Fairchild Corp. of Chantilly, citing a surge of business in its core product lines, debt reduction and cost-cutting, reported net income of $6.1 million (36 cents per share) in the fourth quarter of 1990, compared with a $7.6 million loss in the same quarter of 1989.

Revenues were down 20 percent, to $130 million from $162 million, reflecting the sale of 53 percent of Fairchild's holdings in Banner Aerospace Inc., a distributor of aerospace parts.

Fairchild said that if the 1989 quarter's figures were adjusted to exclude Banner Aerospace, sales would have been $109 million.

On an operating basis, Fairchild's income was up 138 percent to $6.5 million from $2.7 million, adjusted to exclude Banner Aerospace, the company reported.

Its business focuses on the manufacture of aerospace fasteners but also includes industrial tooling and telecommunications.

Fairchild also realized an after-tax gain of $9.5 million from the sale of discontinued business -- primarily the foreign operations of Thompson Aircraft Tire -- during the fourth quarter of 1991.

The report came one day after Fairchild disclosed that it had received an unsolicited and tentative takeover proposal from Mountleigh Group PLC, a British-based real estate and investment group.

Fairchild said its board would respond to the offer if the bid was firmed up.

First Citizens Financial Corp., parent of Citizens Savings Bank of Silver Spring, said yesterday that it lost $5.2 million in 1990, compared with $2.3 million ($1.14 per share) earned in 1989.

The thrift, which operates 14 branches in Maryland, said it lost $43,000 in the fourth quarter, compared with earnings of $42,300 (20 cents) in the same period in 1989.

First Citizens President Enos Fry attributed the year-end and quarterly losses to the continued softening of the local real estate market.

To help protect the thrift against future real estate loan losses, First Citizens set aside $700,000 in reserves in 1990.

Systems Center Inc., a Reston-based computer systems software company, yesterday said it lost $27.9 million in 1990, principally because of a one-time write-down of $26.7 million in the fourth quarter and about $3 million in write-offs earlier in the year.

Revenue was $105.5 million.

In 1989, Systems Center earned $11.2 million on revenue of $78.2 million.

Revenue in 1990 was up 35 percent over the previous year. The fourth-quarter write-down also caused the company to lose $25.1 million in that quarter. Revenue was $36.5 million, up 36 percent from the same quarter in 1989.

Richard Moore, the company's vice president, said the write-down was for the loss in value of marketing rights to Netmaster, a new product the company acquired in the first quarter of 1990.

The product proved harder to sell than it was earlier thought, he said. Systems Center paid $43 million for those rights, plus another $43 million for the company that makes Netmaster, Software Developments International of Australia.

Last month, Systems Center laid off 92 employees worldwide, which was about 10 percent of its work force, in a cost-cutting move.

Moore said the company anticipates profitability through 1991 now that the one-time charges are behind it.