The bizarre encounter last week between a light plane and President Reagan's helicopter in a restricted patch of airspace in the Santa Ynez mountains dramatizes the question of aviation safety, but it is only one of many near-collisions that take place almost daily.

The incident followed five consecutive days of unusually close near-tragedies in which jet pilots flying around New York, Chicago, Los Angeles and Dallas were forced to swerve to avoid other planes. Five of the incidents involved light, general aviation aircraft.

"It lays naked how out of control air traffic really is if we can't even protect the president of the United States," said Anthony Skirlich, an air traffic controller at the Los Angeles Air Traffic Route Center in Palmdale, Calif., which handles air traffic for a vast region around the Southwest. "No other country in the world allows such freedom of airplanes. The 'see and be seen' rules were designed for sailing ships, not high-speed airplanes."

Overall, the number of near-collisions in the first seven months of this year has risen 31 percent over the first seven months of 1986, from 464 to 610. The problems in Southern California, which has one of the highest concentrations of air traffic in the country, are particularly acute.

The Los Angeles basin leads the nation in near-collisions, with a record 51 reported in the last 12 months, more than triple the number in the previous year, according to the Federal Aviation Administration.

California has one of the highest number of small private planes in the country, with 35,464 registered. Air traffic in the Los Angeles basin jumped 20 percent in the first six months of this year, compared to the same period last year, the FAA said.

Earlier in the week, after an American Airlines jetliner carrying 82 people had to dive sharply to avoid hitting a small plane, the FAA issued an emergency order placing additional limits on small planes operating in the Los Angeles area. The FAA also announced it is considering further limits on light plane traffic at 22 other busy airports around the country.

The new rules, which take effect Wednesday, raise the ceiling on restricted airspace around Los Angeles International Airport from 7,000 feet to 12,500 feet and close a north-south corridor directly over the airport through which private planes could fly with no restrictions. To enter restricted airspace, private planes need permission from air traffic controllers and must have a device, known as a Mode C transponder, that reports both position and altitude to air control radar.

The move has rekindled the debate between owners of small planes and commercial airlines who compete for flying room in jammed airspace.

"We have been hearing from dozens and dozens of members out in California, and they are hot. They're incensed," said Patricia Weil, spokeswoman for Aircraft Owners and Pilots Association, which represents private plane owners. "They question, what is this going to do to enhance their safety?"

Weil said private plane pilots now will be compressed into airspace below the restricted area, "much lower to the ground than prudent pilots would like to fly." She said the new upper limit prevents private planes from flying over the top of the restricted area. An FAA air traffic manager suggested that small planes fly around the restricted airspace.

William F. Bolger, president of the Air Transport Association, the lobbying group for the nation's major airlines, praised the move, but said further steps need to be taken. Bolger said the ATA recommends that restrictions not only be expanded to include 23 airports, but an additional 150 less busy airports.

But the debate over restrictions around airports is complicated by other safety issues. Air traffic controllers say more restrictions mean more work and will require additional training for people who are overextended and overworked. The National Transportation Safety Board says the entire air traffic control system operates, to a large degree, on the assumption that light plane pilots will follow the rules.

In the last year, 675,000 aircraft requested to fly through the restricted area. The FAA predicts that the expansion of the restricted airspace will result in a 25 percent increase in air traffic clearance requests from private pilots.

Skirlich said that prediction is too low. Light plane traffic around airports is usually handled by controllers working in FAA facilities around airports. But because of the added workload, much of the new burden will be shifted to the Los Angeles center, where controllers must undergo additional training. Skirlich said the FAA's outdated computers won't be able to handle the additional load any easier than the controllers will.

"What's needed here is a long-range plan," he said.

"We are saturated with IFR {instrument flight rule} traffic now," said Skirlich. Skirlich added that the sudden changes to the restricted airspace will protect airliner traffic around Los Angeles International Airport, but will create more congestion and potential hazard around the Burbank, Ontario, John Wayne and Long Beach airports along the edge of the restricted area.

"We are going to make Los Angeles five times safer than it has ever been," Skirlich said. "It is going to be like an eye of a hurricane. We will have calm in the middle. We've taken the storm to the flanks now."

John Lauber, a member of the NTSB, said the action doesn't address one of modern aviation's central dilemmas: that pilots of private planes are going to make mistakes and blunder into restricted areas. The pilot of the Piper Archer who strayed into the restricted airspace over Reagan's ranch told investigators he did so unknowingly.

"I'm not sure that restrictions are going to address the root of the problem," Lauber said, noting that last year's midair collision between an Aeromexico jetliner and a small private plane over the Los Angeles suburb of Cerritos occurred inside restricted airspace.

In assessing the Cerritos accident, in which 82 people were killed, the NTSB faulted the entire air traffic control system as outdated because controllers are taught to direct planes flying on instrument flight rules and direct small plane traffic only if the workload permits.

"That's why we hit the system the way we did in the Cerritos case. Until the primary job of controllers becomes separating instrument flight rules airplanes from all other aircraft in the system, regardless of whether they are players or not, we are going to continue to see this kind of thing," Lauber said.

New FAA rules also require that small planes flying around the 23 busiest airports be equipped with Mode C transponders by Dec. 1. But Robert Crandell, chairman of American Airlines, has pressed for stricter rules. He wants altitude reporting transponders on all aircraft.

"In my view, anyone who can afford to operate an airplane can afford a transponder," Crandell said in a recent speech at Northwestern University in Evanston, Ill.

The near-collision last Tuesday between an American Airlines jetliner and a small plane just outside the boundary of the restricted airspace prompted the FAA's sudden move. The FAA already had proposed the changes, but when the American pilot had to bank sharply to the left and dive to avoid the small plane, the FAA issued an emergency order putting the rules into effect next week.

The two planes missed each other by 100 feet, FAA officials estimate. The small plane was not equipped with an altitude-reporting transponder and did not appear on air controllers' radar scopes.