After three consecutive years of record losses, the nation's airline industry is making a comeback. But it is weaker than some experts had expected, and is accompanied by trouble at several companies.

"It's a mixed bag," a federal official said, noting such anomalies as a $49.7 million second-quarter operating profit at Pan American World Airways--long considered on the critical list--and a $50.9 million operating loss at Delta Airlines, which for years has enjoyed a reputation as stable, well-managed and consistently profitable.

Adding uncertainty are questions of how well individual carriers will respond to the challenges posed by deregulation, whether they will add more flights and seats than the network can absorb, and the potential drain on profitability of continuing fare wars.

"Indications are that second-quarter airline results are poorer than anticipated," said H. Carl Munson, vice president of strategic planning for the Boeing Commercial Airplane Co., which needs healthy airlines if it is going to sell airplanes.

"The reason, of course, is that yields, while improving on a month-to-month basis, continue to run behind the yields of a year ago. Costs are up with respect to a year ago, so that has negative ramifications," Munson said. Nonetheless, he said, the industry situation is clearly improving and his basic view is one of "guarded optimism."

The Air Transport Association of America (ATA) is predicting an industry profit this year for the first time in four years, but that presumes that a strong third quarter and a minimally damaging fourth quarter will be enough to overcome a record first-quarter loss of $650 million.

History is of little help in forecasting what the industry will do because this is the first economic recovery since the airlines were given the unfettered right to charge what they want, fly where they want, and refuse to fly where they don't want to go.

In years past, analysts looked at a monolithic industry that traditionally followed the economy out of recessions--as soon as the airlines could get fare and service changes through the CAB. Nowadays, airlines can devise their own strategies without checking first with Uncle Sam; thus generalizations about the industry are less likely to apply to individual airlines.

"One of the big differences between now and when the industry was regulated is that cost control has become very important," observed Daniel P. Kaplan, director of the Civil Aeronautics Board's office of economic analysis. "The CAB isn't there to protect the inefficient carriers any more."

Moreover, sharply managed carriers can seek specialized segments of the market and prosper, as People Express Airlines has proven with its no-frills, low-fare approach and despite its primitive reservation and telephone system.

Edwin I. Colodny, chairman of Washington-based USAir, one of the airlines that has performed extremely well with a conservative but consistent growth policy since deregulation, said, "The relative long-term strength of the industry is going to be influenced not only by the present players, but also by how active new entrants become.

"I believe the industry can have some good years ahead of it and I don't see why we're not in the position as the economy stabilizes for carriers to take advantage of it. I believe that the economics of those carriers losing money are going to force them to a conservative posture; they cannot do things or their lenders are going to be on top of them."

Colodny also said that a major question affecting recovery is whether there will be enough passengers to soak up new capacity. Piedmont Airlines has recently based a major expansion at Baltimore-Washington International and United has announced plans for growth at San Francisco International.

Growth is dependent on the Federal Aviation Administration being able to deliver on its commitments to restore full air traffic control services two years after it fired 11,400 striking controllers. So far, the FAA has kept on schedule, despite concerns about safety expressed in some quarters.

Several industry experts interviewed think traffic growth has peaked. "We're expecting the third quarter to come in weak as far as traffic goes . . . " said a West Coast analyst. "We knew it was going to take some time to absorb the new capacity; they may be putting it in a little bit faster than would be prudent."

The airlines traditionally do well in the second and third quarters, buttressed as they are by summer vacation travel. Thus, fares have "firmed for the moment," in the euphemism of one airline executive, which means that competitors do not have to rush out and match Pan Am's $99 ticket to anywhere by tomorrow morning. But it is agreed that big bargains for the careful shopper will reappear in the fall, another factor which makes forecasting dicey.

"We could see some selective brush fire fare wars," said Colodny.

"If everybody's back to $99 fares it makes it a disaster," said Munson.

The $99 fares are gone for the moment, but discount tickets still remain an extraordinarily important factor in the airline profit-and-loss picture. Between 85 and 90 percent of all airline passengers are flying on some kind of a discount, the ATA estimates, and the discounts average 46 percent off full fare. Discounts are no longer the province of just the vacation traveler; business travel offices aggressively search for them as well.

When all the second-quarter results are in, according to preliminary numbers developed by the ATA, industrywide operating profit will be between $80 million and $90 million, down from last year's $123 million second-quarter profit.

But last year's second quarter was not enough to save the industry from its worst annual performance ever, a $733.4 million operating loss on $36.4 billion in revenues.

This year, ATA predicts, the industry will slip back into the black with a profit of at least $200 million and perhaps as much as $800 million. "That's a broad range, but these things are extremely sensitive," said George James, chief ATA economist.

James also is not ecstatic with the return the industry is achieving. "We should be doing between $2 billion and $2.5 billion profit on $40 billion in sales to average the profit margin of the rest of U.S. industry," he said.

Industry experts, granted anonymity, were unanimous in giving American Airlines the highest marks for meeting the challenges of deregulation and fighting through a recession.

"Look at what they started out with," one noted. "A long-haul route structure, a strong presence in the Northeast, and too many jumbo jets," the antithesis of regional carriers such as USAir and Piedmont that have succeeded in deregulation by strengthening hub-and-spoke operations and using smaller planes to connect big cities with little ones.

American gradually shifted its emphasis from the Northeast to Dallas-Fort Worth, where it now has a formidable hub-and-spoke. It has been working to reduce the jumbo component in its fleet. It initiated Super Saver fares, started the frequent flyer programs, and has been aggressively selling its Sabre computer reservations system to travel agents across the nation. Some other airlines feel that the Sabre system and United's competitive Apollo give those industry giants an unfair advantage over other airlines.

Regardless of the reasons, American won the second-quarter sweepstakes with an operating profit of $57.1 million, up from a $17.4 million profit a year earlier.

But that was not as surprising as Pan Am's showing. Its $49.7 million second-quarter operating profit is a remarkable turnaround from a $41.2 million operating loss a year earlier. A Pan Am official worried privately at that time that persistent rumors of bankruptcy were driving the travel agents to book passengers away from the carrier, just as happened to Braniff in the weeks before its bankruptcy.

Then Pan Am began to do the things it had to do; it redesigned its route structure into a domestic-international hub-and-spoke at John F. Kennedy International in New York, raised money, and heavily advertised those $99 fares, making it impossible for travel agents to book around them. Pan Am gets high marks from industry analysts.

Delta was the biggest surprise at the other end of the spectrum. It reported a $50.9 million second-quarter operating loss this year compared with a $34.9 million operating profit in the second quarter of 1982.

When deregulation came, "Delta had what everybody else was trying to create," said one analyst. "But they were a little too big" and received aggressive competition, particularly from Piedmont. Delta has also been unusual in that it has adhered strictly to a no-layoff policy, which has added to its operating costs.

Delta is by no means regarded as threatened. However, that statement cannot be made for some other carriers. Western Airlines is currently high on the analysts' worry list, followed to a lesser extent by Continental, Republic and Eastern, not necessarily in that order. Western lost $20.5 million in the second quarter; Continental lost $14 million; Republic lost $27.5 million and Eastern lost $11.8 million.

Trans World Airlines had been a short-term favorite with Wall Street analysts, then showed a $13.5 million operating loss in the second quarter compared with a $4.6 million profit a year earlier. "At this point we're not sure we understand whatever phenomenom was at work here," said TWA spokesman Jerry Cosley. "We're not at all pleased with these results."

There was unanimity among those interviewed that, somewhere along the way, there will be another airline failure or two, although nobody sees that in the short term.

Franklin K. Willis, deputy assistant secretary for planning and policy analysis at the Transportation Department and DOT's expert on airline economics, said, "the question is whether carriers that are in a more difficult financial situation are going to be able to adapt and improve their situation. My reaction is that, based on how other carriers have done, the opportunity is there. Every carrier stands to benefit as the business cycle improves."

This article is one of a series ap- pearing from time to time, examin- ing the health and outlook of indus- tries coming out of the recession.