A columnist in the Dallas Morning News this week says that a merged US Airways-American Airline would be a boon to the airline's biggest hub, Dallas/Fort Worth.

But how much of the gains there would come at the expense of Charlotte Douglas International Airport, currently US Airways' busiest hub?

Here's the most important passage for Charlotteans from Mitchell Schnurman's column: "A portion of connecting traffic is going to shift to D/FW, with more feed coming from domestic and international,” said the chairman of Boyd Group International in Evergreen, Colo. “A merger would strengthen D/FW in the long run."...He believes that some US Air flights connecting through Phoenix and Charlotte would be more profitable going through D/FW."

That could mean a loss of flights connecting through Charlotte Douglas - a hub that is extremely dependent on connecting US Airways traffic. Connecting passengers make up about three quarters of the airport's 39 million annual passengers, and US Airways accounts for about 90 percent of the airport's daily flights.

So far, Charlotte Douglas officials and US Airways executives have said that the proposed merger would be good for Charlotte, with more flights to and from the city resulting from the combined airline.

But the airline business predicted to grow slowly in the coming years, probably at just around the rate of the economy's growth. So it's possible that the redistribution of flights in a merged US Airways-American would be a zero-sum game, with each hub's game coming as another hub's loss.

In any case, assuming a merger happens, realigning hubs would likely be a multi-year process. Some hubs have lost big in previous mergers (such as Pittsburgh and Cincinnati), while others have prospered (like Charlotte and Houston). Since airlines' largest piece of infrastructure flies, and airlines squeezed by razor-thin margins will shift traffic wherever it's profitable, it's likely hub cities won't know the true impact of a merger for quite some time.