If consolidation wanes, "airports will generally benefit as more carriers (and competition) increase the routes and require more services," Fitch said in a statement.

Small, regional airports stand to benefit the most because they are at greater risk of airline service cutbacks after large carriers merge, Fitch said. Big international airports could also get a boost.

"If US Airways and AMR remain separate carriers, both may separately need to enhance international services to contend with United and Delta," Fitch said, noting American is "better placed to pursue this course given its larger presence at more favorable international gateway airports" such as New York's JFK, San Francisco and Los Angeles.

The picture is complicated by the bankruptcy of AMR, American's parent company.

The suit, filed on Tuesday by the U.S. Department of Justice, six state attorneys general and the District of Columbia, is likely to delay final approval of AMR's plan to exit bankruptcy, of which the merger was a major component.

AMR could decide on a new strategy to exit bankruptcy that includes scaling down its operations, Fitch said. That would leave airports with significant exposure to AMR, like Northwest Arkansas Regional Airport, still susceptible to reductions in AMR's carrier's route network.

(Reporting by Hilary Russ; Editing by Chizu Nomiyama)

Stocks treated in this article : US Airways Group Inc, AMR Corporation