DALLAS — Houston’s decision to allow Southwest Airlines to open five new international gates at Hobby Airport in competition with United Airlines at Bush Intercontinental Airport is a credit positive for the Houston Airport System, according to Moody’s Investors Service.

Southwest has agreed to shoulder the $100 million cost of expanding Hobby. Though financing has not been detailed, bonds backed by Southwest ticket surcharges are considered likely.

Moody’s assigns a Aa3 rating to the Houston Airport System, which operates both Bush and Hobby. Southwest is rated Baa3.

Southwest has shared markets with other legacy carriers in major U.S. cities without significant negative effects on the traffic for those larger hubs, according to Moody’s analyst Laura Barrientos.

This is the first time Southwest will offer international service and compete in the lucrative South American market.

Spirit Airlines has offered low-cost service out of Florida’s Fort Lauderdale Airport to the Caribbean and Central America for a number of years, competing with American Airlines’ service out of Miami International Airport, which is rated A2 by Moody’s, Barrientos wrote in the agency’s weekly credit report.

“Year to date, enplanements at Fort Lauderdale Airport are up 4.5%, and were up 7.0% in 2011, while those at Miami are up 7% year to date and rose 9% in 2011,” Barrientos wrote. “While the two situations are not exactly comparable, given Spirit’s smaller size and less connecting traffic compared with Southwest, it supports the premise that additional service of this magnitude and scope is likely to result in more passenger throughput on the whole.”

A closer parallel exists 240 miles to the north, where Southwest competes on some routes with American Airlines. Southwest flies out of the smaller Love Field while American is the dominant carrier at Dallas-Fort Worth International Airport. Both airports, like Hobby and Bush Intercontinental, are undergoing bond-funded remodelings. Restrictions on Love Field flights end in 2014.

At DFW, Moody’s maintains a negative outlook on its A1 rating based on the airport’s high leverage position and its revenue concentration risk in bankrupt American.

In Houston, the Bush airport is dominated by United Continental Holdings, which opposed the Houston City Council’s May 31 decision to allow Southwest to offer international flights at Hobby.

For the Houston Airport System, which includes the former Ellington Air Force Base, the net effect is positive, Barrientos said.

“More passengers translate into higher revenue for airports through airside charges, passenger facility charges, and related concession and parking revenue,” she added.

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