"Now as always," a large newspaper advertisement proclaimed in major cities yesterday, "Delta is ready when you are." Another said "United Airlines is still flying. So you can keep flying, too."

The ads are designed to counteract news about the air traffic controllers' strike and to convince prospective travelers that they are likely to reach their destinations safely and with little difficulty.

Yesterday, the fifth day of the strike, the airlines continued to operate about 75 percent of their normal schedules and reported generally that traffic has begun picking up.

However, the nation's 300 commuter airlines, which operate a large portion of the shorter-haul routes nationwide, reported that passengers continued to stay away in large numbers. "It's possible some of the smaller carriers could be forced out of business," said Duane H. Ekedahl, president of the Commuter Airline Association of America.

"We're suffering," said a spokesman for Ransome Airlines, a Philadelphia-based commuter associated with USAir. "We're not getting passengers; some planes are taking off with nobody on them."

The commuter lines also are operating about three-quarters of their normal flights. Some industry observers suggested that the traveling public may not understand that the original and much-publicized government strike plan that would have grounded all flights under 500 miles was not put in effect by the Federal Aviation Administration.

Even the so-called "50 percent" plan in effect for the nation's major airports is a misnomer in practical terms. While the FAA asked the airlines to cut their schedules by 50 percent, they also permitted them to ask FAA approval for additional flights. Approval is granted if the flights can be accomodated safely.

So far, the airlines have selected their flight schedules, with no government intervention, according to airline officials. "We cancel those flights that have the fewest number of people on them, those with the least demand," said David C. Frailey, American Airlines' vice president for public affairs.

On Tuesday, the airlines are to meet with government officials to discuss how to set operating schedules through the winter.

Failure of passengers to appear for flights cost as much as $3 million a day for some airlines, although some lost revenue will be offset by reduced costs.

On reduced schedules, airlines will save some money on fuel (which can account for one-third of operating costs), food and liquor, landing fees and travel agent commissions, according to Jon Ash, a vice president of Trans World Airlines Inc.

"Assuming passengers come back, I think the airlines are likely to do well," said Charles Barclay, Senate aviation subcommittee counsel. Noting that many airlines had been flying with too many empty seats, he suggested that some lines will lay off some employes and ground fuel-inefficient planes, "making profits on lower revenues."

Airlines, scaling back services even before the strike, have accelerated or introduced other cost-savings moves.

For instance, Pan American World Airways yesterday began seeking support from its employes for a 10 percent pay cut effective Sept. 1, with wages and salaries frozen until the end of 1982.

Eastern Airlines said 39 executives will take a 10 percent pay cut effective Aug. 16, and Braniff International laid off 1,500 employes.