Federal experts investigating te midair collision that killed at least 144 people here Sept. 25 are concentrating on what they regard as unusual air traffic control practices plus the tantalizing question of whether the pilots saw each other's aircraft.

The Federal Aviation Administration released yesterday the tape recordings and transcripts of final moments of conversation between air traffic controllers and the two aircraft, but they provided little assistance in answering basic questions.

For example, seconds before the crash, the controller at San Diego's Lindbergh Field gave Pacific Southwest Airlines Flight 182 its third advisory that there was a small one-engine plane in the area. "Traffic 12 o'clock, one mile, a Cessna," the towersaid.

"Okay, we had him there a minute ago" a crew member on PSA 182 responded. " . . . Think he's passing off to our right."

Just seconds earlier another controller had told the Cessna that the PSA jet was two miles behind him and "has you in sight," which indeed that controller had been told by PSA Flight 182. Twelve seconds later, the Cessna answered, "Roger."

Nonetheless, the two collided. The planes were on essentially easterly headings. The PSA plane, a Boeing 727 carrying 135 persons, was descending before making a final turn to approach the runway. The Cessna 172, with two on board, was climbing as part of a training procedure and probably was at full throttle.

All of those airborne were killed, in addition to at least seven on the ground. The wreckage smashed through a residential neighborhood. The collision ranks as the worst in North American aviation.

The two planes collided when the Cessna struck from below the lending edge of the Boeing's right wing. Critical mechanical and hydraulic connections were severed, and fuel lines were ruptured. Fire broke out. Although the wing remained with the Boeing until it struck the ground, the crew could not regain control.

"Tower, we're going down, this is PSA" a crew member calmly told the Lindbergh tower.

"Okay, we'll call the (fire) equipment for you," the tower replied. At the same time, a buzzing sound attributed to interference generated by the crash alarm sounding in the tower could be heard on the tape recording.

What did the PSA crew member see when he said, " . . . Think he's passing off to our right?" Was it another plane?

That question, called the "third plane theory," is being debunked by the FAA. "I think we've pretty well dispelled the third plane theory," FAA spokesman Bruce Chambers said.

At least five plannes were being handled by the Lindbergh tower at the time of the crash, the tapes show: the PSA that crashed, another PSA flight that had just landed and a third PSA that was about to take off.

The other two planes were Cessnas - once a twin-engine, the other the single-engine plane that collided. There could have been other planes operating completely without the beneift of air traffic control services, because such control is not mandatory at Lindbergh, as it is at Washington National.

The air traffic control procedures in the San Diego area are complex. The Lindbergh tower handles takeoffs and landings. Planes approaching and departing Lindbergh are handled by a regional "approach control" center at nearby Miramar Naval Air Station.

Miramar's radar, highly sophisticated and computerized, warns controllers when planes are on collision course. Such warnings sounded and appeared on the radar screen at Miramar 17 seconds before the collision.

No action was taken by the controllers because, both FAA and safety board officials said, the two planes had been informed and were aware of each other, therefore, the conflict was considered resolved.