Crown Life Insurance Co., a Canadian firm with its U.S. headquarters in Alexandria, reported second-quarter earnings in U.S. dollars of $16.3 million ($7.05 a share) on revenue of $421.7 million, compared with earnings of $8.2 million ($4.12) on revenue of $291.6 million for the same quarter last year.

For the first six months of the year, the international life insurer earned $28.4 million ($11.98), up substantially from $9.1 million ($4.58) for the first six months of 1984. Revenue rose 22 percent to $818.7 million during the same period.

The company said that income from operations increased by 83 percent, from $23.3 million to $42.6 million for the last six months. Crown Life Insurance earns about 65 percent of its premium revenue in the United States.

* Allied Research Associates Inc. reported a huge leap in net income and sales for its second quarter. Profits grew 325 percent, to $1 million (26 cents a share) from $306,700 for the same quarter last year, while sales increased 364 percent, to $12.8 million from $3.6 million.

For the first six months of the year, the company earned $1.9 million (49 cents) on sales of $21.9 million, compared with net income of $597,000 (15 cents) on sales of $6 million for the same period last year.

The company, based in Severna Park, Md., attributed the growth in sales and profits to the increased demand for infantry weapon systems produced by MECAR S.A., a Belgian subsidiary. MECAR develops and manufactures weapon systems and ammunition.

* At Essex Corp., an Alexandria-based professional services consulting firm, profits fell to $53,000 (4 cents a share) for the second quarter this year from $132,000 (11 cents) in the same quarter last year. Revenue for the second quarter totaled $6.4 million, up from $5.4 million for the same 1984 period.

For the first six months of the year, the company reported net income of $39,000 (3 cents) on revenue of $11.6 million, compared with net income of $285,000 (23 cents) for the same period last year. The earnings per share were computed on a slightly lower average number of shares outstanding, currently 1,240,224, compared with 1,247,241 a year ago.

The company attributed the low income for the last six months to start-up losses in its office systems and personnel placement subsidiaries and "substantial increases in administrative and legal costs associated with an internal examination of our contract cost charging procedures." Essex also said that it soon expects growth in its anti-terrorism training business.

* Revenue increased while earnings declined at QuesTech Inc. for its second quarter. Net earnings for the second quarter were $77,000 (5 cents a share) on revenue of $10.4 million, compared with net income of $295,000 (21 cents) on revenue of $7.7 million for the same quarter last year. The earnings per share reflect the dilution from the company's public stock offering in May 1984.

QuesTech, based in McLean, provides professional, scientific and engineering services in advanced technologies primarily to clients in the fields of national security, aeronautics, energy and industrial modernization.