Washington-based USAir Group, the holding company for USAir airline, yesterday reported 1984 earnings of $121.64 million -- the highest net income in the company's 47-year history.

The airline's profits last year were up 50.8 percent over the $80.64 million it made in 1983.

USAir's 1984 earnings translate into primary earnings per share of $4.92, compared with $3.49 per share in the previous year. Fully diluted per share earnings for 1984 were $4.46, compared with $3.22 per share in the year-ago period.

USAir's 1984 results largely stemmed from lower operating costs and increased long-distance flights, said Edwin I. Colodny, USAir chairman and president. "Our cost per available seat mile continued to decline in 1984, reflecting lower fuel prices," Colodny said. He said that fleet modernization and a "higher level of longer-haul flying" helped produce "a break-even load factor of 51.7 percent, USAir's lowest in many years."

* Martin Marietta Corp. yesterday reported 1984 earnings of $176.01 million ($4.72 per share), an 8.9 percent increase over 1983 profits of $161.62 million ($4.89).

Thomas Pownall, chairman of the Bethesda-based aerospace giant, said the company's earnings last year were helped by an expanding high-technology sales market.

"We expect continued strong growth from the technology-intensive segments of our business to more than replace sales of the discontinued aluminum operation over the next two years," Pownall said.

Marietta's net earnings in the fourth quarter totaled $33.3 million (85 cents), compared with $31.8 million (84 cents) in the fourth quarter of 1983.