Fourth-ranked National Steel Corp. yesterday reported a 93 percent tumble in second-quarter profits from last year's second quarter.

National earned $3.3 million (18 cents a share) for the quarter on sales of $861.2 million. Last year, earnings were $45.2 million ($2.34) on sales of $1.1 billion.

"The severe decline in steel demand, which reached unprecedented lows in the month of April and improved only slightly in May and June, had severe impact on steel operations, revenues and earnings in the second quarter," said Chairman Howard Love.

Losses in the steel operation were offset by strong performance by National's aluminum and financial subsidiaries, Love said.

First-half earnings increased 67 percent to $110.4 million ($5.81) on sales of $2 billion. During the same period in 1979, earnings were $65.9 million ($3.42) on sales of $2.2 billion.

Second-quarter net profits of Aetna Life & Casualty Co. shrank to $1.65 a share from $1.95 a year ago despite a rise in revenues to $3.27 billion from $2.9 billion.

First-half profits slipped to $3.23 a share from $3.49 a year ago as revenues rose to $6.74 billion from $5.51 billion.

Chairman John F. Filer said it was one of the most difficult quarters in Aetna's history as inflation raised costs of every part of the company's activity and increasing price competition reduced premium income growth. High interest rates and increased policy loans aggravated the problems.

Earnings for the second half of 1980 will continue to lag behind those of 1979 because of delays and production costs, McDonnell Douglas Corp. said yesterday as it announced earnings of $43.7 million ($1.10 a share) on sales of $1.6 billion in the second quarter. The figures compare with earnings of nearly $53 million ($1.34) on sales of $1.3 billion in 1979.

The major aerospace company said first-half earnings, totaled $87 million on $3 billion in sales, down from earnings of $98.3 million on sales of $2.6 billion in the first of 1979.

Development and production costs of the DC9 Super 80, additional development costs of the KC10 tanker-cargo plane (the military version of the DC10 commercial airliner) and a substantial decline in interest income adversely affected earnings, the corporation said.

The corporation had a backlog of $8.8 billion on June 30 compared with $6.5 billion in 1979.