CAYUCOS, Calif. (AP) _ Bill Vargas saw a flash of fire in the sky and knew there was nothing anyone could do. Before his eyes a jetliner with 43 aboard plunged into the hillside and exploded.

''I saw a streak drop out of the sky about 3,000 feet. It appeared to be a large flame heading straight down to the ground at a very high rate of speed,'' said Vargas, who was driving on Highway 46 through countryside lush with vineyards, oak trees and rolling green hills.

''It went behind the hill. As we were coming back up the hill, we saw black smoke erupt when it hit.

''It was obvious the plane was going to crash. There was no way the pilot was going to pull it out.''

The 39 passengers and five crew members aboard Pacific Southwest Airlines Flight 1771, a four-engine BAe-146 jet en route from Los Angeles to San Francisco, perished in the crash at 4:14 p.m. Monday.

This portion of San Luis Obispo County is dotted with vineyards, oak trees and rolling green hills, scenery so lovely that newspaper tycoon William Randolph Hearst built his spectacular hilltop palace, Hearst Castle, 15 miles to the north in San Simeon.

Mary Wiley was chatting with a friend on her father's sprawling cattle ranch in the Santa Lucia Mountains when she heard the plane crash and explode.

''We ran to the end of my home by the ridge, and there was one huge flash, then an explosion,'' Mrs. Wiley said. ''There were one or two flashes. We saw pieces of airplane and body parts all over the place.''

Bill Hartzell came home to his Santa Rita ranch about 175 miles northwest of Los Angeles to find it swarming with disaster workers, county sheriff's deputies and firefighters.

''The plane was coming straight down when it hit into a hillside,'' Hartzell said. ''The Sheriff's Department is closing off the area. There's airplane parts and body parts and luggage all over a 15-mile area. I don't know how anyone could have survived.''

Mrs. Wiley said at first she didn't think much of the explosion she heard.

''We have had a lot of military jets coming over lately, and when we heard (what sounded like) a sonic boom, I thought it was one of the planes,'' she said. ''But then my door started shaking, and my friend said, 'That wasn't a Navy jet.'''

Winery manager Jack Strauch was driving home as he came upon a crowd gathered around wreckage. A state Forestry Department employee asked if anyone could quickly scale the hill where more wreckage was scattered.

An experienced runner, Strauch volunteered, hoping to find survivors.

But when he came upon the site he saw that would be impossible. ''It was a mess ... ,'' he said, ''wreckage everywhere, purses hanging in trees.''

''The most poignant thing I found is a little Social Security card I handed back to the Forest Department. When this is all over, I'm going to find his family and see what I can do at Christmas for them ... I was connected to this person for a moment.''