Airline balance sheets don't paint a very pretty picture these days unless you like red.

In 1981 and the first quarter of 1982, losses piled up to produce one of the industry's worst financial periods in history. Even for a business whose results traditionally have fluctuated widely, the losses have been staggering. But airline officials and analysts expect improvement in the bottom line.

Braniff International's shutdown came last week when it ran out of cash--a predicament that experts say other carriers aren't approaching. But other airlines are in financial circumstances that are the reverse of 1978, the year the Airline Deregulation Act was passed and the industry posted record profits.

Earlier this year, auditors for several airlines--Continental, Republic, Western and World, as well as Braniff--qualified their financial statements by noting that they "may be unable to continue in existence."

The statement was included in the airlines' reports to shareholders primarily because of restrictive loan covenants that put some of their debt in technical default; each is continuing to try to restructure its repayment schedule.

Braniff suffered one of the biggest losses in 1981, $160.6 million. Pan American World Airways' net loss was reduced to $18.9 million only because proceeds of the sale of its hotel subsidiary offset a $364 million operating loss for the airline. The year before, it sold the Pan Am Building in New York.

United Airlines, the nation's largest domestic carrier, had an operating loss of $148.8 million last year and a net loss of $104.4 million. Its losses were offset somewhat by profits from a hotel subsidiary, enabling the parent company, UAL Inc., to report a net loss of only $70.5 million. Eastern Airlines lost $65.9 million and Continental lost $60.4 million.

There were exceptions: Delta Airlines, always the industry's moneymaker, earned $91.6 million; USAir reported a $51.1 million profit and American Airlines had earnings of $47.4 million. Other carriers in the black last year were Piedmont, Frontier, Ozark, and Northwest airlines.

Some general factors have affected the industry's recent poor performance: the recession, which caused a downturn in passenger traffic; continuing record interest rates; high--though moderating--jet fuel prices; the restraints imposed on flights as a result of the strike and firings of the air traffic controllers, and discount-fare wars.. Some of the carriers have blamed part of their trouble on the new entrants whose low-fare competition was spawned by deregulation.

The first quarter worsened as passenger traffic continued to decline, harsh winter weather set in, and airlines stepped up fare reductions to attract passengers to keep cash flowing in. Because of the strike-imposed reduction of landing rights at major airports, some airline officials admit they kept half-empty planes flying just to hold onto their airport slots--for a hoped-for summer travel upturn--although they would have put them on the ground in normal circumstances when traffic dried up.

As a result, the first-quarter results were poor even for airlines that were profitable last year, and Pan Am, TWA and United each reported operating losses that surpassed the $100 million mark.

Besides the industry-wide problems, some airlines faced individual obstacles:

* The traditionally profitable Braniff's problems stemmed from a bold route expansion in the late 1970s, just as the economy soured and fuel prices continued to soar. The company didn't retreat fast enough.

* One of Pan Am's problems was the high price it paid to acquire National Airlines and the difficulty it had digesting National's employes and domestic routes.

* Republic Airlines, an amalgam of the former North Central Airlines, Southern Airways and Hughes Air West, is carrying an enormous debt load incurred in its growth. The airline, which had an operating profit in three quarters last year, still ended with a net loss of $46.2 million for the year. The reason was the $108.4 million in net interest payments on its debt.

* Western dallied around after deregulation, waiting for a merger to solve its problems and give it an identity. Western's intended merger partner, Continental, a high-cost airline, let its business flounder while it fought an expensive and ill-fated battle to avoid a takeover by Texas Air Corp.

Despite the industry's problems, few think a bankruptcy of another airline is imminent or even likely in the short term. The peak summer travel season is about to begin, and airline officials and analysts already have been heartened by a significant upturn in passenger traffic that began in March. Many airlines reported that traffic in April showed double-digit increases over April 1981.

Citing the stronger traffic, improving fare yields, lower fuel costs, and an increase in the air traffic system, Mike Derchin, an industry analyst for First Boston Corp., said recently the second quarter is "shaping up to be much improved" over the recent past. Julius Maldutis, an industry analyst for Salomon Brothers, also sees "significant improvement" in the current quarter. "The critical issue will be the economic recovery--and therefore traffic growth," he said.

Many of the airlines also have won wage reductions and freezes and work-rule changes from their union employes that have cut their costs, which bodes well for their profit margins if a traffic increase is sustained.

The demise of Braniff and Laker Airways earlier this year is expected to help the other airlines, allowing them to fill their seats with passengers who might have flown those two carriers. Deregulation gave airlines the flexibility to begin new domestic routes, so existing carriers probably will be able to take up the slack caused by Braniff's bankruptcy.

Also, the disappearance of Braniff and Laker--whose regular prices were lower than their competitors--is expected to result in higher fares on their routes. In Braniff's attempt to survive, it had lowered coach fares by almost 50 percent, and its major competitors, including American Airlines, grudgingly matched them.

The immediate increase in the stock prices of many of Braniff's competitors was the financial market's vote of confidence for the industry in general, one industry observer said.

He suggested that Braniff's failure--and the jet aircraft it puts out for sale in an already glutted market--may keep other airlines' lenders from wanting to call their loans and put even more aircraft on the market. Although some have suggested lenders will be more cautious toward the industry, carriers already have been having difficulty attracting capital.

Some have pointed to the Braniff bankruptcy and the industry's difficulties as a failure of airline deregulation, but that view gets little support in Washington or among airline industry officials, including Braniff's Chairman Howard D. Putnam. "Deregulation gave us the opportunity to try to make it," he said recently.

Echoing that view, Civil Aeronautics Board Chairman Dan McKinnon said there is no reason airlines should be considered different from other businesses. "Some of the airlines are not very successful right now, but we are in a recession," he told a press conference last week. "In this recession, the savings and loan industry is having trouble, the housing industry is having trouble, the lumber people out in the Northwest are having trouble, the agriculture people are having trouble, the steel industry is troubled, the automobile industry has economic troubles.

"I can't understand why the airline industry should be exempt from having a few problems if the whole economy is having troubles as well," McKinnon said.