Two Air Force planes flew perilously close to each other high over Richmond on Dec. 15 during a two-hour, 38-minute failure of the computer that aids air traffic controllers for the mid-Atlantic area, a Federal Aviation Administration official confirmed yesterday.

The computer is part of the immensely complicated air traffic control system located at Leesburg. Military and commercial planes are controlled there as they fly from Maryland through South Carolina and from West Virginia to the Atlantic Ocean.

Angelo Viselli, chief of the Leesburg facility, confirmed that the two Air Force planes were involved in a near collision during the computer outage. "The pilots estimated that they had come as close as 75 feet to each other." Viselli said. The incident happened about one hour and 15 minutes after the computer failed, he said.

A "near miss," as the FAA officially labels such close encounters, occurs if aircraft are closer than one mile horizontally or 500 feet vertically.

The Air Force incident, and other computer difficulties, were reported in a letter to The Washington Post by Michael Krause, of the National Safety Committee of the Professional Air Traffic Controllers Organization (PATCO). Krause is also a controller at Leesburg.

Krause said in his letter that there had been other computer failures on Dec. 16 and Dec. 19. No near collisions were reported on those dates.

Viselli said his log books did not show computer difficulties for Dec. 16, but that there had been a series of "short-span up and down time" computer interruptions on Dec. 19.

A computerized display of the whereabouts of airplanes on a television-like screen has replaced the traditional radar picture with a sweephand and flips in the nation's air traffic control centers. When the computer fails, the traditional system is supposed to be available as a backup.

However, the traditional system does not provide the controller with the same, or as much, information as the computerized version. Furthermore, newer controllers have never worked the old system on a regular basis. When the computer goes down, it is a bit of a scramble to locate and identify all planes on the radar screen.

Krause said in his letter that there is "absolutely no training provided for new controllers on that (old) system and very little opportunity given to remain proficient on the system for veteran controllers."

Viselli sait that old-system training was "limited" and conceded that "if all our people had never used the old system we would have had more trouble." A preliminary investigation has shown the Air Force near-collision occurred because of a communications problem that would not have happened with the computer, Viselli said.

"We do not have an unsafe situation," Viselli said. "I would hate to get the public riled up. But we do have incidents."

The FAA's long-term plans are to used another computer to back up the primary computer.