This coastal border city hasn't had passenger airline service since last year but that doesn't mean the Brownsville-South Padre Island International Airport stands idle.

In one 12-hour period recently we recorded $100,000 worth of usage fees, said Larry Brown, director of transportation services for the City of Brownsville. On another day we had 25 Lear jets backed up at one time on a

runway hauling cargo.Thanks to increased use of maquiladora plants in nearby Matamoras, this city has managed to carve a new growth sector for itself in air cargo.

Linking Mexican component assembly plants with automobile plants in the Midwest, air freight business here has more than doubled while generating expansion at companies involved.

And participants in the air cargo boom here expect the growth to continue.

With a foreign trade zone site and a recovering economy in Mexico, the air cargo business of Brownsville appears poised for a prosperous takeoff as Texas itself steps toward a new era of diversification.

The airport is moving 2.5 million pounds of air cargo a month, Mr. Brown said. That includes an increase of 500,000 pounds in just the last four months.

And it represents a boost of 1 million pounds over the monthly average in the last four years.

Our economy here is tied to Mexico's, Mr. Brown said. In 1983, when the peso suffered a significant devaluation, our economic leaders decided to take a deliberate approach to improve industrial development.

Securing a foreign trade zone site for the airport, they positioned the city to take advantage of the impending growth in maquiladora operations just across the border.

And they moved quickly to provide a modern facility for air cargo operations, beginning construction in 1985 on the Air Cargo Freight Terminal.

A $1 million structure completed that same year, the terminal boasts 30,000 square feet divided into separate sections for leasing.

Trucking companies have efficient access to aid in the link between air cargo carriers and the twin plants across the border.

Currently, Emery Air Freight, Burlington Air Express and the Bazaar Corp. have air operations there. Other tenants are the freight handling firm of Marks & Rodriguez and an office of the Clarion Corp., which rents 5,000 square feet to warehouse electronic components for General Motors cars.

Air cargo has increased here tremendously, agreed David Lorenz, city manager for Burlington at the facility.

It's doubled in the last couple of years. We have one plane per day. It arrives in the morning with deliveries and goes out in the afternoon. Most of the cargo is headed for the maquiladoras - about 90 percent of it.

But Mr. Brown said general trade with Mexico also has contributed to the higher air cargo tonnage.

He lists the Bazaar Corp., a local firm, with carving a niche in that traffic. Bazaar executives declined interviews but they did acknowledge they can handle air freight deliveries over the border. And they, too, must be thriving.

Bazaar is proposing to add another 25,000 square feet and will lease three acres from the city for expansion at a cost of $500,000, Mr. Brown said. They're looking to have the expansion complete by next year.

Typically, Mr. Brown explains, parts arrive from overseas into the foreign trade zone at the air cargo terminal.

Trucks carry them through Brownsville to one of the 200 twin assembly plants in Matamoras where Mexican workers assemble them into radios, windshield wipers or any one of a number of particular components.

The finished products then return to the air cargo terminal in containers by truck for further shipment to auto assembly plants in Detroit, Kansas City, St. Louis and Kokomo, Ind.