A rash of incidents at Los Angeles International Airport, including a near-collision between a Mexicana Airlines jumbo jet and a commuter airliner, has prompted a National Transportation Safety Board (NTSB) investigation into flight operations there.

The pilot of Mexicana Flight 906 reported that his DC10 jet missed hitting a United Express commuter plane by 30 feet as both aircraft approached the airport for landings Thursday night.

The pilot of the commuter plane, operated by Westair of Fresno, Calif., told investigators that he estimated the distance between the planes at 1,000 feet.

The incident was one of four Thursday. The others involved errors made by air traffic controllers and a minor collision when a plane being moved by maintenance workers struck a parked plane.

On Tuesday, a United Air Lines DC10, carrying 298 people and traveling 175 mph, narrowly avoided hitting a Delta Air Lines jet on the runway in front of it. The United jet was traveling fast enough and the Delta jet, a Boeing 737, was far enough down the runway that the DC10 took off and cleared the 737 by 200 feet.

The NTSB, which had begun an investigation of Tuesday's near-collision, broadened its probe after the incidents Thursday, NTSB spokesman Alan Pollock said. The board dispatched two investigators from its headquarters here to study human performance and air traffic control, he said.

The heavily-congested airport has some of the nation's highest traffic volume.

The first incident occurred at 9:20 a.m., when controllers failed to maintain proper separation between aircraft and a Los Angeles County Sheriff's Department helicopter on a police call while flying through a restricted area.

At 6:38 p.m., a controller directed a taxiing United Express plane across a runway too soon after a Delta Boeing 727 began its takeoff roll. The United Express plane, a 19-seat Bandierante, had just landed after a flight from Fresno.

The two planes passed within 1,500 feet of each other, with the Bandierante crossing the runway after the jet took off, according to Barbara Abels, a Federal Aviation Administration spokeswoman. The Delta jet was carrying 38 passengers and a crew of six on a flight to Anchorage.

Three hours later, at 9:16 p.m., came the near-collision involving the Mexicana jet and United Express plane.

The United Express plane was following the Mexicana DC10 as they approached parallel runways, 25 Left and 25 Right. The Mexicana plane was carrying 153 people on a flight from Puerto Vallarte and Mexico City.

A few miles beyond the airport, controllers directed the Mexicana plane to "side-step" to Runway 25 Right, Abels said. The United Express plane was directed to continue to Runway 25 Left, Abels said.

The near-collision occurred before the Mexicana jet made the move, while both planes were five or six miles from the airport.

"The copilot spied it and alerted the captain," said Ruth Shari, a Mexicana spokeswoman. "The captain saw it, and they veered to the right, then came in for a landing."

But Westair Chairman Timothy P. Flynn told the Los Angeles Times the planes did not come closer than 1,000 feet.

"Westair was anticipating, apparently, that the Mexicana would move over, and it didn't happen," he said. He called it a "lack of communications between the Mexicana plane and the tower."

The last incident occurred at 10 p.m., as a Pacific Southwest Airlines BAE146 collided with an unoccupied Eastern Airlines L1011 parked away from the runway.

Abels said the PSA plane was being handled by mechanics to check the brakes, which failed at the time of the accident, Abels said. She said that the wing of one plane was slightly damaged and that the vertical fin was knocked off the PSA plane.