UNTIL AN OLD back injury began to give him trouble, John Rhoads was a good police chief. He presided over a difficult department -- Prince George's County's -- and he was beginning to solve some of its serious problems. But the back trouble became too much for him, he said, and he retired two months ago at age 43 on a $29,700 tax-free disability pension.

In those two months, Mr. Rhoads' back has improved enormously. His doctor says his problem was aggravated by stress, which is quite possible, and vacating the hot seat behind the chief's desk in Prince George's County should relieve a lot of stress. Now, Mr. Rhoads is in contention for a job as deputy sheriff in an urban county in Florida. If he gets it, he will be paid $30,000 a year -- substantially less than he was making in Prince George's -- but he will continue to draw that disability pension.

It is obvious why many public officials in Prince George's County are outraged at this sequence of events. They think they have been taken for a ride and there is no place they can get out. The county's taxpayers will go on paying that pension regardless of what Mr. Rhoads does until he dies or gives it up.

The anger, however, ought not to be aimed so much at Mr. Rhoads personally as at the system that permits such things to happen. It is quite possible that his physical condition will not permit him to function as chief of a department full of controversies but will permit him to function as the No. 2 man in a sheriff's office in a much quieter county.

Those who have followed the history of the District of Columbia's police and fire departments know this story all too well. Until the rules were tightened this year, retirement on disability was almost routine in those departments and new jobs, some of them requiring equal physical ability, often provided a second income to the "disabled." The same pattern exists elsewhere, including in the military establishment.

Any way you look at it, such arrangements are rip-offs. Disability pensions -- as distinct from those that are earned through years of service -- are designed to compensate workers for their inability to earn a living or the kind of living they might have otherwise achieved. When these are awarded, there should be some mechanism in the system to permit their being lowered, raised or eliminated as physical conditions change. That's where Prince George's County went wrong in the case of Chief Rhoads. It shouldn't take that county's officials as long to spot this loophole in its system as it took the District's officials to recognize the farce that disability pensions had become in the city.