Continuing a trend that has been sustained for 10 successive quarters, Fairchild Industries Inc. reported a sharp increase in profits for the July-September period this year.

The aerospace and communications firm, based in Germantown, posted earnings of $5.9 million ($1.23 a share) in the recent period compared with $3.1 million (66 cents) in the same quarter last year. Sales rose to $133 million from $103 million, Fairchild said yesterday.

For the first nine months of the year, Fairchild's profits more than doubled to $17 million ($3.61) from $6 million ($1.29), and sales soared to $383 million from $287 million.

The major factor propelling Fairchild's profits upward is a contract with the Air Force for A-10 close-air-support fighters, for which production has been increased. In the third quarter, Fairchild delivered 24 of the planes, bringing the 1978 total to 64. In 1977, the firm delivered 56 A-10s.

In addition, company officials cited higher sales of commercial aircraft at the Swearingen Aviation subsidiary and a substantial reduction in satellite lease costs at American Satellite Corp., a Fairchild domestic communications firm.

Earnings for the nine months of 1978 also included $1.4 million ($29 cents) from the exchange of $875 million of new debentures for an existing series.

Union Trust Bancorp and First Maryland Bancorp, two Baltimore-based bank holding companies, reported that third-quarter profits rose by one-third.

Earnings of Union Trust were up 33 percent to $1.9 million (79 cents a share) from $1.45 million (59 cents) in the same period a year ago, not counting securities transactions. Nine-month earnings were $6.2 million ($2.53) vs. $5.2 million ($2.13), an increase of 19 percent.

Chairman J. Stevenson Peck yesterday said increased interest and non-interest income and controls over non-interest expenses were primary factors in the improved earnings. During the past year, Union Trust assests rose 6 percent to $1.2 billion, deposits were up 5 percent to $892 million and loan volume increased 5 percent to $718 million.

First Maryland, the parent firm of First National Bank of Maryland, reported third-quarter profits of $4.2 million ($1.13 a share) compared with $3 million (84 cents) a year earlier, a gain of 34.5 percent. Nine-month earnings, also not counting gains or losses from sales of securities, totaled $11 million ($3.01) vs. $8.7 million ($2.41).

Assets on Sept. 30 were $1.7 billion, an increase of 7 percent over the same date last year. Deposits increased 8 percent to $1.3 billion, and loan volume grew by 12 percent to $1.1 billion.