Despite a new merit pay plan for 150,000 mid-level bureaucrats, a legal safety-catch makes it impossible for big bosses to deny some aides full raises, even if they get low performance marks.

Carter administration officials are aware of the wrinkle in the merit pay plan being introduced for Grade 13, 14 and 15 workers. Nearly 65,000 in metro Washington are candidates for the system. It was designed to provide pay incentives, and distincentives, for mid-level managers. Some agencies will begin it this year. All will have it in place by October 1981.

Under the civil service reform act, GS 13 through 15 personnel designated as "managers" or "supervisors" will be subject to merit pay. It guarantees them only half of the regular October adjustment other civil servants routinely get.

To qualify for full October raises, or amounts even higher, those employes must demonstrate superior qualities -- as measured by bosses. Idea is to punish the marginal with half pay raises and give out bigger boosts (from the same salary pot) to persons who demonstrate the ability to walk on water rather than just float through careers.

In fact, however, many supervisors and managers will continue to get full raises (unless they are fired or replaced) simply because another pay law says nobody at the minimum pay rate for any grade can be denied the full amount of an October raise.

Minimum pay for Grade 13 is $29,375. At Grade 14 it is $34,713. Grade 15 has a minimum rate of $40,832. Nobody at those rates can be denied the full amount of the upcoming October raise, or future increases, so long as they remain at the bottom pay level.

Merit pay will be fully operational in 1981. Some agencies plan to use it for the upcoming October raises to give people more, and hold others to 50 percent of the regular raise.

The federal pay "comparability" law says persons at the minimum rate of each grade cannot be denied full increases. It means no supervisor-manager at the minimum can get less than the full percent raise set for other civil servants. It sets the stage for people at the minimum, even if they are marginal, to get full raises while other workers with better ratings could get only half the raise.

Unless a way out of the pay bind is found, many supervisors and managers eventually could wind up at the bottom of the pay totem pole for their grade -- thus being guaranteed full raises each year. In efect it exempts people at the bottom, for whatever reason, from merit pay.

The merit pay plan has troubled many federal workers who fear that it will be applied unevenly and unfairly, and provide an additional lever on the bureaucracy for political appointees. Now there is a new wrinkle.

Originally, administration officials said the merit pay system would be applied to only supervisors and managers. In fact many agencies simply are designating EVERYBODY at those grades, whatever their job, as managers and supervisors. One big Navy unit in suburban Maryland recently informed all of its Grade 13 through 15 people, including scientists and engineers that they are now managers and will be placed under merit pay.

Navy officials told this column that 17,000 of their 18,000 Grade 13 through 15 employes will be placed under merit pay.Navy is the second largest employer in metro Washington.

Army brass say "at least" 80 percent of their GS 13 through 15 employes will be put under merit pay. Being listed as managers or supervisors also effectively excludes workers from active roles in unions.

It is clear the Carter administration plans to expand merit pay to include nearly all GS 13 through 15 aides, whether they are managers or supervisors. What is not clear is how merit pay is going to work if the people at the minimum, who remain at the minimum, are going to be affected by the merit raise ratings that will hit all others.