on the flights that best fit their schedules.
But the low-cost carriers nonetheless force the big airlines to figure out a way to draw the most price-sensitive fliers in any given market — those who scour the internet for the cheapest tickets possible. Those customers make up a significant portion of travelers, meaning the major carrier cannot just ignore them.
“Those passengers certainly are important,” said David Weingart, an economist at the aviation consultant GRA. “The larger airlines have proven that in how they’ve reacted, in how they’ve tried to capture or recapture those passengers.”
Delta, American and United Airlines have all rolled out “basic economy” fares. Such tickets are priced competitively against Spirit and Frontier, but do not offer the amenities that most consumers have come to expect on a flight, like receiving a seat assignment ahead of a flight or obtaining a refund for a ticket.
Of all the major carriers, United is fighting on price the most aggressively.
Scott Kirby, who was appointed as United’s president a year ago, has shifted the carrier’s strategy toward the low-cost airlines, mirroring one he helped to drive when he served as a top executive at American.
Pushing back against Wall Street’s wishes to limit capacity growth, United is adding seats in a number of its major markets across the country. It has, for example, swapped out smaller jets for larger planes to increase the number of seats it has available to sell, and matched fares offered by low-cost carriers.
By expanding capacity, United aims to get back to what Mr. Kirby calls its “natural” share of passengers in some of its hubs. The carrier now expects to increase its seat capacity in the domestic market by as much as 4.5 percent this year over last year, double the 2 percent growth Delta has forecast. American does not expect its capacity to change.
“We’re just returning to where United’s natural market share is,” Mr. Kirby told stock analysts in April. “We’re going to be very careful to calibrate how it’s working and how we’re doing.”
United’s new approach has put it into direct competition with Spirit in Newark, Houston and Chicago, according to analysts and executives. Spirit has certainly noticed.
“While we are not surprised that the environment remains very competitive, it is surprising to see our competitors resort to the unusual level of discounting we are currently seeing, especially since we are still in the summer peak period,” Matthew Klein, chief commercial officer at Spirit, said during a call with analysts in July.
Denver has also emerged as a battleground. Frontier, a privately held carrier, announced in July that it planned to add 21 flights from Denver International Airport, mostly to smaller cities like Albuquerque; Louisville, Ky.; and Charleston, S.C. The company said that it planned to double the number of nonstop routes it operates to 314 and its total number of routes to 1,000 by next summer.
Because of the underlying strength of the economy in the Denver area, United, Southwest and Spirit have added flights there in recent years as well. Kim Day, the chief executive at the Denver airport, said fares at the hub are now 15 percent below the national average.
She said that she believed each of the airlines that use the sprawling facility 25 miles from the city’s downtown is drawing different kinds of passengers.
“They’ve all found their niches,” she said. “If they can make money here, they’re going to continue to flow connecting passengers in here.”
The current skirmishes do not amount to a broad-based fare war. Many routes in the United States are dominated by a single carrier, insulating them from price competition.
The cost of a round-trip domestic ticket averaged more than $490 in the first half of the year, up slightly compared with 2016, according to Airlines Reporting Corporation, a company that settles flight transactions between a number of carriers and booking services like Expedia.
The jostling, however, has left airline investors skittish. As the publicly traded airlines in July reported earnings for the second quarter, shareholders sold off their shares, worried about the fight over fares and capacity increases.
But anxiety among investors is good news for fliers. Travelers on routes that are competitive will probably be able to snap up good deals.
“I don’t see the prices really rising that much from year to year, based on that ability to have that competition and keep prices at a normal level,” said Merritt Pullam, a real estate agent who lives with his wife and two young children in Denver and flies to places like California and Hawaii for vacations.
An earlier version of this article gave an incorrect surname for a traveler who lives in Denver. He is Merritt Pullam, not Pullman.
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