And that may very well not be a good thing for Charlotte travelers going to Mexico City using US Airways’ existing nonstop flight. Here’s why: The current aviation treaty between the U.S. and Mexico restricts the number of airlines per nation that can fly a particular route to two (to Mexico City) or three (to tourist destinations in Mexico) with “route” defined by market, not airport. In addition, Mexico City International Airport (MEX) is capacity (slot) constrained and its difficult to get additional landing and takeoff slots at reasonable times.

Current daily US Airways flights to Mexico City are: Charlotte 1, Phoenix 2
Current daily American Airlines flights to Mexico City are: Dallas/Ft. Worth 5, Miami 4, Chicago O’Hare 1
American Airlines is not among the pair of U.S. airlines allowed to fly to Mexico City from either New York or Los Angeles though it does codeshare on Alaska Airlines’ LAX-MEX service.

The details of the new deal between the two countries, which goes into effect on Jan. 1, 2016, haven’t been announced yet. However, the rumors are that it will include a lifting on the cap on airlines on a particular route. If that’s correct, then CLT is definitely at risk of losing its Mexico City flight. The new treaty isn’t going to eliminate slot caps at MEX, which means that the new American still can have only 13 flights a day to there. And let’s be honest here: It’s questionable whether keeping the existing Charlotte – Mexico City flight would be the best use of one of those limited takeoff and landing rights, if both NYC and Los Angeles become possibilities 13 months from now for the merged airline.