Avemco Corp., an aviation insurance and financial services firm, reported record third-quarter and nine-months earnings, resulting largely from low underwriting losses from fewer accidents and more premiums written on larger aircraft.

Net earnings for the quarter ended Sept. 30 were $665,000 (26 cents a share) compared with $109,000 (5 cents) for the same quarter last year. Revenues were $7.9 million compared with $5.9 million in 1980.

For the nine months ended Sept. 30, net earnings were $2.6 million ($1.04) compared with $1.8 million (72 cents) for the first three quarters of 1980. Revenues were $22.9 million compared with $17.6 million.

"The successful year to date can be attributed to continued high premium growth, with aviation losses remaining generally level, plus the benefit of increased income from investments," Avemco Chairman Arnold H. Johnson said.

Atlantic Research Corp. of Alexandria reported third-quarter net income of $964,000 (52 cents), a 130 percent increase from $419,000 (21 cents) for the same period last year. Revenues were $21.6 million compared with $19.5 million.

For the nine months ended Sept. 30, net income was $2.5 million ($1.36) compared with $1.7 million ($1.01) for 1980. Revenues were $67.4 million compared with $55.5 million.

ARC, which develops sophisticated small rockets and gas generators, using solid propellants and is involved in several aerospace-related industries, last year won a major multiple launch rocket system contract from the Army. At the time, company officials expected that program to generate more revenues than any other contract it had.

Ethyl Corp. of Richmond reported third-quarter net income of $20.96 million ($1.05) compared with $19.15 million (95 cents) this time last year.

For the first nine months, net income was $65.1 million ($3.26) compared with $65 million ($3.25) in 1980.