The National Transportation Safety Board tentatively ruled yesterday that the mid-air collision between a Pacific Southwest Airlines (PSA) jetliner and a small private plane that killed 144 people in San Diego Sept. 25 was primarily the fault of the PSA crew.

In a carefully worded statement that the board spent most of yesterday polishing, it said the PSA crew had failed to maintain "visual separation clearance" from the small plane. In other words, crewmembers failed to keep the small plane in sight.

Furthermore, the board said, the PSA crewmembers failed to notify air traffic control that they had lost sight of the light plane.

The board reached its tentative key finding yesterday, then adjourned before deciding how many "contributing causes" should properly be assigned to the accident and what final safety recommendations it should sent to the Federal Aviation Administration. The sately board has no regulatory powers; it can only advise.

Possible contributing causes still being considered by the board include:

Inadequacy of the air traffic control procedures in effect at San Diego at the time of the accident.

Failure of the small plane, a Cessna 172, to maintain its assigned neading.

A lack of strict discipline and attention in the cockpit of the PSA jetliner, a three-engine Boeing 727.

The board is deeply split on a number of these technical points and that fact itself points up the difficulty of assigning "probable cause" of most aviation accidents to one individual or event. The board has split in the past on major accidents.

Board Member Francis H. McAdams pushed yesterday to assign some blame to the student pilot and the instructor in the Cessna 172. The Cessna and the Boeing collided as both were on a northerly course.

The Cessna was climbing out of the San Diego airport, the Boeing was flying parallel to the runway in preparation for a 180-degree turn and landing. Each plane was advised by air traffic control of the other's presence.

McAdams said that the Cessna "has the equal responsibility to see and avoid" the other plane. "The mere fact it is a little difficult" made no difference, he said.

James B. King, board chairman, said, "I feel the [Cessna] was a victim." The crew in the Boeing jet, he saidm "had the clear responsibility" to avoid the Cessna.

The Boeing came down on top of the Cessna. It is difficult to see down from the cockpit of a Boeing, because its normal approach and landing attitude is with the nose up. It is difficult to see up from a Cessna because the wings are over the cockpit.

Tape recordings of air traffic condoubt in the minds of the PSA cres that they had the Cessna in sight.

Inside the cockpit, however, a tape recording of crew conversations showed, there was substantial concern.

"Are we clear of Cessna?"

"Supposed to be."

"I guess."

"I hope."