Fairchild Industries, stunned by an Air Force decision to halt orders for the A10 jet fighter that is a mainstay of the Germantown firm's business, marshalled enough support in Congress and the administration in the past two weeks to keep the A10 program alive in curtailed form, company and congressional sources said yesterday.

Termination of the A10 would have been a heavy blow to Fairchild and to the City of Hagerstown, where the plane is assembled. More than half of Fairchild's annual sales of about $1.2 billion are in military contracts, according to company figures, and the A10 Thunderbolt is the biggest component.

The Air Force, anticipating budget reductions of up to $8 billion in the 1982 and 1983 fiscal years, decided two weeks ago to drop the A10. It had been scheduled to buy 60 of the planes in 1982 at a cost of more than $9 million each.

But Reps. Beverly Byron (D-Md.), whose district includes Hagerstown, and Joseph P. Addabbo (D-N.Y.), chairman of the House defense appropriations subcommittee, rushed to the plane's defense and succeeded in salvaging some of the program, company officials and congressional sources said.

After reports of the A10 curtailment were published, Fairchild stock dropped a point to 13 3/8, its low for the year, in trading on the New York Stock Exchange yesterday. The high was 33 7/8.

Appearing before Addabbo's subcommittee yesterday, Defense Secretary Caspar W. Weinberger spoke of "revamping" the A10 program but gave no details. Fairchild's Washington representative, Hal Howes, said the Air Force now has agreed to buy 20 of the A10s in the 1982 fiscal year and another 20 in 1983.

That is far below the peak annual production of 144 planes that Fairchild delivered in the late 1970s, but should be enough to keep most workers on the job while Fairchild bids for the contract to build a new jet trainer aircraft later in the 1980s, defense experts said.

An aide to Byron noted that the production schedule already had begun to "taper off" because Fairchild's total agreement with the Air Force called for delivery of 693 planes, which would have been achieved by the end of 1983. He said that anything other than an abrupt halt would give Fairchild time to plan an orderly adjustment of its work force. Jobs at the Hagerstown plant would begin to be affected in another year if no new contracts are awarded to Fairchild, he said. Hagerstown already has one of the highest unemployment rates in the region.

The A10, an unglamourous plane of limited capabilities, never has been a high-priority item with the Air Force because its primary function is to provide close combat support to ground troops, defense experts said. Fairchild, however, has argued that production should be maintained because in its two-seater form the plane is usable for training and also has a potential for export sales to less-developed countries.