Profits of USAir Inc., one of the main beneficiaries to date of airline industry deregulation, took off last year and never stopped climbing.

For 1980, profits were up 81 percent to a record $60.4 million ($4.71 a share) compared with $33.4 million ($2.64) a year earlier, the previous record.Revenues increased 33 percent to $972 million, while operating expenses were up 30 percent to $880 million for an enterprise that has become one of th fastest-growing corporations based here.

As recently as 1975, USAir (then named Allegheny) had a net loss of $10.6 million on revenues of $377 million.

In the fourth quarter alone, profits of the airline were double those of the 1979 period: $18.3 million ($1.42) vs. $9.2 million (69 cents) as revenues rose to $267 million from $206 million.

Chairman Edwin I. Colodny, who earlier this week shook up his industry with a speech that suggested closing National Airport after a specific program aimed at promoting Dulles International as the primary replacement, said yesterday that the 1980 results are "especially gratifying in light of the general industry downturn."

USAir now has recorded profit gains over year-earlier periods for six consecutive quarters, partly because of new routes launched in the absence of federal regulations over such decisions.

Colodny also announced yesterday that USAir plans to offer 1.5 million new shares of common stock to the public, probably in mid-February. A Wall Street syndicate headed by Lehman Brothers Kuhn Loeb Inc. will handle the stock sale, for which a registration statement was filed yesterday at the Securities and Exchange Commission. The company has 12.3 million shares currently outstanding.

USAir revenue passenger miles rose 8 percent last year to 5.58 billion although the number of passengers carried was up just 1 percent to 14.3 million.

U.S. Fidelity & Guaranty of Baltimore reported that operating earnings before capital gains or losses in the fourth quarter fell to $45.5 million ($1.62 a share) from $65.2 million ($2.34) a year earlier. For the year, operating earnings of the diversified insurance firm were little changed at $228.5 million ($8.81) vs. $229 million ($8.27). Including capital loss of $5.4 million in 1980 and gains of $89,000 in 1979, final net income was $223 million ($7.99) vs. $229 million ($8.28).

Earnings reports from regional banks continued to show general fourth-quarter gains as well as higher profits for the full year at most institutions, but there were exceptions. Among the financial institutions:

Mercantile Bankshares Corp. -- a Baltimore holding company that owns Mercantile-Safe Deposit & Trust, Potomac Valley Bank and Bank of Southern Maryland among other institutions -- reported that operating earnings last year rose 13 percent to $16 million ($2.86 a share) from $14 million ($2.53) in 1979. Fourth-quarter operating profits rose to $4.3 million (78 cents) from $3.6 million (64 cents). Average deposits rose 5 percent to $984 million.

Union Trust Bancorp of Baltimore said, in contrast, that fourth-quarter net income fell to $1.8 million (73 cents a share) from $2.1 million a year ago; no per-share figure or operating earnings data were published. For the full year, net income declined to $7.3 million ($2.97) from $8.8 million ($3.61). The holding company for Union Trust Co. attributed the declines to the cost of funds to support such assets as consumer loans and fixed-rate mortgages, as deposits rose 5 percent to $949 million and loans expanded 3 percent to $792 million.

Central Fidelity Banks Inc. of Richmond reported that income before securities transactions for the fourth quarter was $4 million (74 cents a share) compared with $3.8 million $69 cents) for the same quarter in 1979. Income before securities transactions for 1980 was $15.8 million ($2.88) compared with $15.4 million ($2.77) in 1979. Directors increased the quarter dividend on each share of common stock from 25 cents to 28 cents.

Equitable Bancorporation of Baltimore said fourth-quarter operating profits rose to $2.9 million (74 cents a share) from $2.7 million (68 cents). For the year, operating earnings fell to $9.6 million ($2.43) from $14.2 million ($3.56) because of higher reserves for possible loan losses and the higher cost of funds. Directors approved a regular quarterly dividend of 19 cents a share, payable April 1 to owners of March 13.

Bank of Columbia said earnings increased last year to $462,558 ($1.05 a share) from $381,387 (87 cents), a gain of 21 percent. Year-end assets of the Washington bank were $49 million.