Tucked into the $160 billion defense appropriations bill passed by Congress Friday is a provision that will for the first time provide funding for the expansion of Fairchild Industries Inc. A10 ground support air craft program and permit the company to supply the Air Force with a two-seat version of the plane.

The bill expanded the government's purchase of A10s by adding an additional $44 million to the program. Of that appropriation, $20 million is for research and development for the A10 and another $24 million for actual expansion of the plane from a one-seat to two-seat version.

Although Congress had already committed the Air Force to the 1981 fiscal year purchase of 60 A10s, the appropriations bill calls for 30 of those planes to be converted to the larger model. The expanded aircraft will cost about $600,000 more per plane than the single seat version, bringing the total cost of the fiscal 1981 A10 appropriation to about $556 million.

The appropriations bill is particularly significant for the Germantown, Md.-based defense, communications and technical equipment company because the A10 is easily Fairchild's largest single government contract. In addition, the company hopes to develop a foreign market for the A10, and Fairchild officials said the two-seat version should aid in that sales process.

With the aggressive support of Rep. Beverly Byron (D-Md.), whose district includes a Hagerstown, Md., plant where the plane is assembled, the National Guard Association and the Reserve Officers Association, Fairchild has argued that by using the two-seat version, more of the military's existing A10 fleet would be freed for tactical use.

Use of the single seat version meant that trainees had to be followed by another plane. With the two-seat plane, a pilot's training can be supervised by a pilot sitting in the same cockpit. The two-seat version also permits the plane to be operated in combat by one pilot.

In addition, Fairchild has argued that as a result of the simpler training program, the government will save about $20 million a year in training costs and would also save about 8 million gallons of fuel per year.

The A10 is assembled at the company's Fairchild Republic Co. plant in Hagerstown after parts of it are constructed at another Fairchild facility in Farmingdale, N.Y. The plane is designed to provide air support for ground troops in combat situations.

"This money is very important to the long-term health of the program," said John Williamson, Fairchild Republic's vice president and director of the two-seat A10 program. "We are convinced that one of the nation's strategic defense needs is the ability to cope with armored threats. The two seater offers the attractive business proposition of being able to free up the airplane following the trainee and allow for more defense on the front if it is ever needed."

Under the government's previous authorization plans, there was to have been $512 million for 60 single seat A10s in the defense budget. The current contract calls for construction of 633 A10s for the government.

The funding bill approved Friday brings the total to 693, and the company plans to sell about 40 more to the government before the program runs out in 1983. Fairchild has already delivered 429 of the planes to the Air Force, the company said.

The work on the expanded A10s will be carried out essentially by the same company teams that have built the existing supply of A10s. The new planes could be in service by 1983.