By the time the air traffic controllers at Miramar Naval Air Station realized they had a problem, it was too late. Pacific Southwest Airways Flight 182, a jetliner with 136 persons on board, was already falling like a rock after colliding with a small plane.

"Low altitude alert, PSA 182," the controllers radioed futiley. The collison in the air here Sept. 25 killed at least 144 persons and became North America's worst air disaster.

Investigators from the National Transportation Safety Board are concentrating on the Federal Aviation Administration's air traffic control procedures in their study of this accident. Those procedures, based on evidence already obtained, may be seriously inadequate both here and at many airports nationwide.

Investigators and outside experts, citing information from air traffic control tapes and other sources, report these findings:

The small airplane, a Cessna 172, was practicing instrument - guided approaches for landing on the same runway that all other aircraft were using, but the Cessna was flying in the opposite direction. "That's a perfect set-up for a head-on collision," a knowledgeable FAA official said privately last week. "It's not a safe procedure." The collision here was not head on, but the fact the fact the Cessna was there in the first place can be traced to the procedure.

The air traffic controller at regional radar center at Miramar Naval Air Station ordered the Cessna to fly a heading that [WORD ILLEGIBLE] evitably across the path of inbound [WORD ILLEGIBLE] such as PSA flight 182, a Boeing 727.

Maintaining a safe distance between the Boeing and the Cessna was made the responsibility exclusively of the pilots despite the fact that full radar coverage was available and that each plane was in the worst possible position to be seen from the cockpit of the other.

An electronic alarm, warning of a possible collision, sounded in the regional radar center 17 seconds before the collision. The controllers took no action and apparently thought none was required.

In the moments immediately before the collision, radio contact with the two airplanes was divided between the Lindbergh Field tower at San Diego's major commercial airport and the regional radar center at Miramar, 12 miles away. Therefore, neither controller had the full picture. The Linbergh tower did not know that the collision alarm had sounded. The regional center did not know that the PSA pilot had apparently become uncertain as to the location of the Cessna.

"It is my view," said Washington attorney Donald Madole, "that the traffic controllers set up that collison. They had one airplane climb through the airspace used by another." Madole specializes aviation crash litigation and may represent the estates of some of the victims.

Lindberg Field's habit of permitting practice landings against the traffic flow is unusual, perhaps even unique, in the words of one inspector. It stems from the fact that Lindbergh's main runway is fully equipped to provide guidance for instrument landings in only one direction.

Therefore to practice instrument landings, a pilot must sometimes fly against the flow or go elsewhere. The closet other instrument runway is 40 miles away.

Lindbergh is instrument in only one direction because a mountain and several high-rise office buildings would foul up the electronic guidance for approaches from the opposite direction even though the between-the-buildings and over-the-mountain route is the most common into San Diego because of prevailing winds.

The other findings associated with the San Diego collison are common throughout the FAA's airspace control system. Control of airplanes in the same general area is regularly divided among controllers, but they are supposed to talk to each other.

The eyesight of pilots is regularly used in clear weather as the primary means of separating airplanes, including airliners, even when radar surveillance by air traffic controllers is fully available. The technique of "visual separation" is known commonly as "see and be seen" or "see and avoid."

The FAA officially regards it as equally "appropriate" as instrument-guided separation, according to FAA air traffic control official Gene Lawing.

"Flying is obviously not as safe as it could be," said Rudolph Kapustin, who is heading the safety board's investigation here. "As long as you depend on "see and be seen," then air traffic control is not as sophiscated as it could be."

The problems are apparent in the accident here. PSA Flight 182 was approaching San Diego from Los Angeles after starting the morning in Sacramento.

The official FAA transcript of pilot-controller conversation shows that PSA coming in from the coast, was cleared by Miramar controllers for a "visual approach to Runway 27" - the over-the-mountain route. That meant that PSA 182 would fly east, parallel to and north of the main runway, until it was well over central San Diego, then would turn and come back in to the runway for a landing.

About the same time an instructor and an experienced "student" pilot seeking to upgrade his instrument rating, were practicing instrument approaches to Runway 9 in the Cessna. The Cessna broke off the approach before touching the runway, as directed, then was assigned a heading of 70 degrees - east northeast.

The PSA flight was told by Miramar that a Cessna was three miles in front of him, dead ahead. "Traffic in sight," the PSA crew responded by radio. The Miramar controller then directed the PSA crew to change radio frequencies and contact the Lindbergh tower, and "handoff."

The Cessna pilot was told by Miramar that the PSA jet was about two miles behind him and "has you in sight." The Cessna said, "Roger,"

Those exchanges mean that each pilot was told of the other's presence and they were now on their own.

Six seconds later, the Lindbergh tower told PSA that the Cessna was straight ahead at one mile.

"OK," the PSA crew said, "we had him there a minute ago."

"Roger," the tower said.

"Think he's passing off to our right," PSA said.

"Roger," the tower said.

Critics claim those last two transmissions should have told the tower that the PSA crew had lost sight of the Cessna. But there was no alarm in the pilots' voices.

Meanwhile, back at Miramar, a new computerized device called a conflict alert sounded. The two planes possibly on a collision course were identified by blinking signals on the radar screen. A bell sounded in the radar room.

The controllers did nothing according to both safety board and Miramar officials, because they "thought conflict had been resolved" when both thePSA and Cessna acknowledged they knew of each other.

Another factor may be reliability problems that have accompanied the installation of much of the PSA's new computerized radars across the country. Irvin Vodovoz, chief of the Miramar facility, said in an interview that the conflict alert sounded as often as 12 times a day while it was being tested and broken in, but that "since commissioning" on Aug. 7, the average was two to three times a day.

"We don't have false alarms," said Vodovoz, "we have nuisance alarms." The Miramar controller took no action to call the Lindbergh took no action to call the Lindbergh tower or either airplane. Vodovoz said, "because in his mind he was satisfied the conflict had been resolved . . . If the conflict had not been resolved he will take some action to resolve it."

As FAA officials replayed the tape of the Miramar tower communications for reporters, the first hint of tension in the controllers' voices came later, when another computer alarm went off. Then, the controllers' voice crackles with urgency when he says:

"Low altitude alert, PSA 182."

The low-altitude alert system designed to warn controllers when planes are lower than they should be at a given point, was pushed into service in the nation's air traffic system after TWA Flight 514 smashed into the mountains West of Dulles International Airport on Dec. 1, 1974, and killed all 92 persons aboard.

Norman Belsterling, the safety representative of Professional Air Traffic Controllers Orgainzation unit at Mirama, said, "Both controllers at both facilities did everything they could within the system."

So, despite all the radar equipment at Miramar, visual separation was being employed. Both pilots were flying directly into a morning sun. The collision occurred at 9:01 a.m.

The controllers in the Lindbergh tower, who were also looking into the sun told investigators they did not see the collision. They assumed, when they saw fire break out, that PSA had lost an engine.

When the PSA crew radioed, "Tower, we're going down," they called the crash equipment.

The PSA Boeing, descending for a landing, was in a nose-up configuration, meaning that the pilot's vision was blocked both below him and to the front - where the Cessna was located.

The Cessna had a high wing, over the cockpit, meaning its crew could not see up and to the rear - where Boeing was located.

What did the Boeing pilot see when he said, "Traffic in sight?" The safety board is still carefully checking radar tapes to see if a third or a fourth plane might have been sighted. The tapes show there was at least one other Cessna in the area.

The PSA jet hurtled to the ground almost directly below where it was struck. It is still something of a mystery why the pilot could not regain control, because, although the right wing and a hole in it, it remained intact.

It was 18 seconds from the collision to the time the right wing of the Boeing struck Harold Krill's home at 3611 Nile Street. The plane exploded and cartwheeled through a city block, obliterating houses and killing seven people on the ground. Bodies were strewn for blocks. Krill was not injured.

The tragedy was a heavy one for San Diego. PSA is based here and 38 PSA employes - flight attendants, mechanics, pilots - were on the jetliner. Almost all passengers were Californians. Everybody interviewed here knew somebody who had died in the accident, from the reporters at the San Diego Union to the waitress at the coffee shop.

"This was not an accident caused by the crowd skies," said attorney Madole. "This was caused by the failure of human beings to use the equipment that was assigned to them to do the job. We will see more ofit.

"One reason we will see more is that after a disaster the FAA takes a litigation stance instead of a safety stance."

At a press conference here last week, with FAA's top crash attorney sitting on the stage, FAA spokesman Bruce Chambers assured everyone that flying has an excellent safety record and that the American public should continue to fly, with confidence.

"We have not changed our procedures," Chambers said. "If we know of some correction that could be made, we would make it immediately."