Under the current pension system -- not really a system but a collective name for a vast assortment of different public and private pension programs -- some people luck out and some people don't.

Rita McIlwain, now 68, who lives in the Pittsburgh area, was one of those who didn't luck out, and her case illustrates some of the gaps in pension coverage.

She was a medical technician and she "earned good income" working at hospitals and other medical institutions for more than 20 years.

But many of the hospitals had no pension systems, and she didn't have any chance to gain credits toward a future retirement benefit.

Other hospitals did have pension systems, but she didn't work in any one long enough to "vest" in future pension benefits, that is, earn a permanent right to a benefit. Until federal legislation was passed in 1974, a person did not automatically vest in five or 10 years on the job even if working in a pension-covered job. The employer could set his own terms.

Even under these new rules, she wouldn't have done very well, because while her total work time exceeded 20 years, she didn't work in any one place with a pension plan for more than a few years. The upshot was that she ended with no pension at all -- only Social Security.

So when she and her husband split up after 40 years, she had to go back to work at 68. She finally landed a 20-hour-a-week position with the Pennsylvania Green Thumb job-training program for older people. Even so, her income from the job and Social Security combined is less than $8,000.

Robert J. Rosenthal of Washington, on the other hand, is one who did well. Now 75, he is a lawyer. He worked 35 years for the U.S. government and was a supervising attorney at the National Labor Relations Board. Because government pensions (like many in private industry) are based on length of service and salary during final years of work when most people are at their top pay, he ended with a pretty good pension.

Moreover, federal employee pensions get annual cost-of-living increases, something extremely rare in private industry. The outcome for him is a government pension of $40,000 for 1990, which will go up 5.4 percent in January when the 1991 cost-of-living increase kicks in.