At least 142 persons were killed yesterday when a Pacific Southwest Airliners jetliner collided in midair with a single-engine Cessna 172 that apparently was practicing landing approaches.

It was the word air disaster in U.S. history.

Witnesses said the jetliner, a Boeing 727. lost an engine and exploded after being struck on the right wing by the smaller plane. All 136 aboard the PSA plane were the two persons in the Cessna. The other four known fatalities came when pieces of debris struck persons on the ground as the wreckage ignited more than a dozen homes in the North Park area of central San Diego.

PSA Flight 182 originated in Sacramento with only 29 passengers, far fewer than usual but took on 100 more in Los Angeles. The plane was at about 2,000 feet. banking for its landing turn into San Diego's Lindbergh Field, when the collision occurred.

"I looked up and it was like the Fourth of July." said Susan Quinn, an attorney at the University of San Diego who was driving to work. "There were balls of fire, and then it turned into black smoke. Then you could see the outline of the flames, and I knew it was a big plane . . . You saw it and you knew there was nothing you could do, nothing. It was a feeling of total helpessness."

On the ground the scene was one of carnage resembling a wartime bombing raid. Fragments of bodies and metal were everywhere. A pall of smoke hung over the smoldering ruins of burned homes in what had been the heart of a quite middle-class residential neighborhood inhabitated mostly by elderly persons.

Police, struggling to keep thousands of onlookers away from the crash scene in temperatures of more than 100 degrees. said the death toll could go higher. It was not immediately possible to ascertain, they said. whether pieces of bodies found in the wreckage were from the homes or had fallen from the sky.

At least 10 persons were arrested for looting bodies of rings and wallets. But at a nearby hospital more than 900 persons responded to broadcasts calls for blood donations to aid those injured by the airborne debris.

The death toll could well have been higher. Witnesses said that the burning plane appeared to be heading for St. Augustine's High School, which now has been set up as a morgue for the crash victims.

James Clifford, vice principal at the all-boys school, was standing outside and talking to other faculty members when he noticed the jetliner banking for its landing turn.

"All of a sudden we noticed a little plane ascending underneath it. I said, "Oh, oh, they're too close," at which point the little plane struck the jet in the right wing. The wing burst into flame but the plane continued to bank and started coming toward the school. I yelled for everyone to get the hell out of there but the plane dropped down a ways away."

Clifford and other priests raced to the crash scene and gave the last rites of the Catholic Church to the victims.

The crash, occuring at 8:59 a.m. Dt, was witnessed by thousands of commuters.

One of them, nurseryman Frank Antonicelli, said he saw the plane making a turn to approach the airport.

"The engine . . . fell from the plane," Antonicelli said. "It was a big ball of fire and it threw the balance of the plane off. There was no possible way the pilot could have tried a belly landing or anything else. It went down on its side and then nose first just like a roller coaster."

Why the two planes collided was a mystery. Air traffic controllers were not available for comment. Bill Gibbs, a director of the Gibbs Flight Service, which owned the Cessna, said that the pilot of the small plane was a Marine with a Commercial pilot's license who was receiving instrument rating training.

Gibbs said the pilot was being tutored by an instructor who also had a commercial pilot's license.

At the time of the crash, according to Gibbs, both the Cessna and the PSA plane were on radar screens of San Diego Approach Control, the division of the Federal Aviation Administration that covers the area from the Mexican border to Oceanside north of San Diego. Both planes abruptly disappeared from the radar screens.

It was not known whether anyone at the San Diego control tower had attempted to warn either plane.

The crash, the first fatal accident in the 29-history of PSA, was a special tragedy for the airline, which has its headquarters in San Diego. At least 25 persons on the airliner, including the crew of seven, were PSA employes. Several were stewardesses returning to their homes in San Diego.

One of the passengers on board was Los Angeles attorney Valerie Kantor, wife of Democratic fund-raiser Mickey Kantor. Aides of Gov. Edmund G. (Jerry) Brown Jr. said the governor had flown to Lindbergh Field to be with Kantor, who already was in San Diego.

Mayor Pete Wilson of San Diego, flying over the crash scene in a helicopter, termed the site "incredible and beyond description."

Down below what had been a neighborhood of white frame homes and broad streets had become instead a scene of smoldering rubble.

Hal Brown, a reporter for San Diego radio station KSDO, a few blocks from the crashs site, said that people were running around the street after the crash, dodging metal and pointing to the sky.

"It was absolute anpdemonium," he said.

In one grisly scene on the ground, a body fell from the rooftop, and the woman inside the house ran outside and away from the street. Her husband arrived home soon afterward and broke down, thinking that the body was that of his wife.

Police poured into the area, attempting to aid survivors, keep out curious onlookers and recover bodies, which were taken to St. Augustine's.

The National Traffic Safety Board immediately dispatched a team of investigators from Washington to take charge of the after-crash investigation. Teams from San Diego and Los Angeles alreay are on the scene and are known to have recovered the in-flight recorder which will be used in an attempt to reconstruct communications before the crash.

PSA, which links California's largest cities, is the nation's busiest commuter airline.

The San Diego-based airline has 200 flights a day and carries more than 7 million passengers a year. It flies exclusively within California.

PSA, founded in 1949, flies between San Francisco, San Jose, Oakland, Monterey, Fresno, Stockton, Sacramento, Lake Tahoe, Hollywood-Burbank, Los Angeles, Ontario, Long Beach, and San Diego.

The airline has gained popularity with highly competitive fares. "Redeye" flights between Los Angeles and San Francisco, departing each night after midnight, cost $20 one way.

The 727 jet is one of PSA's most common aircraft. The plane, which has all three engines mounted at the tail, is manufactured by Boeing Aircraft of Seattle. It carries about 130 passengers and was first introduced around 1965.

The collision revived a 50-year-old controversy over the location of Lindbergh Field.

The crash occured as the PSA 727 was entering a steep descent to land at the airport, which lies on the edge of San Diego Bay just west of the downtown section.

This approach pattern is over the tallest buildings of downtown - 20 story banks and resot hotels built on a hill above Lindbergh Field.

The accident was the first fatal crash at Lindbergh, which handles almost 80,000 fights a year. But the steep landing descent over downtown - up to 40 degrees at times - has been a matter of concern ever since the airport opened in 1927.

Two years ago, John McLucas, head of the FAA, called for relocation of Lindbergh, saying it had "the potential for a tragic accident."

Its location near the heart of the nation's ningh largest city made the crash threat ominous, McLucas said. Airline pilots agreed privately that Lindbergh potentially was one of the world's most dangerous airports.

But over the years many have argued that the route over hilltops, power lines and San Diego's tallest buildings made the pilots especially careful and that the landings were safer as a result.

Yesterday's collision was one of the worst aviation disasters in history.

The worst - killing more than 500 persons - occurred on the ground last year in the Canary Islands.

Here is a list of major airplane collisions in the past 30 years. With the exception of the Canary Islands collision, they all occurred in the air as did yesterday's accident:

March 27, 1977 - Pan American Airways 747 and KLM 747 jumbo jets collided on runway at Tenerife in the Canary Islands, killing 579 in the worst disaster in aviation history.

Sept. 10. 1976 - Yugoslav charter jet DC-10 and British Airlines Trident collided near Zagreb, Yugoslavia, leaving 176 dead. World's worst midair collision.

July 30, 1971 - Japanese airliner struck by fighter plane over Japanese Alps near Honshu, leaving 162 dead.

Sept. 9, 1969 - Small plane and Allegheny Airlines DC-9 collided near Indiannapolis, killing 83.

July 19, 1967 - Piedmont Boeing 727 collided with private plane near Hendersonville, N.C., killing 82.

Feb. 1. 1963 - Turkish Air Force plane and Lebanese Viscount collided near Ankara, Turkye, leaving 95 dead.

Dec. 16, 1960 - United Airliners DC-8 and TWA Super Constellation collided over Staten Island, killing 134 persons.

Feb. 25, 160 - U.S. Navy plane and Real DC-3 collided near Rio de Janeiro, killing 61.

June 30, 1956 - TWA Super Constellation and United Airlines DC-7 collided over the Grand Canyon in Arizona, killing 128.

Aug. 11, 1955 - Two U.S. Air Force planes collided near Edelweiler, West Germany, leaving 66 dead.

March 27, 1952 - Two Soviet planes collided over Tula Airport near Moscow, killing 70.

Nov. 1, 1949 - Bolivian fighter plane and Eastern DC-4 collided over the Washington, D.C., airport, leaving 55 dead.