Pittsburgh International Airport's bid to regain trans-Atlantic nonstop service could get a shot in the arm early this spring when a new agreement gives European airlines far greater freedom to fly to cities in the United States.

Some experts believe the "open skies" accord between the United States and the European Union, which takes effect at the end of March, could improve chances for medium-size cities like Pittsburgh to get nonstop flights to Europe, at least at some point.

The city has been without such service since November 2004, when US Airways dropped flights to London and Frankfurt as part of a dramatic downsizing at Pittsburgh International.

"I think this improves those prospects. Every carrier in Europe can look at every market from Europe to the U.S. Therefore, I think Pittsburgh has a wide array of possibilities," said Pat Murphy, partner at Gerchick-Murphy, a Washington, D.C.-based aviation consulting firm. "I'm not predicting that it will happen overnight, but I think it increases the likelihood of that in the future."

Perhaps the chief advantage of open skies to cities like Pittsburgh is that it gives them a chance to aggressively go after virtually any European airline to pitch service, analyst Terry Trippler said.

In the past, those airlines were restricted to flying from their home country to the United States. Now they can fly from any city in the European Union. So Lufthansa, for example, could fly from London to Pittsburgh and British Airways could fly from Frankfurt to Pittsburgh.

"It's got to be the right airline, the right city and the right circumstances, but it does open up an opportunity that would not have been there without open skies," Mr. Trippler said.

The new agreement takes effect as the region's political, civic and business leaders renew a push to regain trans-Atlantic service. The airlines at the top of the wish list are KLM Royal Dutch Airlines and Northwest Airlines, which has a partnership with KLM.

Allegheny County Chief Executive Dan Onorato and local business and civic leaders met with KLM officials about the possibility of a direct flight between Pittsburgh and Europe during a recent weeklong overseas trip.

While the open skies agreement wasn't a major focus of the discussion, Mr. Onorato believes the accord will have a positive impact because it will open up more slots in Europe and provide more options to carriers looking to start service in the United States.

"It definitely doesn't hurt our cause, put it that way," he said.

But that doesn't mean it will be easy.

In the view of some experts and airline officials, Pittsburgh doesn't have a large enough population to support a nonstop flight to Europe. And without a hub to feed expensive trans-Atlantic service, few airlines are willing to take the chance, they said.

Local officials have tried repeatedly to persuade US Airways to resume flights to London or Frankfurt from Pittsburgh, but to no avail. On a recent conference call with analysts and reporters, US Airways President Scott Kirby explained why.

"It is [an] unfortunate reality of the economics of the business, particularly with high fuel prices, that small and medium-size markets simply can't support service internationally," he said. "Pittsburgh is a great market and has some great business there. It's just not large enough to support international service."

Darryl Jenkins, an airline consultant, said Pittsburgh may end up picking up some seasonal service to Europe, but not much more than that.

"If I were to draw up a list of the Top 10 hot spots, Pittsburgh would not be on it," he said. "When you lose connecting traffic, you're less likely to get nonstop service to Europe."

"[Open skies] is not a big benefit to Pittsburgh," aviation consultant Mike Boyd added. "There isn't enough traffic to make it work. That doesn't mean you can't get charter or seasonal service. But with a mainline carrier like Air France ... it's very unlikely -- not impossible, but very unlikely."

But Michael Langley, chief executive officer of the Allegheny Conference on Community Development, said experts who focus on the region's local traffic "just haven't looked at the numbers like we have."

Through surveys, the conference has found that on average 325 people a day leave Pittsburgh for destinations in Europe and beyond and 325 people return each day.

That's more than enough, he said, to support a flight.

"We have a market that needs to be served. That's the most important and positive discriminating element that we as a region can put on the table," he said. "It's not wishful thinking. It's not emotional desire. It's a real financial benefit to a carrier to begin service directly to Europe."

The 325-passenger estimate includes a "very high mix of business class traveler" that airlines typically look for to support trans-Atlantic service. Mr. Langley said that's not surprising given the number of local firms, such as Bayer, U.S. Steel, Reed Smith, PPG Industries, Westinghouse Electric and Medrad, with business overseas.

A number have committed to booking flights on KLM if it starts trans-Atlantic service from Pittsburgh.

"Almost all of the major employers are doing global business. That's not a trend that will diminish. That's an ever-increasing trend," Mr. Langley said.

He also said the conference has found that many people would rather fly nonstop from Pittsburgh into the heart of Europe and then connect from there to a London destination rather than to go through airports in Philadelphia, New York, Washington or Charlotte, N.C., "because it's too much hassle on this side of the Atlantic."

Another factor that could work in Pittsburgh's favor is that some airlines are now using narrow-bodied aircraft, such as the Boeing 757, to make the trip across the Atlantic. Those planes typically hold 160 to 170 people. It would be easier to fill the seats on one of those planes, compared to a wide-body that can carry more than 300 passengers.

Northwest, in partnership with KLM, for example, is offering service from Hartford, Conn., to Amsterdam using an extended range Boeing 757.

"There are opportunities for those aircraft to operate in a market the size of Pittsburgh," said Kent George, the former executive director of the Allegheny County Airport Authority. "I think there's enough market in Pittsburgh that it can support a narrow-bodied aircraft expanding service internationally."

With open skies, some also see the potential for low-cost carriers on both sides of the Atlantic to jump into the competition.

European low-fare carrier Ryanair has said it intends to start flying to the United States in 2010, with service to five or six cities, including Baltimore and Providence, R.I. And Southwest, Pittsburgh's second largest carrier, has expressed an interest in European service. Low-cost carriers could "offer real potential" for cities like Pittsburgh, Mr. Murphy said.

But not everybody is sold on the theory that the open skies agreement offers the potential for more service, whether it's New York or Podunk.

Mr. Hazel, for one, said he expects the biggest impact to be in the largest markets and at London Heathrow, where U.S. carriers were severely restricted in the past but which will now be open to any airline that can get a landing slot.

Beyond the major markets, however, Mr. Hazel believes the reach of the open skies accord has been overstated. Even before the agreement, countries like Germany, France and the Netherlands already had open skies deals with the United States.

Others are more optimistic.

Mr. Langley said he believes the region has made a good case for international service. He expects KLM to make a decision this year as to whether to initiate a flight from Pittsburgh. But even if it says "no," he believes the region ultimately will persuade some airline to fly nonstop to Europe.