Higher premium rates and unusually good weather across the country, which reduced the number of serious weather-related air accidents, helped boost Avemco Corp. profits to record levels in the first half of 1979.

The Bethesda company, whose major subsidiary is the nation's largest insurer of private aircraft, said profits jumped 66 percent to $1.7 million (67 cents a share) from $1 million (41 cents) in the 1978 period.

The second quarter produced a similar gain, with earnings up 69 percent to a record $1 million (42 cents a share) from $619,000 (25 cents) a year earlier. Results for the recent quarter included capital gains of $133,000 (5 cents a share).

Avemco Insurance Co.'s premiums written in the second quarter totaled $6.7 million, a gain of 32 percent over last year. The firm also is engaged in finance services and product marketing, and Chairman Arnold Johnson said yesterday that all subsidiaries showed "substantial" production gains in the recent period.

Speaking to analysts in New York, Johnson said further that:

- The largest contribution to 1979 profit improvements came from better underwriting results.

- Avemco would not suffer from either the current energy crisis or projected recession.

Because major airlines are not expected to expand services and commuter lines will concentrate on feeding hub airports, use of general aviation insured by Avemco will continue to expand, Johnson forecast.

Ethyl Corp., a Richmond-based manufacturer of petroleum andindustrial chemicals, reported record sales and profits in the second quarter and first six months. Earnings for the recent quarter were $26.3 million ($1.34 a share) compared with $21.3 million ($1.10) a year ago as sales rose to $413 million from $364 million.

Six-month profits rose to $47.5 million ($2.41 a share) from $39 million ($2.01) as sales increased to $784 million from $684 million.

Bank of Virginia Co., a Richmond-based bank holding company, posted second-quarter operating profits of $3.8 million (82 cents a share) compared with $3.2 million (70 cents) last year. Six-month earnings rose to $7.5 million ($1.61) from $6.3 million ($1.37).

Government Services Savings & Loan Inc., of Bethesda, reported a decline in profits for its first fiscal quarter ended June 30. Net income was $295,739 (35 cents a share) compared with $655,180 (77 cents) a year ago.Savings deposits were up slightly to $273 million from $270 million in mid-1978. Directors voted a dividend of 12 cents a share, payable Aug. 15 to owners of record July 25.

Atlantic Research Corp., of Alexandria, reported a 17 percent increase in six-month profits to $734,000 from $625,000 a year earlier. Per-share earnings were 63 cents vs. 62 cents, reflecting a greater number of shares now outstanding after a recent public offering. Sales rose to $25 million from $21 million, and the backlog of orders now totals $48 million compared with $26 million last year.