USAir reported yesterday that 1981 was the second best earnings year in the company's history despite a drop in profits for both the fourth quarter and the year.

For 1981, the Washington-based airline reported net earnings of $51.1 million ($3.13 a share), a 15 percent decline from 1980's record earnings of $60.4 million ($4.71). Revenues for the year totaled $1.1 billion, up 14 percent from $971.8 million in 1980. Last year was the first time USAir's revenues surpassed the $1 billion mark.

"Our 1981 results are gratifying in light of the recessionary economy and the negative impact of the Patco work stoppage," USAir Chairman and President Edwin I. Colodny said yesterday.

USAir's fourth-quarter net earnings totaled $12.6 million (73 cents), a 31 percent decline from $18.3 million ($1.42) in the fourth quarter of 1980. Revenues totaled $294.3 million in the fourth quarter, an increase of 10 percent over the $267.3 million in revenues in the same period of 1980.

USAir's revenue passenger miles--one revenue paying customer flown one mile--fell 1.1 percent last year; the number of passengers carried fell 5.6 percent to 13.5 million.

The airline said that 1981 results include an after-tax gain of $1.9 million on the sale of tax benefits in the fourth quarter. Cash proceeds from the sale of the tax benefits amounted to $8.4 million.

Per-share earnings in 1981 were computed on 15.8 million average shares of common stock outstanding, 3.7 million more than in 1980.

Separately, other Washington-area companies reported earnings:

Nuclear Support Services Inc. of Woodbridge, which provides technical services to the nuclear utility industry, reported net income of $181,880 (18 1/2 cents a share) for the three months ended Dec. 31, 1981, an increase of 46 percent from $123,984 (13 cents) for the same quarter of 1980.

The company, which had its initial public offering of 350,000 shares last month, reported that revenues in the October-December period increased 32 percent to $5.4 million from $4 million in the same period of 1980.

Ryland Group Inc., a Columbia-based home builder, reported earnings of $3.02 million (99 cents a share), compared with earnings of $6.57 million ($2.12). Sales were $230.88 million, down 18 percent from $258.04 million a year before.

Fourth-quarter earnings were $565,000 (19 cents), compared with $2.16 million (69 cents) for the fourth quarter of 1980. Sales were $53.74 million, down from $85.22 million.

Ethyl Corp., a Richmond-based producer of performance chemicals, posted earnings of $90.89 million ($4.56 a share) on sales of $1.75 billion for 1981, compared with earnings of $89.68 million ($4.49) on sales of $1.74 billion in 1980.

The fourth quarter produced net income of $23.85 million ($1.20) on sales of $430.33 million. That was down slightly from earnings of $24.68 million ($1.24) on sales of $431.59 million in the fourth quarter of 1980.