A large chunk of metal from a disintegrating flywheel punctured the floor of an empty Metro subway car and struck the ceiling in a bizarre accident that Metro safety officials are still investigating, The Washington Post has learned.

The July 10 accident was detected at 7:40 p.m. on a train parked in a siding at the Dupont Circle station, according to Metro safety officer John J. Flynn.

The flywheel -- about a foot in diameter before it broke apart -- was part of the single air-conditioning compressor unit that serves each of Metro's two-car units, Flynn said. All 150 such units have been inspected since the incident, Flynn said, and no other faulty flywheels have been found. s

The floor of a Metro car is about 2 1/2 inches thick and contains three strips of aluminum, a sheet of plywood and a layer of insulation. The chunk of metal that came through that floor was described as about 1 1/2 inches by 2 inches by 6 inches. It could have seriously injured passengers if any had been aboard the car.

The piece of flywheel punctured the floor in the center aisle near the middle set of doors.

Flynn said that early inspections of the failed flywheel indicated that lugs holding it to a shaft had slipped and that the wheel had simply come loose rather than fracturing.

He said consideration was given to grounding the Metro fleet of 150 two-car units until all could be inspected but that, because the problem was thought to be mechanical rather than metallurgical, inspections continued along with train operations. Flynn said loose lugs "could be detected by visual inspection" without dismantling the compressor assembly.

Flynn said this was the first accident of this type, another reason officials felt it was safe to continue operations while inspections were made. "I think everything prudent was done immediately," Flynn said.

However, neither Flynn nor other members of his safety office were contacted before the decision to run uninspected cars during the Friday, July 11, morning rush hour was made, Flynn said. He said that Metro officials had attempted to call him the night of the incident, but he was not at home.

It could not be learned yesterday who made the decision to continue operations. General Manager Richard S. Page said he was not told of the matter until the next day.Should he have been told sooner? "Probably," Page said.

As an added precaution, the disintegrated flywheel has been shipped for a special metallurgical inspection. Furthermore, Metro has devised a new shield to surround the flywheel and keep it from penetrating the car floor should one give way again. Parts for the new shield have been ordered, Flynn said, but none has been installed.