The government's private-pension insurance plan, which protects the pensions of 33 million workers, is headed for trouble.

Your individual pension remains safe. Back in 1974, Congress guaranteed that most earned benefits will be paid, even if your company goes broke. That is a promise that Washington intends to keep.

What's in trouble is the system Congress chose for guaranteeing pension payments. Under the law, corporations have to pony up whatever money is needed to cover pension plans that fail.

But unless we get extraordinarily lucky, this system is headed for a breakdown--because some companies are abusing the system, and because the potential bill for failed plans is too big. At some point in the future, we as a nation may have to decide either to cut back on private pension insurance or to back it up with public funds.

"No other nation insures the performance of private pension plans," says Barnet Berin of the pension-consulting firm of William M. Mercer, Inc. "It's a high-risk social experiment that did not anticipate the enormous cost of guaranteeing pensions in declining industries. The acid test for pension insurance is whether it will be able to survive, in its present form, during these hard times."

You are generally covered by the government's Pension Benefit Guaranty Corp. (PBGC) if you work for a private company (above a certain size, depending on the situation) and if yours is a "defined-benefit" pension plan. Defined-benefit plans promise you a specific level of pension payments at retirement. Open-ended profit-sharing plans are not insured.

The PBGC guarantees that if your company drops its pension plan for any reason, or if it goes bankrupt, retirees will continue to receive their monthly pension payments, up to a maximum this year of $1,380 a month. That maximum rises every year.

Current workers, not yet retired, are insured to the extent of their vested pension benefits (meaning those benefits permanently assigned to their pension accounts).

Ideally, a terminated pension plan should be well enough funded to cover all retirements benefits. If it isn't, the PBGC can seize up to 30 percent of an employer's net worth (and even more, in some plans) to meet the company's pension obligations. If the company doesn't have enough money, the PBGC has to make up the shortfall itself.

As things now stand, the Pension Benefit Guaranty Corp. does not have enough assets to cover its known future obligations to 71,200 workers and retirees in the 659 failed plans which it is currently administering. Preliminary statistics show that pension-plan terminations rose sharply last year, reversing four years of decline.

The PBGC is supported entirely by corporate assessments. The insurance plan has a $100 million line of credit on the U.S. Treasury, but so far it has not been used. By pooling their risks, private companies are supposed to insure each other without having to look to the government for help.

So much for the dream. Here's the reality:

1. Some very large companies are in financial trouble, among them International Harvester, Chrysler and Braniff. The failure of the White Motor Corp. helped push the deficit in PBGC's single-employer insurance fund to $188.8 million for fiscal 1981, up from $94.6 million in 1980. To cover this liability, PBGC wants to increase the annual company payments into the fund to $6 per worker, from $2.60 today.

Two or three more large pension-plan terminations could overwhelm the PBGC's resources and generate strong resistance among the companies called upon to pick up the pieces. What would Congress do then?

2. Some companies are seeking ways of terminating under-funded pension plans and dumping them into the lap of the PBGC. In effect, the offending company gets out from under the heavy cost of its pension plan by charging off the liability to all the other participants in the insurance fund.

Many of these plans are small, but some are large. The PBGC is currently suing International Harvester for allegedly dumping a $45 million pension liability in its now-bankrupt Wisconsin Steel Division. Harvester denies the charge.

Most private plans are well-funded and likely to remain so. But the growing number of weak plans, especially among large companies, amount to a blank check on the bank accounts of stronger companies and, potentially, a debt that the U.S. Treasury might be forced to redeem.