When Harris McMann of Fairfax took his last Federal Aviation Agency Flight physical, he passed easily and was certified fit to fly commercial aircraft.

A month later, United Airlines told McMann his 29-year career was over. Not only couldn't he handle the cockpit, he couldn't keep the desk job he'd held for the past 12 years.

Between the two evaluations, McMann turned 60. Under United's pension plan, that was the mandatory retirement age.

"Age discrimination," McMann fumed, and he called a lawyer. That was four years, one union grievance and two court decisions ago.

Now McMann's contention that discrimination based on age, like discrimination based on race or sex, is going to the United States Supreme Court. This winter or next spring, the high court will try to tackle the legal question of mandatory retirement in private employment.

Simultaneously, Congress will come to grips with other aspects of required retirement, which the Bureau of Labor Stastics estimates affects 31 million Americans.

For the Supreme Court, the question is whether United's retirement policy violated the Age Discrimination in Employment Act. The court also could decide the broader question of whether mandatory retirement laws in general deny older Americans their constitutional rights to due process and equal protection of the law.

For Congress, the issues are more complex. Despites extensive federal hearings and academic research, some of the most important questions about mandatory retirement remain unanswered.

What impact does forcing workers to retire have on their physical and mental health? How much does mandatory retirement cost the Social Security system? To what extent doest the inevitability of mandatory retirement lead workers to choose early retirement? And what should Congress do about it?

These questions were raised - if not totally answered - in hearings before the House Committee on Aging, whose chairman is Rep. Claude Pepper, (D-Fla).Pepper, who is 76, frequently notes that he has been re-elected to Congress by voters, many of whom are considered too old to work.

Before the August recess, the House Education and Labor Committee sent to the floor a bill by Pepper (H.R. 5383) which would begin to restrict mandatory retirement.

The bill would eliminate mandatory retirement based solely on age for federal employees. Civil Service workers now generally must retire at age 70 after 15 years of service,most Foreign Service employes must retire at 60, and the maximum age for federal law enforcement agents is 55.

Pepper also proposes extending protection of the Age Discrimination in Employment Act to persons up to 70 years old. Currently the law bans age bias against persons between the ages of 40 and 65. Finally the bill would ban mandadatory retirement by private employers before age 70.

The later proposals are regarded as merely redefining ageism by some lawmakers. They contend Congress should totally prohibit any age discrimination including mandatory retirement. Such sweeping measures are unlikely to be considered before the Supreme Court hears the McMann case.

One reason mandatory retirement is under such fire is the financial straits of the Social Security system, which paid out $3.2 billion more in benefits than it collected in taxes last year and which is expected to run up a $4.5 billion deficit this year.

Commerce Secretary Juanita Kreps, whose responsibilities include Social Security, suggested the problems of Social Security deficits and mandatory retirement might be tackled jointly by raising the minimum age for collecting full Social Security benefits from 65 to 68. When Pepper and other old-people activists howled in protest, Kreps backed off.

A Social Security system spokesman said no studies have been done of how much private mandatory retirement rules cost the system, and how much might be saved if required retirement were banned or the age raised.

One member of the Aging Committee, Rep. Mairo Biaggi (D-N.Y.), estimated that 400,000 workers are retired solely because of mandatory retirement rules and that 60,000 workers are added to that list each year. More than half of them, 34,000 workers a year, would continue working if they could, Biaggi said. If these workers were paying Social Security taxes rather than collecting benefits, the system soon would save $600 million a year, he contends.

Other estimates of the number of involuntarily retired workers vary. Pepper's committee quotes Dr. James Schultz, a Brandeis University economist, who surveyed a group of retired men. He found 54 per cent of them worked under mandatory retirement rules, and 14 per cent said they had been unwilling to retire. While some were unable to work for health reasons, 10 per cent could have continued to hold a job. But less than a third of them were able to find a new job, leaving 7 per cent unable to keep working.

Schultz estimates that making these workers quit cost the economy three-tenths of 1 per cent of the gross national product, or more than $4.5 billion last year.

The Aging Committee quotes the American Medical Association and University of North Carolina researcher Dr. Susan Haynes as claiming mandatory retirement is detrimental to the health and even the life expectancy of those who want to keep working. In an interview, Dr. Haynes, now a government researcher in Washington, said that while her study alone is not evidence that forcing workers to retire harms their health, it does support prevailing theories about retirement.

The theory is that retired workers first go through a "honeymoon" phase in which they revel in idleness, take vacations and generally enjoy themselves, but become dissatisfied two to three years later.

Studying retired workers from two Akron, Ohio, tire factories with mandatory retirement programs, Haynes found that three years after the workers were forced to quit, their mortality rates jumped upward. In one of the plants, the change was too small to be statistically significant, but in the other the death rate was 30 per cent higher than it should have been.

Dr. Haynes said her studies of mortality and health provided no evidence to support forcing workers to quit at age 65. That age apparently was picked when Social Security was started because it approximated that life expectancy of an American during the 1930s.

Longer life expectancy is believed to be one cause for a trend that seems to run counter to objections to mandatory retirement, but which may well be related to it - early retirement.

Early retirement is both a cause and an effect of the Social Security system's problems. Early retirement increases the number of persons collecting Social Security and is encouraged by the availabity of those benefits.

Since 1956, when women were allowed to collect reduced Social Security benefits at age 62, and 1961 when the same option was given men, the number of people quitting work early has climbed steadily. Today almost 65 per cent of Social Security recipients are persons retired early and 70 per cent of the new Social Security claims last year were for early retires.

One reason many workers chose to quit before they reach the usual mandatory retirement age is believed to be a desire to avoid the feeling of rejection that comes when a worker is told he or she no longer is fit to hold a job because of age.

It was just such frustration that led Harris McMann to take United Airlines to court over his retirement, the long time pilot recalled in an interview.

"I felt good. I enjoyed my job. I wanted to keep working," McMann said. "There wasn't any reason why I couldn't keep working except for the retirement policy.

"I was physically and mentally able to fly," said the Fairfax pilot, who soloed in 1934 and flew for a living for three decades before taking a management job with United.

"After 34 year, the aviation business was all I knew, all I was interested in and I wanted to continue."

McMann protested repeatedly about his forced retirement, and was told he'd agreed to quit at 60 when he signed up for the airline's pension plan in 1964. The Age Discrimination in Employement Act's ban on discriminating against persons up to 65 didn't apply, United contended, because a provision of the law specifically says it is not meant to interfere with any private pension plan.

That provision, McMann and Rep. Pepper agree, was written into the law not to allow pensions to be used as an excuse for forced retirement, but to do the opposite - to allow companies to hire older workers who were past 65. Pepper's bill would change the provision.

After a union grievence brought no results, McMann's lawyer, Francis McBride, then a University of Virginia law school graduate student, took the case to federal court in Richmond. In 1975, Judge Albert Bryan Jr. dismissed the case, citing the provision of the law exempting retirement under pension plans.

McMann appealed to the Fourth Circuit, which said any pension plan using that provision of the law to force workers to retire before age 65 must be presumed to be "a subterfuge to evade the purposes of the act," and upheld McMann. United then asked the Supreme Court to review the case.

The issue has been in the courts for so long that McMann soon may be too old to get what he wants - his job back - if the Supreme Court rules in his favor. If he turns 65 before the case is decided, the Age Discrimination in Employment Act no longer will guaranted him a job, though McMann will be eligible for back pay.

Since he lost his job at United, McMann has continued to work part time in aviation - teaching younger pilots how to fly.