For the last 15 years the Federal Aviation Administration has been attempting to devise a radar system that would enable air traffic controllers to do on the ground what they do in the air: keep airplanes apart.

No sooner had FAA officials begun installing the first machines in the multimillion-dollar ground radar system than the agency discovered a major flaw. The system did not work in heavy rain, a time when ground controllers at airports need help.

So the FAA put the system on hold, hoping to devise modifications that would work. But when the latest prototype was installed in Pittsburgh, FAA officials found the system's rapidly revolving antenna tended to fly apart, causing double images and other problems.

Monday's collision of a DC-9 and 727 at Detroit Metropolitan Airport, an accident that killed eight people and that most experts said could have been avoided, brought renewed criticism yesterday that the FAA is once again moving too slowly to ensure air travel is safe.

"They really have difficulty finding the {necessary} technology, although they have people knocking at their door with it," said Roger Fleming, senior vice president for technical development and planning with the Air Transport Association of America, the airlines' Washington lobby.

Fleming said his group and others have argued for years that the FAA needs to revise its procurement rules to enable it to more quickly secure the sophisticated equipment needed to safeguard the nation's air traffic.

The FAA also has been hampered by government personnel rules "and the FAA's inability to manage complex procurements," Fleming said. "However, that's true of most systems that the FAA puts in the field."

"They've got to reduce cycle time between the discovery of a need and satisfaction of the need," he said.

Paul Steucke, a FAA spokesman, strongly denied that the agency has been slow to secure a new ground radar system, although he acknowledged that the Pittsburgh prototype had fallen about six months behind its previously announced timetable.

"How can anybody say we're dragging our feet when we are developing this radar? . . . It's fortunate that the FAA has had the foresight to move as rapidly as we have," he said.

The Pittsburgh prototype, when fully operational, will be known as Airport Surface Detection Equipment 3. The FAA has installed an earlier version, Airport Surface Detection Equipment 2, at 12 airports, but Detroit is not one of them.

The Detroit airport is among 20 scheduled to get the $2.9 million ASDE 3, with installation set for next May in Detroit, officials said.

If the system works as planned, officials say it should not only give airport ground controllers a clearer view of how planes are moving about the airfield but will be linked to approach radars and give controllers warnings of potential collisions with incoming aircraft.

"I have seen the ASDE in operation and it's impressive," said Steucke. ". . . We're working the bugs out of it."

Dulles International and Andrews Air Force Base in Maryland are among the 12 airports with ASDE 2. Washington National and Baltimore-Washington are among the airports scheduled to get ASDE 3 within the next three years, FAA officials said.

"I don't know whether we could have done it faster, better," said Steucke. The FAA views "runway incursions," the term the agency uses to describe incidents in which objects or people stray onto runways, as "a serious problem and we are addressing them with a variety of programs," he said.

FAA figures show that as of the end of October, the 220 incursions recorded this year appear to be on a pace that would exceed the 223 listed in 1989. There were 179 recorded in 1988.

At Detroit, pilots whose planes were on the ground were believed to have been responsible for 11 incursions in 1987, nine in 1988, eight in 1989 and two this year. In most cases, the pilots entered a runway without clearance from the airport's control tower, the FAA said, a finding that mirrors national results.

Not all incursions involve two airplanes, as did Monday's accident. Snow plows, trucks, pedestrians and even animals are involved in some of the incidents, Fleming said.