USAir Inc., formerly Allegheny Airlines, yesterday reported a banner year in 1979 and announced plans for a $19 million expansion of its maintenance center at Pittsburgh's international airport.

Profits totaled $33.4 million last year compared with operating profits of $22.45 million the previous year as revenues increased to $729 million from $567 million. Earnings per share were $2.64 compared with $2.75 -- a reduction that reflects an increase to 11.3 million from 7.5 million in shares outstanding.

In 1978, a one-time tax credit of $9.85 million had resulted in net income of $32.3 million. Moreover, the 1978 earnings included gains from equipment sales of $11.37 million compared with similar gains of $2.4 million last year, indicating an even more substantial gain in profits from airline operations.

In the fourth quarter, USAir earned $9.2 million compared with operating earnings of $3.7 million a year earlier as revenue rose to $206 million from $146 million.

"Our results for the quarter and year, while somewhat counter to industry trends, reflect a policy of controlled growth to several important cities in the first year under [airline] deregulation, combined with the positive impact of improved financial structure, fleet modernization, and effective marketing programs designed to compete at compensatory fare levels," said USAir Chairman Edwin Colodny.

The expansion at Pittsburgh, where Allegheny traffic increased 24 percent last year to 2 1/2 million passengers, will include a 112,000-square-foot hangar and a jet engine assembly building. Some 135 new jobs will be created, the Washington-based company said. Overall, USAir carried 14.15 million passengers last year, up 9 percent.