Two major air cargo carriers announced this week plans to move their hubs to new locations in the Midwest, enabling each to expand their operations significantly.

American International Freight, which operates an overnight service catering to domestic freight forwarders, will move in May to Terre Haute, Ind., from Willow Run International Airport in Ypsilanti, Mich.Bob Hunter, AIF president, said it will be able to double its capacity by moving to Terre Haute. The carrier currently operates 12 aircraft to 18 cities but will expand its network to 18 jets serving 31 cities once it makes the move, he said.

Meanwhile, Southern Air Transport, which primarily operates as a charter carrier, will relocate from Miami to Rickenbacker International Airport in Columbus, Ohio.

Lawrence Garrison, executive director of the Rickenbacker Port Authority, said Southern Air plans to operate freighters between the hub and the Far East. The carrier also will use it for some maintenance operations, he said.

Gov. George Voinovich of Ohio was scheduled to announce the move at a press conference Thursday afternoon.

Limited Inc., the nation's largest retailer, is based in Columbus, a fact that undoubtedly contributed in large part to Southern Air's decision, said Brian Clancy, a cargo analyst with MergeGlobal Inc., an Alexandria, Va., consulting firm.

During the peak fall shipping season, as many as 24 747 freighters arrive each week in Columbus with imported cargo destined for retail shelves around the country.

AIF's expansion will benefit domestic forwarders who have become increasingly dependent on AIF to move their cargo because of cutbacks in widebody lift by passenger airlines.

"They're our primary carrier for next day overnight freight," said Dave Pillets, vice president of operations at Pilot Air Freight of Lima, Pa.

Terms of the agreement have not yet been finalized, "but this is a done deal," Mr. Hunter said in a telephone interview Thursday.

The 7,600-foot runway at Willow Run was too short for AIF to operate at full capacity. It had planned to move to Toledo, Ohio, in October, but that would have required the construction of a new ramp to serve the hub building and hangar.

Hulman Regional Airport in Terre Haute has a 9,000-ft. runway and a ramp large enough to accommodate AIF's expansion plans, Mr. Hunter said.

The airport also serves as the hub for Roadway Global Air, an integrated air carrier that began service in September 1993.

Both Mr. Hunter and Donald Berger, RGA president, said the two companies have no plans to merge or form a joint venture.

"We're glad they're coming. The more aircraft operating there, the better," he said. The two carriers do not compete because AIF sells to forwarders, while RGA sells directly to shippers.

"We'll have to see what the future brings in terms of cooperation," he added. RGA, a subsidiary of Roadway Services Inc., operates 14 aircraft.

Mr. Hunter declined to discuss prospects for cooperation, but said, "We intend to be a good neighbor."

Freight forwarders hailed the AIF plan.

"We can only expect good things to come of it," said Brandon Fried, chief operating officer of Adcom Express Worldwide of Alexandria, Va.

Adcom currently books some 15 percent of its traffic with AIF, but Mr. Fried said he expects that figure to reach at least 30 percent as AIF expands its service.

Southern Air officials could not be reached for comment, but wire service reports quoted Bill Langton, its president, as saying it was attracted by the twin 10,000-foot-long runways at Rickenbacker and the availability of land for development at the sprawling former Air Force base.

One of the busiest carriers at Columbus is Polar Air Cargo, a Long Beach, Calif., all-cargo airline that initially used Southern Air to operate Polar's chartered 747 freighters. Polar took over its own flight operations last fall, but the experience probably encouraged Southern Air's move. Over the past year, it has acquired two 747 freighters formerly owned by Japan Airlines.