Some outstanding government managers in the future will be getting pay raises four to five times larger than their "mediocre" colleagues under a White House merit incentive salary plan nearing approval in Congress.

Part of the President's civil service reform package calls for elimination of longevity pay raises for nearly 73,000 federal managers in Grades 13 through 15. About 25 percent of those managers live and work within the Metro Washington area.

Under the reform bill - being worked out by Senate-House conferees - longevity pay would be out, replaced by a system giving managers half the annual raises that go to other civil servants, plus the chance to get merit increases in the area of 12 percent.

Pay for those employes will range from $27,453 a year to $47,500, after the October raises become effective.

Under the Carter plan, those managers would be judged by their superiors each year and given merit pay raises which, at maximum, could amount to around 12 percent. Agencies would be given ceilings on both the number of managers who could get merit raises, and also on dollars they could use for the merit pay pool.

If the system had been in operation in 1977, for instance, when rank-and-file civil servants got a flat raise of about 7 percent, some managers could gotten as little as half that amount, while more outstanding individuals would have received combined regular and merit pay raises totaling almost 16 percent.

There are many more workers in Grades 13, 14 and 15 than the nearly 73,000 who would be stripped of automatic longevity pay raises and put under the merit pay plan. The bulk of workers in those grades who are not considered "managers" or supervisors would continue to get regular annual pay raises based on similar jobs in private industry. (The October raise due this year will be 5.5 percent).

Within the management group, the Civil Service Commission says there are 31,822 in Grave 13; 24,947 in Grade 14 and 15,688 in Grade 15 who would be under the merit pay plan. Average salaries for people in that group now, according to CSC data, is $29,765 for GS 13; $35,102 for GS 14 and $41,816 for GS 15 personnel. Those averages will jump next month when the new 5.5 percent raise goes into effect. At that time GS 13 will begin at 27, 453 and go to $35,688; GS 14 will be $32,442 to $42,171 and GS 15 will have a range of from $38,168 to $47,500.

Workers in those grades who remain outside the merit pay system will continue to get longevity pay raises - between 2 percent and 3 percent - under the present semiautomatic system. In addition, they will also get the full regular October salary adjustment for other government workers.

The "managers," however, will get only about half the regular adjustment on an automatic basis. For any amount over that, special pay increases will have to be approved for them by their bosses - within ranges for their pay grade and under guidelines for the amount of the maximum merit raise, and number of employes who may get merit pay boosts. That has not been worked out yet. But insiders say a figure of 12 percent for the maximum merit boost is likely.

The changeover - which is expected to approve shortly as part of civil service reform - will mean an end to automatic pay raises for length of service for 72,000 managers. It also will mean some managers will get bigger raises than others, and some - if they get only the minimum increases over a period of years - will soon wind up making the same or less money than some of their subordinates, and many of their colleagues in the same pay grades who are not part of management, therefore not under the merit pay plan.

If the system works well, it should mean the best people will be rewarded for being good managers, and that mediocre or average managers who now get longevity raises simply for remaining alive will get minimum increases. If the system is abused, perish the thought, and favoritism rears its ugly head, it will means some unfair or politically-minded bosses will have a new pipeline into the federal cash register to reward their friends and punish the less cooperative. Perish the thought.