A new study made public yesterday by Ralph Nader's Aviation Consumer Action Project shows that the Federal Aviation Administration has substantially underreported the number of near-collisions in the air in the past two years.

Nader attributed the underreporting to either "negligence or deliberate policy," and said it means the FAA has "not conveyed the full range of risk to the American public and has underestimated the need for air traffic controllers of full experience."

The FAA's air traffic control system by all accounts is still recovering from August 1981, when 11,400 controllers struck and were fired by President Reagan. Nader said that Reagan has "punished" the fired controllers long enough and that some should be rehired to take the pressure off the system. Administration officials have adamantly rejected that course.

To prove his allegations, Nader's group presented detailed summaries of 63 near-collisions reported by pilots in 1983 and 1984 that never found their way into the FAA's central computer and thus never were counted. In those two years the FAA reported a total of 585 near-collisions.

The 63 were found by Nader's ACAP researchers in checking reports from three of the FAA's nine geographical regions page-by-page against the central FAA file. Nationwide, Nader posited, it can be expected that the number of unreported near-collisions is higher.

Further, ACAP and ABC News, working together, discovered that another 39 near-collisions reported by controllers but not by pilots in the same two years were not being counted as near-collisions even though they meet the FAA's definition: two planes within 500 feet of each other.

Therefore, Nader said in a letter to Transportation Secretary Elizabeth Hanford Dole, "it appears there were 102 near midair collisions in 1983 and 1984 which were not reported to the public as such."

There is a discrepancy of 19 between Nader's total and the FAA's, but FAA officials said yesterday there is no question that Nader and ABC have found something and that the agency is moving to correct it.

FAA Administrator Donald D. Engen has ordered procedural changes to assure that pilot reports are counted and checked and to merge pilot and controller reports so that all near-collisions wind up in the same column, FAA spokesman Ed Pinto said yesterday.

According to the FAA, there were 299 reported near-collisions last year, a slight increase over the 286 in 1983. Engen, in several congressional appearances, has reported with pride that the trend over the years has been downward. There were 585 reported near-collisions in 1980, the last full year before the strike.

Since then, there have been major changes in the way the system " . . . 102 near midair collisions in 1983 and 1984 were not reported to the public as such." -- Ralph Nader handles air traffic. In addition to hiring new controllers, the FAA has improved computer surveillance and techniques have changed, partly to protect green controllers from the heavy traffic situations their predecessors complained about.

Nader made several recommendations in his letter to Dole, including a suggestion that near-collision statistics be compiled by an outside agency such as NASA or the National Transportation Safety Board.

The majority of reported near-collisions involve private planes, not commercial airliners, statistics show. The last collision in the air involving a major airliner occurred in September 1978, when 144 people were killed in San Diego in the crash of a small private plane and a PSA jetliner.

More recently, in August, a Wings West commuter airliner and a private plane collided near San Luis Obispo, Calif., and killed a total of 17 people.