The House Armed Services Committee has voted to expand Fairchild Industries' A-10 aircraft program by $50 million. The A-10 is the biggest project currently in production at Fairchild.

The funding, if adopted by both houses of Congress, would allow production of the plane to be expanded from a one-seat to a two-seat format. Such a change in the A-10 configuration could enable Fairchild to successfully market the plane to foreign customers.

The amendment to the defense spending bill was introduced Tuesday by Rep. Beverly Byron (D-Md.), whose districts include Hagerstown where Fairchild Republic Co. has a major plant. The committee passed the amendment by a 25 to 13 vote.

Byron, who said she personally had flown the new A-10 plane, said passage of the amendment is "great for the district and great for the country."

"The plane has great capabilities," she said. Byron said Fairchild officials say that introduction of the two-seater could save the Defense Department $10.5 million a year in fuel costs because a second plane would not have to follow the current single seater on training flights.

Although Fairchild has already been authorized $494 million in the 1981 fiscal year budget for production of 60 one-seat A-10s, the additional appropriation calls for the Germantown, Md.-based company to produce 60 one-seaters and 30 two-seaters.

About $30 million of the additional procurement costs are for the production of the two-seater. An additional $20 million has been allocated by the committee for research and development, bringing the fiscal 1981 total funding for the project to $544.5 million.

In addition to the added seat, the new plane has the engineering capability to fly in bad weather and at night. "In recent years, U.S. and NATO officials have become increasingly concerned as to the adequacy of our combat forces' capability at night and in poor weather conditions," Fairchild said in its 1979 annual report.

The company tested the plane for five months last year and the prototype was turned over to the Air Force for testing at Elgin Air Force Base. The Air Force has completed tests and is assessing the results.

Hal Howes, Fairchild's government relations executive, said the company has a "very real interest in foreign sales" of the modified version of the A-10. Howes said that within State Department guidelines, Fairchild already has briefed foreign government representatives about the plane. the U.S. government would have to sell the planes

The new planes could be in service as soon as 1983 because they will be assembled by the same teams at Hagerstown and Farmingdale, N.Y., where another Fairchild facility is located.

The recurring cost of the expanded A-10 is $600,000 more than the single-seat aircraft. The original model was designed to accommodate the two cockpits.

In 1979, Fairchild delivered 129 A-10s to the government, a package that brought the company about $426 million in revenues. Congress has funded total production of 627 of the A-10 aircraft.