USAir Group Inc. yesterday reported second-quarter net income of $29.6 million ($1 a share) on revenue of $458.5 million, a 4.7 percent decline from a year ago for the parent company of USAir.

During the second quarter of 1985, the company had net income of $49.5 million ($1.64) on revenue of $481 million.

Net income for that quarter was the best for a single quarter in the company's history and resulted partially from a strike against United Airlines.

For the first six months of 1986, USAir Group's net income was $20.6 million (78 cents) on revenue of $865.5, down from $59.5 million ($2.08) on revenue of $875.5 million for the first half a year ago.

"USAir's cost per available seat mile declined by 9.5 percent, partially offsetting a drop in yield per revenue passenger mile flown earned revenue of 8.8 percent for the quarter," said Edwin Colodny, the firm's chairman and president. "Major factors contributing to the drop in cost were a significant reduction in fuel price and increased use of the fuel-efficient Boeing 737-300 aircraft," he said. Fairchild Industries Inc. reported profits of $3.8 million (5 cents a share) on revenue of $242 million for the second quarter ended June 29, compared with losses of $82.3 million on revenue of $188.1 million for the same period last year. For the six months, the Chantilly, Va., aerospace and communications company reported profits of $11 million (35 cents) on revenue of $464.2 million, compared with a loss of $93.4 million on revenue of $364 million for the same period a year ago.

The negative results reflected a year ago were due to cost overruns in Fairchild's T46A jet trainer program for the Air Force and the Saab SF340 commuter aircraft venture between Fairchild and Saab-Scania AB. Fairchild, now a subcontractor for the venture, is expected to conclude its participation in early 1987.

Fairchild said the 1986 six-month results were due to brisk sales in its aerospace fasteners and communications and electronics products businesses. Fairchild Republic Co., the Long Island, N.Y., division that makes the T46A and also makes assemblies for the Boeing 747 and the C5B, the Air Force transport, had sharply increased sales, while Fairchild Aircraft Corp., a commercial aircraft division based in Texas, continued to be profitable.

But, the company added $20 million to reserves on the T46A trainer-aircraft program, resulting in a loss from continuing operations of $9.3 million before income taxes and the reversal of a tax credit taken in the first quarter. Dynalectron Corp. of McLean reported that profits nearly doubled in the second quarter, rising from $1.3 million (13 cents a share) in 1985 to $2.5 million (24 cents) for the three months ended June 26.

For the year to date, net earnings from continuing operations totaled $3.5 million (35 cents), up from $1.7 million (18 cents) for the first half of 1985.

Revenue from continuing operations rose 21 percent in the second quarter, from $153.5 million to $186.3 million. Revenues for the first half of the year totaled $348.2 million, up 18 percent from $295.1 million for the first six months of 1985.

The company said the improved earnings resulted from a turnaround in the company's U.S. electrical contracting business.

Electrical contracting is part of Dynalectron's Specialty Contracting Segment, which contributed $1.3 million to operating income in the first half of 1986.