The Professional Air Traffic Controllers Organization (PATCO) charged yesterday that breakdowns in the computerized radar system used to direct airplanes are becoming so frequent they are "a serious menace to the safety of air passengers and crews."

John Leyden, president of PATCO, the union that represents most of the nation's air traffic controllers, based his charge, he said, on nationwide statistics PATCO gathered after a collision between two loaded jetliners was narrowly avoided Oct. 31. The incident occured during a computer failure at the air route traffic control center in Leesburg in suburban Virginia.

Leyden said yesterday that there were 143 computer outages at Leesburg between April 2 and Oct. 3, and he cited outages totaling in the hundreds at other air route centers around the country.

When asked to explain what it all means, however, Leyden was less certain. At first he said that the figures showed a "dramatic increase in the number of outages." When asked to provide back-up data, he said that was difficult to obtain. When asked again to justify the "dramatic increase" statement he said, "I don't know."

There is no question, however, that there are problems in the computer, that they irritate controllers and sometimes contribute to unsafe conditions.

The computer at the center for the New York City metropolitan area was "down" for three days last week, for example, resulting in delays to air traffic up and down the East Coast. Washington passengers using the shuttle to LaGuardia Airport were among those affected.

FAA spokesman Fred Farrar said yesterday that in 1978 there were an average of 8.15 computer outages per week at each of 20 centers, and that 91 percent of those outages lasted for more than a minute. So far this year, he said, the average is 7 outages per week per center, with 88 percent of those outages exceeding one minute.

"We think the system provides an adequate level of safety," Farrar said.

The problem with a computer failure is that it forces controllers to use a back-up system that is much more difficult to read and with which fewer and fewer controllers have long experience.

The $700 million en-route system is being fitted with a new back-up system that is regarded as an interim solution at best, according to Farrar. There are long-range plans for an all-new air traffic control computer by the late 1980s or early 1990s, he said. That system is estimated to cost $1 billion to $2 billion.

Leyden said that the air traffic computer was having increased difficulties because it is getting old, there were no spare parts for some items and the number of maintenance personnel has been reduced by the FAA.

Farrar said that it is true that IBM is planning to stop manufacturing spare parts, but "we have told them we will negotiate a spare parts inventory with them" before they stop. The number of personnel required for maintenance has been reduced, he said, "because we have been replacing vacuum tubes in the system with solid state equipment, which requires less regular maintenance and is easier to fix."

raining for the back-up system is accomplished for all controllers at centers on the midnight shifts, Farrar said. PATCO representatives challeneged that assertion.

The Air Line Pilots Association (ALPA) issued a statement supporting "the growing concern" of PATCO.