Maybe the third time will turn out to be the charm.

United Airlines Inc. parent UAL Corp. and US Airways Group Inc., according to sources, are once again engaged in merger talks, mulling a transaction that was agreed to but eventually scuttled in 2000 as an antitrust challenge loomed and then discussed again as recently as 2008. Although some of the significant hurdles that have kept the carriers apart in the past remain, there is reason to think that this time around something might actually come of the talks.

Chicago-based United has been exploring its options seemingly since emerging from Chapter 11 protection in 2006 and is also reportedly talking to joint venture partner Continental Airlines Inc. about a deal. This latest round of discussions comes as airlines are hopeful that an improving economy will spur increased business travel and boost revenues but amid persistent concerns that there are simply too many carriers in the sky for the industry to achieve sustained profitability.

The call for consolidation is not unique to the United States, with Giovanni Bisignani, director-general of trade group International Air Transport Association, telling reporters last week that "no other industry is so fragmented" and urging his members to "consolidate in order to build more efficiency." Airlines operating within the European common market have made great strides in recent years consolidating around three global alliances, most recently with British Airways plc and Iberia Líneas Aéreas de España SA finalizing plans this month to combine under the International Airlines Group banner.

American carriers are no strangers to dealmaking either. But consolidation for most of the industry's history has been a regional affair, with US Air, for example, an amalgamation of Allegheny Airlines of Pittsburgh, Piedmont Airlines of North Carolina and America West Airlines of Phoenix, among others. As a result, the United States now has five large national carriers offering similar, if not redundant, service.

Making matters worse is that since the 1980s those so-called legacy carriers have been joined first by Southwest Airlines Co. and more recently by the likes of AirTran Airways Inc. and JetBlue Airways. The rise of the discounters has sapped whatever pricing power the legacies have left.

Few businesses are as burdened as the airlines, which toil in a capital-­intensive industry with huge fixed costs and at the mercy of such variables as fuel prices, labor relations, regulation that can be politically tinged and even the weather. Airlines over the past decade have tried to shrink their way to profitability -- a combined United and US Airways today, for example, would have 23% fewer employees than standalone United did in 2000, according to J.P. Morgan research -- but with at least a half-dozen competitors on major domestic routes, those cuts have still not translated into profits.

The nine major passenger airlines in the U.S. collectively lost more than $3 billion in 2009, which was actually an improvement over the $19 billion they lost in the prior year.

With that as a backdrop, the financial community has long cried out for deals. Barclays Capital analyst Gary Chase, in a note responding to the latest reports, called the potential for additional airline consolidation "an unambiguous positive," with potential benefits going far beyond just the participants. In fact, it may turn out that domestic airlines not involved in dealmaking will be the real winners, benefiting as merger partners take capacity out of the system.

"The rationalization process is likely to create revenue benefits for all industry participants, not just combining carriers," Chase wrote. "For those not involved, these benefits will come without any cost or risk associated with implementation."

A large-scale merger would likely send ripples across the industry, affecting smaller as well as the major airlines. There may be more pressure on regional affiliates providing small-jet flying, a business already under duress, as the majors have sought ways to draw down flying during the recession without having to haggle with their own employees.

But if the financial community favors dealmaking, a number of other key constituents are opposed. Labor groups, politicians and passenger advocacy groups worry, rightly, that rationalization means fewer airline jobs, reduced service to smaller communities and potentially higher fares.

Labor groups have already taken it on the chin, with managers using a series of bankruptcies in the past decade to slash pay, cut pensions and rewrite work rules. They're in no mood to bargain: The Association of Flight Attendants, which represents workers at United and US Airways, put out a statement last week as rumors of a potential deal surfaced describing "the notion of joining together two airlines in turmoil" as "absurd."

An airline merger, then, is a leap of faith that those groups can all be appeased without management promising away the cost-cutting benefits of doing a deal. That is no easy task. "The airlines are caught between a competitive rock and a political hard place," one industry banker says. "As a result, you hear a lot of complaining about the lack of consolidation, but very few people are willing to do anything about it."

In spite of the obstacles, though, there is reason for deal advocates to have hope. Antitrust hurdles have receded, thanks in part to the success of Southwest and JetBlue. The $5 billion or so in fresh funding or refinancing that analysts think US Airways and United would need during integration looks to be available. United CEO Glenn Tilton, long an advocate of airline consolidation, has in public comments over the years discussed the substantial lag between when a deal is closed and when benefits kick in.

And industry sources can point to the relatively successful combination of Delta Air Lines Inc. and Northwest Airlines Corp. in 2008 as a template for how to get an airline deal done.

Management teams historically have left it to labor groups to settle thorny integration issues like seniority. US Airways is an extreme example, still operating with two bickering pilot groups five years after America West bought it out of Chapter 11. But Delta engaged its pilots before a deal was announced and nearly backed away from a transaction when it appeared aviators from the two companies might be unable to forge a compromise.

The pilots eventually agreed on the process of merging seniority lists prior to the airlines consummating a deal, allowing Delta to extract efficiencies, including revamping the combined route network. Analysts credit the proactive approach and the strength of leadership at the companies for clearing the Delta-Northwest deal for takeoff.

"The Delta-Northwest combination was blessed to have a pilot leader in place that understood consolidation and globalization are not only inevitable but are important to the success of his company and therefore the pilots he represents," William S. Swelbar, a research engineer at the Massachusetts Institute of Technology's International Center for Air Transportation, wrote in a recent note.

It remains to be seen whether United and US Airways can get their workers on board. Swelbar, for one, is skeptical. Combine "the dysfunctional relationship between the US Air and America West pilots with the entitlement mindset of the United pilots (and all United employees, for that matter)," he says, "and you have a mess."

Others, though, have more hope.

"United's pilots are under new leadership and have toned down their rhetoric, and US Airways pilots are adrift," the industry banker says. "The timing is perfect for management to step in and offer a clear vision of the future, and engage the pilots on how to get there."

The signs are there. After a meeting last week between union leaders and US Airways managers including CEO Doug Parker and president Scott Kirby, the union released a statement saying that they had emphasized the group's "willingness to work cooperatively with our company's management to facilitate any transaction that brings true value to all the potential stakeholders."

It has taken a decade, and there's still plenty to sort through. But as long as everyone keeps talking, this deal may actually get cleared for landing.