USAir turned a first-quarter operating loss into a profit increase of 74 percent thanks to the sale of tax leasing benefits.

The Washington-based airline yesterday reported net income of $10.8 million (55 cents per share), up from $6.2 million (37 cents) in the same quarter of 1981. Revenues during the quarter just ended increased by 11 percent to $279.2 million.

Without the sale of leasing benefits on seven DC9s, the carrier would have suffered an operating loss of $4.9 million, primarily because of bad weather. Company officials declined to identify the purchaser of the benefits. This type of sale is permitted under the revised 1981 tax code; however, its use has become so controversial that Congress has threatened to close this loophole.

Other first-quarter profits reported yesterday showed mixed results. Ethyl Corp., which manufactures gasoline additives and other chemicals, experienced a 19 percent decline in earnings, a drop the company's chairman blamed on the general economic malaise. Net income fell to $17.1 million (86 cents) from $21.2 million ($1.06) a year earlier. At the same time, sales decreased by 5 percent to $402.6 million compared with $425.2 million in the first quarter of 1981.

The Student Loan Marketing Association(Sallie Mae) registered a 95 percent jump in earnings and a 61 percent increase in assets. Sallie Mae, a government-chartered stock corporation, provides a secondary market for guaranteed student loans. Net income for the quarter was $7.6 million ($7.60), compared with $3.9 million ($3.89) a year earlier. Total assets rose to $5.6 billion from $3.5 billion.

President Edward A. Fox attributed the gains to the continued demand for loans, which are in turn sold by lenders in the secondary market to investors. The volume of loans to students has increased dramatically in the past two years. Now the administration has warned that it wants to cut back on the growth in such loans.

Easco Corp., a Baltimore producer of hand tools and aluminum shapes, registered a slight increase in sales and operating income; however, its net income declined after inclusion of an extraordinary gain last year. Net income from continuing operations rose by 5 percent to $2.7 million (71 cents) on sales of $101.3 million compared with $2.5 million (75 cents) on sales of $97.6 million the previous year.

However, net income during the first quarter declined by 27 percent from $3.6 million a year earlier. First quarter net income in 1981 was pushed up by a gain of $1.1 million from the disposal of its engineering segment. In January 1982 Easco acquired K-D Manufacturing Co, thus increasing the company's shares by 12 percent.

Richmond, Fredericksburg and Potomac Railroad Co. had earnings of $3.7 million ($10.39) on revenues of $13.2 million, down from $3.9 million ($10.99) on revenues of $13.6 million in the same period last year.

Tonnage handled by the general commodity carrier declined 12 percent below the level moved in the first quarter of 1981, reflecting the weak economy. However, said President John Newbauer, piggyback business remained very strong in that time.

United Services Life Companies realized net earnings of $3.3 million (58 cents) in the first quarter versus $3.1 million (56 cents) last year. Total income, including premiums, investments and variable annuities, was $30.5 million, up from $29.3 million.