American Airlines named a new president yesterday and reported a loss of $34.8 million in the second quarter, in contrast with last year's profit of $95.3 million (3.22 a share).

The big second-quarter profit in 1979 was caused largely by bonanza business resulting from the strike against United Airlines and a large insurance recovery on a lost airliner. A 22 percent cut in scheduled traffic caused by the recession was also a big factor in the 1980 loss, Chairman Albert V. Casey said.

The new president is Robert L. Crandall, who joined American as a financial vice president in 1973 after working for Hallmark Cards, Eastman Kodak and Trans World Airlines. Casey had been acting as both president and chairman since 1974.

Revenues in the second quarter rose 4.6 percent from a year ago to $901.3 million from $861.9 million, but Casey said fuel costs rose 59.7 percent from a year ago and American continued to have to pay more for its fuel than its competitors. Casey said this differential ranged from 1.6 cents to 7.6 cents a gallon.

The loss for the first half was $76.7 million on revenues of $1.747 billion, compared with earnings of $88.4 million ($2.84) a year ago on revenues of $1.54 billion.

The 1980 first-half loss is after a capital gain of 84 cents a share on the sale of various hotel properties. The first-quarter profit in 1979 included a 44-cent-a-share investment tax credit.

Bankers Trust Co. reported second-quarter profits of $62.7 million. Per-share earnings of $5.42 were up from $2.34 a year earlier.

The profits included a $4.1 million gain on the sale of 12 bank branches. Operating profit of $5.05 a share was up 116 percent from a year ago.

First-half net income was $106.9 million. The per-share earnings of $9.17 were up from $4.54 a year earlier. Operating profit for the half was $8.19 a share.

Chairman Alfred Brittain iii said higher interest income, strong securi- ties trading profits, net gains on foreign investments and higher commissions on various activities caused the gains in operating profit. But he said the loan loss provision of $25 million for the quarter was twice that of the 1979 quarter.

Signal Cos. Inc. reported a 34 percent decline in second-quarter net income and attributed the lower earnings to losses in its truck-manufactuing division.

The multi-industry company said income from continuing operations was $35.5 million (92 cents a share), down from $54.1 million ($1.40) in the same 1979 period. Sales were down slightly to $1.5 billion from $1.6 billion.

Signal Chairman Forrest N. Shumway said the company's high-technology divisions had performed well but noted that the Mack Truck division had suffered a net loss of $10.4 million in the second quarter. Mack earned $22.6 million in the second quarter of 1979, Shumway said.

"Mack's earnings are well known to be cyclical in nature," Shumway said. "However, the industrywide decline in demand for heavy-duty trucks has been so severe it resulted in a loss at Mack for the second quarter."

Gannett Co. Inc. reported earnings in the second quarter of 1980, its 51st consecutive improved quarter.

The company said its earnings rose 11 perncent to $40.5 million ($1.13 a share) in the second quarter from $36.5 million ($1.02) in the second quarter of 1979.

First-half net income rose to $67.7 million, and operating revenues increase over the comparable period last year. Earnings per share in the first half were $1.88, up from $1.70 in 1979.