Layoffs in the airline industry began to mount yesterday as carriers sought to trim expenses to match their lighter strike-induced flight schedules.

Following on the heels of Braniff International's layoff of at least 1,500 employes last week, American Airlines yesterday notified its unions that 1,650 of its 36,000 employes will be furloughed or asked to take voluntary leaves of absence over the next month. On Thursday, Washington-based USAir said it had laid off 700 of its employes.

Meanwhile, in an effort to alleviate strike-related cash-flow problems of commuter airlines, the Civil Aeronautics Board eased its flight requirements but said it would pay subsidies at the maximum ceiling amount for the next month. The payment will be adjusted later, based on actual service provided, the board said.

American Airlines spokesman Joe Moran said 850 ground workers and 200 pilots would be laid off and that 600 flight attendants would be asked to take voluntary leaves or be furloughed. Leaves allow an employe's seniority to continue to accrue, he said. The 1,650 includes some layoffs that always come at the end of the peak vacation season, he said.

Moran said the number of layoffs is not firm but could change in a few days. "We don't know yet how many there will be because we don't know how big an airline we're going to operate until the FAA tells us," he said.

The schedules of American and the other airlines basically are set through Sept. 8; American now is operating at 72 percent of its normal capacity. Within the next two weeks, the Federal Aviation Administration is expected to tell the airlines what they can operate through the winter.

USAir said it laid off about 700 of its 11,000 employes from all job categories -- pilots, flight attendants, customer service agents and clerical help. Because of the controllers' strike, the airline is operating about 75 percent of its normal seat capacity, according to David H. Shipley, a USAir vice president.

More than half the USAir employes laid off were part-time and temporary workers; the rest were the most-recently-hired employes who had not completed their probation, six months or a year depending on the job category, Shipley said. "Our basic work force of full-time employes is still very much intact, and we do not anticipate any additional separations," he added.

Trans World Airlines has told 200 flight attendants they will be furloughed effective Sept. 3. However, TWA spokesman David Venz said the layoffs were unrelated to the controllers' strike. The attendants were hired for the heavy vacation season and would have been terminated after Labor Day in any event, he said. TWA has 5,600 remaining attendants.

However, TWA is looking at potential manpower cutbacks that would be more related to the reduced schedule caused by the strike, Venz said.

Commuter airlines, including Mississippi Valley Airlines and Air Wisconsin, also have announced some layoffs.

Besides labor cutbacks, airlines are looking for other ways to cut their costs for the duration of the strike. For instance, Republic Airlines employes agreed to take part of their pay in stock, at least through August. Under the plan, unionized employes and management will take common stock in place of 15 percent of their salary for time worked this month. The program will reduce Republic's cash outlay by an estimated $6 million, the Minneapolis-based airline said.

Eastern Airlines, one of the largest employers in the industry with 39,500 employes, hasn't determined a course of action yet, according to spokesman Jim Ashlock. He said Frank Borman, Eastern's chairman, has asked the unions for their ideas on how to cut costs, including the possibility of instituting salary adjustments. Eastern's 39 top executives agreed to a 10 percent pay cut last week.

Eastern, like many other airlines, is waiting to see what flying levels will be permitted after Sept. 8, Ashlock said. The Miami-based airline usually lays off between 1,200 and 1,400 employes around Labor Day to accommodate the 5 percent to 7 percent cutback in operations after the heavy summer season. If the airline's operations are cut back 15 percent to 20 percent because of the strike, "We'll probably have to go beyond that seasonal adjustment," Ashlock said. "The question mark is how much and how might you minimize it by some other action."

"It's like any other high-volume business," Ashlock said. "If you have a big hotel, a lot of rooms and a lot of business, you need more employes. If you close off one wing, you don't need so many people."