Fairchild Industries eventually will close Fairchild Republic Co., the division that makes the T46A jet trainer, if Congress fails to reinstate the plane in the 1987 budget, a top Fairchild executive said yesterday.

"Without the T46 there will be no Republic," said Tom Turner, vice president of marketing and corporate communications at Fairchild Industries of Chantilly, Va. If the plane is not restored, "You'd have to assume layoffs," he said.

Fairchild Republic Co. in Farmingdale, on Long Island, provides 3,500 jobs, 1,400 of them directly related to research on and production of the T46A. The division had anticipated $4 billion to $6 billion in domestic and foreign military sales.

Turner could not speculate when layoffs might begin or how severe they might be because of uncertainties in the federal budgeting process.

Last month, the Reagan administration asked Congress to eliminate virtually all funding for the jet in fiscal 1987. The Air Force needs a new trainer, but the Defense Department decided to cancel the T46A last September because of "numerous management and production deficiencies" and cost overruns.

But production problems have been corrected, according to company officials. A trainer prototype has been flown successfully at Edwards Air Force Base, and its minor deficiencies are almost ironed out, according to test pilot Wendell Shawler. "I think it'll be a super trainer," he said. "The procedures are simple, and it's going to be a great training environment."

The company does have a contract with the Air Force to deliver 10 of the trainers. But $206.5 million in fiscal 1986 appropriations for 33 more planes has been put on hold, and those funds will not be released if the trainer is left out of the fiscal 1987 budget, Turner said.

In trying to reach a compromise with the Air Force, Turner said the company was trimming its requests. Instead of making 44 planes for $350 million, the company is asking to make 11 planes for $105 million, $31 million of which would be paid out in fiscal 1988.

It is not clear whether the offer will be enough to persuade Air Force Secretary Russell Rourke to reinstate the plane. Rourke has said he will make a decision on the jet this month.

Fairchild Republic already is losing some of its top people who are looking for other jobs, Turner said. It has hundreds of millions of dollars at stake in sales to foreign countries as well, he said.