U.S. airline behemoths United Airlines of Chicago and Delta Air Lines could be joined by a third mega-carrier, as talks to merge American Airlines and US Airways heated up heading into the Labor Day weekend.

On Friday, AMR Corp., parent of American Airlines, and US Airways Group announced they signed a non-disclosure agreement and would exchange confidential information in an effort to merge their companies. A merged airline would have complementary route networks and achieve the scale necessary to compete head-to-head with the largest carriers, US Airways officials have said.

For consumers, the effect of a potential American-US Airways combination might not be benefit.

"It is pretty clear that a smoothly executed merger -- a historical coin flip at best -- would probably create a stronger business entity," said Rick Seaney, CEO of FareCompare.com. "Unfortunately for passengers, we would likely see an acceleration in the current trend in reduced choice, packed planes and higher ticket prices, albeit finally on newer aircraft."

Brett Snyder, who writes the Cranky Flier blog, said it's easy to predict that fares would rise, but there's a lot of uncertainty right now. "We have no idea what a merger will look like at this point, so it's impossible to say who will be in charge and what will happen," he said.

The non-disclosure agreement means the companies won't be providing more information regarding the status of discussions until there's a deal or they call off talks, the airlines said. The airline companies said they would work in "close collaboration" and "good faith" to evaluate a merger, including working with the creditors committee of AMR, which is in bankruptcy protection.

The companies note there's no guarantee that a merger will result.

American, the No. 2 airline in the Chicago region after United, also said it signed non-disclosure agreements with other parties but would not name them. Reportedly, one is with the parent of British Airways, International Airlines Group, which potentially seeks to buy a stake in American Airlines, according to the Associated Press. It can only buy a stake and not the whole company because foreign entities by law can control only 25 percent of a U.S. airline.

However, analysts have said the most likely hookup would be with US Airways.

Although, for a long time after AMR's bankruptcy last November, CEO Tom Horton insisted the company was not interested in a merger while it was restructuring, saying it was too complex to do both. Meanwhile, US Airways CEO Doug Parker aggressively and publicly campaigned for merger talks.

"I think it would be tremendously unwise to get distracted with talk of combination inside a restructuring," Horton told the Tribune in March. However in July, Horton said the company had "greater clarity on our revenue outlook and cost structure" and would begin evaluating strategic options, including mergers.

In a letter Friday informing American Airlines managers about the non-disclosure agreement, the company said, "This will be an objective, thorough, fact-based process. Although we will proceed with these discussions in good faith, there can be no assurance that any transaction will take place."

Parker and independent analysts have said an American-US Airways combination would allow the airlines to better compete with the two largest U.S. carriers, United Continental Holdings, parent of United Airlines, and Delta Air Lines. Those airlines in recent years have grown bigger by their own mergers.

"You would have the international operations of AMR and the lower cost structure of US Airways," Morningstar analyst Basili Alukos said.

However, combining airlines is no easy feat. The most recent evidence is the mega-merger of United Airlines and Continental Airlines, which combined in 2010. The new United Airlines continues to struggle with integration. Notably, its March combination of United and Continental passenger reservation systems has resulted in numerous serious glitches and rampant flight delays. And the airline is still struggling to strike labor agreements with its unions from United and Continental.

However, a merger of American and US Airways has the support of AMR labor unions. In April, the unions, including pilots, flight attendants and ground workers, said they struck independent deals with US Airways in anticipation and support of a merger.

American's flight attendant union, the Association of Professional Flight Attendants, said in a statement Friday it continues that support. "APFA's position remains consistent regarding a possible merger with US Airways: we support a strategic alternative that includes a management team with a proven record of effective leadership. Combining our networks will allow American Airlines to grow and compete with our industry's dominant global carriers."

 The American Airlines pilots union, Allied Pilots Association, saw Friday's non-disclosure agreement as a good sign. "APA sees it as a positive move forward," said spokesman Dennis Tajer. "We look forward to more progress in the days and weeks ahead."

At least one airline industry analyst was reading between the lines of the joint news release Friday.

"We noticed the press release was sparsely worded and lacked the typical happy speak such as 'we are pleased to announce' or 'we look forward to' whatever, which no doubt is because AMR has been dragged kicking and screaming to the table," wrote analyst Vicki Bryan of Gimme Credit in a research note Friday. "Yet there it now sits because, as we have speculated, AMR's stakeholders are becoming universally convinced that a merger presents the best chance today and in the future for the resulting carrier to compete with any reasonable success against Delta and United-Continental as well as global behemoths emerging overseas."