Fairchild Industries Inc. said yesterday that its $13.5 million gain in pension assets was a major factor in boosting the aerospace contractor's second-quarter net earnings to $18.8 million, compared with $3.8 million for the same period in 1986.

Fairchild Industries reported second-quarter net earnings from continuing operations of $9.1 million (47 cents a share), compared with $5.2 million (15 cents) for the second quarter last year.

Sales for the quarter were $168.1 million, down from $172.5 million in the same period in 1986.

"Fairchild's financial results are improving," said Edward M. Murchie, the company's senior vice president and chief financial officer.

Murchie said sales, "although down in comparison to a year ago, have shown approximately a 20 percent increase from the first quarter this year.

In addition, profit margins in the aerospace-related operations have improved as compared with the first quarter of 1987." USAir Group Inc. yesterday reported record quarterly net and operating income in its 1987 second-quarter earnings statement.

Net income was $72.7 million ($2.32) compared with $32.2 million ($1.08) for the same period in 1986. Operating income for the quarter was $99.1 million on revenues of $619.5 million, up from $49.3 million on revenue of $458.5 million for the same quarter a year earlier.

The second-quarter net profit figure included $21.4 million representing USAir's 55.5 percent equity share in the earnings of Piedmont Aviation. USAir has agreed to acquire Piedmont and is awaiting regulatory approval.

USAir Group net income for the first six months of 1987 was $96.2 million ($3.13) compared with net income of $25.4 million (93 cents) for the same period of 1986.

For the first half of 1987, the company had operating income of $127.7 million on revenue of $1.08 billion, up from operating income of $35.3 million on revenue of $865.5 million for the first half of 1986.

"Results for the second quarter reflect strong traffic at USAir as well as Piedmont and Pacific Southwest Airlines," said USAir chairman and president Edwin Colodny. "We expect the third-quarter results to reflect a continuing strong travel season through Labor Day."

USAir Group includes USAir, Pacific Southwest, Pennsylvania Airlines, Suburban Airlines and USAir Leasing and Services as wholly owned subsidiaries.

Acquisition of PSA was completed in late May.

Potomac Electric Power Co. reported second-quarter earnings of $42.2 million (42 cents), a 32.4 percent decrease from the $62.4 million (63 cents) from the same period a year ago. However, the 1986 results were inflated by a $21.7 million one-time gain from the sale of the company's Virginia service area to Virginia Electric and Power Co.

For the first six months of 1987, the company reported net income of $80.7 million (81 cents), compared with $103.1 million ($1.03) in the first six months of 1986.

The 1986 per-share results were restated to reflect a 2-for-1 stock split effective May 4