All four persons aboard a single-engine airplane were killed this afternoon when their plane crashed and burned after attempting to take off from a private grassy air strip three miles north of here.

"They were just taking off," said Charles Foushee, 50, a truck driver from Culpeper. "I saw the plane in the air. It took a nose dive to the left. Then I looked up and it was gone."

The plane, a Piper Pacer, plunged into an empty field 1,000 feet from the airstrip.

"When it hit the ground, it was in flames," said Foushee, burying his head in his arms on the hood of a car. "It wouldn't have done any good to go over there."

State police Sgt. M.D. Dowdy identified the victims tonight as Ronald E. Smart; his wife, Susan Thayer Smart; their daughter, Cheryl Ann Smart, and a French exchange student, Sandaine Colas of Paris.

Ronald Smart was a professional pilot, friends at the scene said. Cheryl Smart would have been a senior this fall at Culpeper County High School.

Volunteer firefighter Robert Thomas said the crash touched off a brush fire that "covered a lot of ground. It covered the whole area . . . about the size of a baseball infield."

Mechanical difficulties were suspected as the cause of the crash, State Trooper W.O. Miller said.

He added that the plane's troubles may have been related to the heat and humidity, the type of plane, and its load.

The crash occurred at 12:30 p.m., as the plane took off from the privately owned airstrip on Berryvale Property Owners Association property, authorities said.

The plane appeared slow in leaving the ground, ascended to about 100 feet and plunged into an empty field owned by Joel Shankle. The crash site was off Virginia Rte. 625 near the intersection of Rte. 685.

Foushee, like most of the other 15 or so witnesses and neighbors who gathered at the edge of the field yesterday afternoon, was emotional.

"It tears me up," he said. "Four people out there and I couldn't do anything about it. No, I didn't know them. You don't have to know people when they die. It still hurts me so."

By midafternoon, investigators from the Federal Aviation Administration, the National Transportation Safety Board, the Virginia State Police, the county sheriff's office and local volunteer firefighters were at the twisted rubble. It lay in an acre-sized patch of oily and charred grass, bounded by yellow police tape.

The bare metal frame of the tail section was still intact, but all else was a jumble of burned metal. Slowly, the investigators pried it apart, removing pieces of the plane.

Several witnesses said they believed the pilot and passengers were en route to nearby Orange, Va., for lunch, but authorities would not confirm this.

Miller said the crash is believed to be the first at the Berryvale strip.

He did not know how long the runway has been there, or whether the plane made any radio contact with the ground before it crashed.

The landing strip is located in a subdivision of country-style homes that neighbors described as a popular place for commercial airline pilots and plane enthusiasts, many of whom work flights out of Dulles International Airport.