While Fairchild Swearingen Corp. battled it out before the International Trade Commission in Washington last week against a Brazilian commuter aircraft maker, it was engaged in a battle here, to.

Fairchild Swearingen, a subsidiary of Fairchild Industries of Germantown, Md., is one of dozens of American companies seeking sales in the international aviation marketplace called the Farnborough Air Show.

"We're selling all our products, but what we're pushing here is a military aircraft," explained Anthony J. Spuria, Fairchild's senior vice president and head of its commercial aviation group. The plane is the new "Air Sentry," a special-mission aircraft fitted out for maritime surveillance for exhibit purposes here.

The plane utilizes the same airframe as Fairchild's Metro commuter aircraft and its Merlin executive aircraft, but is configured and equipped primarily "to define non-friendly ships at sea," Spuria said. The plane is currently undergoing wind-tunnel testing by the Swedish Air Force, which would use the plane, should they buy it, with laser radar antenna that can see 17 miles on each side of the aircraft, he said.

The plane can also be equipped for other missions, including air ambulance, all cargo, flight inspection, 25-seat high-density transport, navigation training, aerial photography and remote sensing uses.

The plane "sells to governments -- small governments -- and quite a few of them come to Farnborough," Spuria said in an interview in Fairchild's hospitality "chalet." The company wasn't participating in this market while it was developing its executive and commercial aircraft business, he said.

"Now that volume has fallen off, we have plenty of capacity to build special-mission aircraft," he noted. "It's something our competition is doing."

The new Sentry is one of three aircraft Fairchild brought to exhibit here at Farnborough, about 35 miles southwest of London. The other two planes on display are its latest-model Merlin executive transport and the A-10 Thunderbolt II, its "multimission defender," a single-seat close-support aircraft shown here with all the lethal weapons it can carry.

Fairchild is also pushing the 34-seat Saab-Fairchild 340, a commuter aircraft being produced with Saab-Scania of Sweden. In the 50-50 venture, Fairchild builds the wings and tail, and its Fairchild Burns subsidiary makes the seats. Saab builds the fuselage in Sweden where the final assembly takes place.

The new twin-turboprop aircraft is set to roll out of the factory Oct. 27, with first deliveries set for the second quarter of 1984, after certification in the United States and Europe. Spuria said the venture would break even when 200 aircraft are sold. They have orders for 100, including some to Air Wisconsin and Air Midwest, he said.

Right now, Fairchild has no open orders -- no backlog at all -- for its 19-passenger Metro commuter aircraft, the plane it contends is losing out to the Brazilian Bandeirante because of alleged "unfair competition." The 18-passenger Bandeirante, increasingly popular with U.S. commuter airlines, is made by Empresa Brasileira de Aeronautica S.A., commonly known as Embraer (pronounced embra-air).

In its case before the International Trade Commission, Fairchild is contending that Embraer is subsidized heavily by the Brazilian government, which owns 51 percent of the company, enabling Embraer to offer preferential financing terms to U.S. airlines buying the aircraft.

If Fairchild were successful, American purchasers of the Bandeirante would have to pay an import duty of 44 percent on the plane's $1.8 million purchase price, industry sources said.

Spuria contends that in some cases, the financing Embraer offered the American commuters means their payments per month are 40 percent lower than anything Fairchild can offer.

There are currently 235 Metros in service with about 49 commuter airlines, Spuria said. Embraer has sold more than 400 Bandeirante aircraft worldwide.

Fairchild contends the two planes are comparable, but commuter industry sources say some airlines have opted for the Bandeirante because it was available when they needed it and because it was an unpressurized aircraft good for the short distances the commuters operate.

The Bandeirante also is reputed to have lower maintenance requirements than the more sophisticated Metro, a pressurized, high-speed, high-flying, longer-range aircraft with a history of engine trouble.

Spuria admitted that for a period, there was a problem with the Metro's engines but he said it was solved with an engine modification.