USAir Inc. reported yesterday its first profitable January-March quarter in the company's more than 40 years of business.

Separately, Maryland National Corp. owner of that state's largest bank, said earnings were flat in the first quarter of 1980.

Washington-based USAir earned $5.1 million (32 cents a share) in the recent quarter, compared with a loss of $6.9 million in the same period in 1979. The domestic carrier, formerly Allegheny Airlines, historically has suffered a first-quarter loss because of winter weather over much of its basic northeastern U.S. routes structure.

However, with new routes now stretching virtually across the country coupled with a mild winter, USAir passenger volume soared in 1980. Revenues jumped 50 percent to $211 million.

A company statement pointed to improved yield per passenger mile as well as strong traffic in the Florida and Arizona markets. Revenue passengers increased more than 10 percent to 3.37 million.

Maryland National, parent of Maryland National Bank, said first-quarter operating profits were $6.8 million compared with $6.7 million a year ago, or 94 cents a share in both periods.

Chairman Robert D. H. Harvey said the flat performance reflected an increased loan volume but a narrowing of interest margins on loans. Deposits rose to $2.19 billion from $2.06 billion a year ago as loan volume jumped to $2.28 billion from $1.93 billion.

Ethyl Corp., of Richmond, reported first-quarter profits of $28.5 million ($1.40 a share) on sales of $438.6 million, compared with $21.1 million ($1.03) on $370.6 million in sales for the first quarter of 1979. Both figures were the best for any first quarter in the petroleum chemical company's history, with the increase in net income and earnings per share being assisted by a capital gain from the sale of land in Louisiana.

BDM International Inc. reported a 41 percent increase in profits for 1979, compared with 1978. Profits rose to $2.05 million ($2.46 a share), compared with $1.45 million ($1.87) for 1978.

Sales for the McLean-based professional services firm jumped 36 percent to $65 million in 1979.

Computer Data Systems Inc., a Washington-based consulting and data processing firm, had net income from continuing operations of $447,600 (80 cents) for the nine months ended March 31, compared with $282,600 (51 cents) for the same period last year.

Revenues from continuing operations for the period totalled $10.5 million, an 86 percent increase over the $5.7 million for the first three quarters last year. Income from discontinued operations added another 5 cents a share for the period ended March 31.