After a seemingly endless series of bankruptcies, mergers and general corporate restructurings, it appears the U.S. airline industry may finally reach a state of corporate equilibrium.

The last hurdle appears to be American Airlines, the last airline to undergo a major restructuring, merging with US Airways, a survivor of 2002 and 2004 bankruptcies. The merger will combine two of the world’s 10 largest airlines, leaving America with only four mega-sized airlines.

It could mean trouble for air traffic hubs like Charlotte. A merger of this size can act as the first domino in a series of recalibrated routes and redefined market power – in the end, leaving certain aviation markets with sharply reduced connectivity.

Certain hubs rarely feel the effects of airline consolidations. They include metro areas that attract routes based on their economic clout, like New York, or tourist-centric places, like Las Vegas. These markets have too much local and external demand to experience reduced connectivity. But there is another type of hub, the airline-centric one, where a merger of this size can reverberate.

The problem is an over-reliance on transfer passengers. These passengers bring benefits. They increase aircraft operations, are stuck shopping in concourses, and sustain more connectivity than their markets would otherwise enjoy. The problem is their lack of sustainability. All it takes is a shift in airline location patterns, and routes can quickly disappear.

The past decade serves as a cautionary tale. St. Louis, Cincinnati, Memphis and Pittsburgh all experienced major drops in connectivity, and then passenger levels, due to airlines relocating hubs. Between 2003 and 2011, their domestic-only passengers dropped by at least 20 percent, and more than 50 percent in Cincinnati’s case. The drops left empty gates at the airports and extended travel times for locals.

So what does the proposed American-US Airways merger mean for their current hub markets? New York, Los Angeles, Miami and Washington all sustain great connectivity, first and foremost, on the back of local business. The merged airline will need those markets as much as the markets need it.

For the remaining five metro hubs warning bells should be ringing.

Charlotte most vulnerable

The biggest concern is in Charlotte. Charlotte is the country’s smallest metropolitan hub as measured by local population. More than half of the airport’s domestic passengers are bound for other destinations, and over three-quarters of the airport’s international passengers don’t start or end their trip in Charlotte. The airport may be one of the fastest-growing in the country, with capital plans to sustain the momentum, but too much of its business simply recirculates passengers.

Charlotte’s pressures really mount when considering the firm’s coastal competition. Miami and its Latin American business is a major asset, and its tourism industry attracts domestic connections. New York and Philadelphia offer broader connectivity to Europe, and do so with larger local customer bases. Could those markets pick up Charlotte’s slack?

The other hubs could also see changes. Philadelphia’s role could be marginalized with New York’s presence. The always-crowded Chicago could cede Midwest transfers to Dallas – especially if the new headquarters stay in Texas. Similarly, Phoenix could see a drop if it loses US Airways’ headquarters.

But each of these metros has a local customer base to support connectivity, regardless of airline changes. The same cannot be said in Charlotte.

Of course, nothing is certain. The new firm can maintain commitments to Charlotte, using other hubs as international transfer points and leaving Charlotte to do the heavy domestic lifting. Or it could even blaze a new path, maintaining all the hubs and routes. And no matter what happens, long-term leases and other commitments mean changes are years away.

In the case of domestic air travel, it’s not a question of whether you can get there, it’s a question of how long it’s going to take – and nothing changes that equation like airline operations. With another tectonic shift on the horizon, only time will tell which hubs are left standing.