The sky was clear as two small planes drifted toward the main runway of Manassas Municipal Airport which handles only business and private flights but is among the busiest airfields in the Washington area.

Then, 100 feet over the field, the planes collided. The aircraft plummeted to the field, injuring all three persons aboard.

It was an accident that safety experts say is all too typical of the scores that occur each year at the nation's 11,000 uncontrolled airports -- the relatively small fields that lack control towers.

Federal and industry officials are now devoting a lot of attention to an experimental control tower, automated and small enough to fit inside a broom closet, that is being tested at the Manassas airport.

If the device had been in operation there when the collision occurred five years ago, the pilots would have been warned of each other's presence by a mechanical voice that is triggered by radar blips on a screen and broadcasts their location.

"It's really a tremendous asset," said Virginia State Sen. Charles J. Colgan (D-Prince William), owner of Colgan Airways Corp., which operates the Manassas Airport under a lease agreement. "Those planes were completely demolished and I believe it (the new system) would have prevented that collision."

The system, developed by the National Aeronautics and Space Administration and the General Aviation Manufactures Association, costs about $50,000 and uses a minicomputer to give wind, weather, and runway information as well to aircraft using the small airport.

Tests on the automated system began in June and officials at the Northern Virginia airport say they're encouraged by the results so far. If the system continues to work as well as it has and the FAA is persuaded to accept it, it may be utilized at unmanned, uncontrolled airports throughout the country, says Chuck Ponder, a technician with the project.

The project, called APAS (for Automated Pilot Advisory System), was begun in 1971 with studies that showed that "pilots were making good landing approaches in uncontrolled airspace, but were simply unaware that there were other aircraft nearby," said Lloyd Parker, director of safety at NASA's Kennedy Space Flight Center.

Radar for the system was developed at NASA's Wallops Island Space Flight Center on the Eastern Shore in a cooperative venture with the General Manufacturers Association. The Research Triangle Institute, a North Caroina think tank near Raleigh, designed the computer program.

The five week test program began last June and John Parks, NASA project manager at Manassas, said the system's accuracy has been "above 95 percent" throughout, handling between 250 and 300 aircraft each week day and up to 10 aircraft at one time.

"The amount of weekend traffic has doubled those figures," Parks said. "This is actually higher than the amount of traffic that APAS is designed to handle at a smaller airport, but we assumed that if it could pass the tests here, it would certainly be able to do so at airfields with less traffic."

The system is not without its bugs. One of the functions is the selection of a "most favored runway" for takeoffs or landings, based on weather conditions and wind speed.

"It was working too rapidly once. By the time the pilot had reached the preferred runway, he was being told to try another and had to taxi back and forth . . . they (pilots) really didn't appreciate that," Parks said.

About 70 percent of the pilots using the test system have reacted favorably, Parks said.

"All a pilot has to do is tune in the frequency on a VHF radio, listen and observe," Parks said. Other air traffic control devises currently under FAA scrutiny require the aircraft that use them to have on board navigational devises that APAS does not need, said John Myer, a spokesman for the General Aviation Manufacturers group.

Parks said the system is still in the experimental stage, but could be available as soon as 1982 or 1983.

Myer also suggested that the federal government's Aviation Trust Fund, which has a $3.6 billion surplus, could be used to help fund APAS or other air traffic control systems.

"This will result in greater safety for the flying public," Myer said. "And that's why we think that APAS is the way to go."