Federal officials investigating the private plane crash Aug. 2 that killed Richard Obershain, then the Virginia Republican nominee for the U.S. Senate, have found that the pilots had little night-time flying experience and that a key navigational aid at Chesterfield County Airport was inoperative.

Those facts, and many others, are contained in a report released yesterday by the National Transportation Safety Board. The board will use the report to determine officially the probable cause of the accident.

It is clear that Obenshain, in the tradition of campaigning Virginia politicians, placed his life in the hands of an inexperienced crew. It is possible that a pilot not fully qualified under federal regulations was acting as the pilot in command.

Only three people were on the plane when it crashed into the trees about 11 p.m. near the Chesterfield County Airport southwest of Richmond. In addition to Obenshain, 42, they were pilots Richard F. Neel, 42, of Alexandria and Ronald Alan Edelon, 27, of Camp Springs in Prince Georges County. It has never been determined which pilot was actually in control of the plane.

Before the plane took off from Winchester, however, Neel told Obenshain that he was receiving instruction from Edelon on his instrument rating and asked if Obenshain minded, according to the report. Obenshain said he did not.

Neel, who had at most 19 hours total of night-time flying, did not meet the requirements of being pilot-in-command at the time of the accident. His medical certificate had expired and he did not have the required number of night-time takeoffs and landings in the 90 days preceding the flight, the report said.

Edelon had the appropriate pilot ratings, but only 12 hours of night-time operations in a multi-engine airplane and a total of 42 hours of night-time flying in any kind of plane. "Inexperienced is a fair characterization," an experienced general aviation pilot said yesterday.

It took Neel and Edelon two practice runs to get into the Winchester airport to pick up Obenshain in the first place. They made the two passes "to get the lay of the land," one of them told witnesses.

They encountered fog and clouds on the route to Chesterfield, and once told the Richmond air traffic control center they were lost. The center gave them some help on the radar, got them relocated and put them within sight of the Chesterfield airport.

"We have it in sight, we're goin' in," the crew responded. The crew then canceled its radar help. The plane made one pass over the aiport, turned to the north, began to lose altitude and crashed into the trees.

Investigators found no problems with the plane's engines.

A navigational aid at the Chesterfield Airport, a device known as a Visual Approach Slope Indicator (VASI), had been out of operation for two days because of a blown fuse, according to the report. A VASI is an arrangement of lights that tells a pilot approaching a runway whether he is too high or too low. The Federal Aviation Administration said yesterday that the VASI was owned by the Chesterfield Airport and the maintenance responsibility was the airport's, not the FAA's.

On a night with bad visibility, a VASI could be particularly useful landing an airplane, according to knowledgeable aviation sources.