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Their recipe for success has been straightforward: fewer airlines, fewer planes and fewer seats combined with higher ticket prices and more fees.

While the United States economy is showing signs of strength, the airlines have indicated in their latest earnings reports that they intend to stick to the formula. Oil prices last year averaged about $100 a barrel, about the same level as in 2008. But the airlines delivered higher-than-expected profits for 2011, while they lost 17 cents for every dollar of revenue they generated in 2008. In those three years, Delta completed its purchase of Northwest Airlines, United merged with Continental, and Southwest bought AirTran Airways.

Meanwhile, American Airlines, left out of the latest round of consolidation, filed for bankruptcy protection in November after losing billions of dollars in recent years. The carrier hopes to be able to pare its costs during the restructuring process and emerge as a leaner, profitable carrier. That prospect has also set off renewed talk about a possible bid for American Airlines by one of its competitors, perhaps Delta or US Airways.

“What the industry has done in three years is remarkable,” said William S. Swelbar, a research engineer in the Massachusetts Institute of Technology’s International Center for Air Transportation. “Airlines seem to be much more stable than I can remember in decades. Consolidation is having a significant impact on pricing, no doubt. And the industry has rid itself of unprofitable routes.”

The airlines hope they can continue to raise revenue this year faster than fuel prices rise, while cutting capacity to offset weak demand.

Labor relations are likely to take center stage in 2012. As part of its restructuring process, American is seeking to renegotiate its work agreements with pilots and other labor groups in a bid to cut its costs, which are the highest among its peers. These talks may put pressure on Delta and United, also facing labor negotiations this year, to address costs that have been slowly rising in past years.

United, the nation’s top carrier since its 2010 merger with Continental Airlines, said Thursday that it cut its capacity, or the number of seats flown, by 2.5 percent in the fourth quarter. Meanwhile, it increased ticket prices by 9 percent over the same period a year earlier. As a result, United narrowed its fourth-quarter net loss to $138 million, an improvement from its loss of $325 million in the year-earlier period, the company said.

The loss was largely attributed to costs associated with the merger. Revenue was up 5.5 percent, to $8.9 billion, in the quarter. Excluding one-time items of $247 million, United reported a net income of $109 million in the fourth quarter. The company recorded a full-year profit of $840 million, down 12 percent from 2010.

United’s shares rose 6.3 percent to $21.70 on Thursday. Shares of Delta also rose for a second day after the company said Wednesday that fourth-quarter profit surged to $425 million, up from $19 million in the year-earlier period. In the quarter, which the company said was its most profitable ever for that period, Delta filled nearly 82 percent of its seats while it reduced capacity by 3.5 percent. As a result of higher ticket prices and higher fees, Delta increased its overall revenue by 11 percent in 2011, even as it operated 30 fewer planes than the previous year, according to Richard Anderson, the company’s chief executive.

Delta reduced its capacity to most of its destinations, especially Europe, which saw a 10 percent drop. Latin America was a rare exception: Delta increased capacity there by 5 percent. The company said it would continue to reduce its capacity in the first quarter by 3 to 5 percent.

At Southwest Airlines, net income grew 16 percent in the fourth quarter, to $152 million, with a 32 percent jump in revenue to $4.1 billion. US Airways said its net income declined by 35 percent to $18 million in the fourth quarter. Even though its revenue in that period rose 8.5 percent to $3.2 billion, fuel costs rose faster.

Alaska Airlines and JetBlue Airways also posted a profit in the last quarter. JetBlue is one of the industry’s few exceptions in raising its capacity in 2011 — the airline increased seats by 10 percent in the fourth quarter, with new flights to the Caribbean and to Boston. It expects to keep adding more flights to its schedule this year.

The airlines said they expected these gains to continue in the first quarter of 2012.

“If anything, the new year has seen a step-up in business demand,” the US Airways president, Scott Kirby, said on Wednesday. “The pricing environment remains strong and the industry is successfully recovering high fuel prices.”

Separately, US Airways confirmed this week that it was interested in exploring a deal with American Airlines. A combination between US Airways and American would create a carrier roughly the same size as United, Delta and Southwest.

Service cuts, fare increases and the retirement of less fuel-efficient planes have aided airline profits even as high oil costs and economic uncertainty have threatened demand for flying.

Southwest, which is based in Dallas, forecast strong passenger revenue for the current quarter.

“The industry has set itself up to continue to grow in 2012, but the wild card here is the general economy,” said Matthew Jacob, an analyst with ITG Investment Research. “We’re not seeing any indication that demand for air travel is slowing at this point.”

Southwest, the traditional low-fare leader among major American airlines, acquired AirTran last year and gained entry to East Coast cities, including Atlanta. Southwest-branded flights will start in Atlanta next month.

Southwest is also upgrading its planes. It reached a deal late last year to buy 208 Boeing 737s, including 150 of the soon-to-come 737 MAX planes that will have new engines. Southwest said this week that it would upgrade cabin interiors and add six seats to more than 350 older planes.

The AirTran purchase “is going to be a major game changer for them,” said Helane Becker, an analyst with Dahlman Rose Company. Still, she said Southwest’s biggest challenges were managing merger expenses and higher energy costs while keeping worker productivity high.

Fourth-quarter net income at Southwest was $152 million, or 20 cents a share, compared with $131 million, or 18 cents a share, in the period a year earlier.

Excluding items tied to fuel contracts and acquisition costs, net income was $66 million, or 9 cents a share, Southwest said. Analysts expected 8 cents, Thomson Reuters said.

Revenue rose 32 percent, to $4.1 billion. Operating expenses were up about 37 percent, with fuel and oil costs up 59 percent from the period a year earlier.

Southwest’s shares rose 3 percent, to $9.30.