WASHINGTON - Nearly a year after John F. Kennedy Jr.'s plane spiraled into the ocean, killing him, his wife and her sister, federal investigators have concluded what they suspected from the start: The son of the late president became disoriented in the night sky and lost control of the aircraft.
The National Transportation Safety Board, in a report that may be released as early as next month, is expected to conclude that pilot error caused the July 16, 1999, crash off Martha's Vineyard, Mass., according to government and industry sources with close knowledge of the investigation.
The wording in the report, however, is expected to blame the crash on ''spatial disorientation,'' confusion in the brain that results from a loss of balance in the inner ear, said the sources, all of whom spoke on the condition of anonymity.
Killed were Kennedy, 38, his wife, Carolyn Bessette Kennedy, 33, and her 34-year-old sister, Lauren Bessette.
''I think it's pretty much what we had been thinking,'' one source said of the findings.
Federal investigators found no problems with Kennedy's Piper Saratoga II, and autopsies on all three victims found no abnormalities.
Contributing to Kennedy's disorientation, said the sources, were the night sky, a lack of visible horizon over the open water and a haze that blanketed Kennedy's flight path from New Jersey over Long Island Sound to the Vineyard.
Without visual references, a pilot can easily become disoriented boring through the night sky. One pilot likened the conditions Kennedy faced to riding Walt Disney's ''Space Mountain'' ride, a twisting, turning roller coaster, in the dark.
The safety board is not expected to make any sweeping safety recommendations, except more publicity about spatial disorientation and the need for training to cope with it, one source said.
''You can recognize that and stop chasing cues that are misleading because your ear is screwed up,'' the source said.
Typically, pilots in trouble are taught to ignore physical sensations and instead follow a recovery regime that includes focusing on key navigation equipment. Kennedy, who received his pilot's license in April 1998, did not have an instrument rating.
''What happened to him we've seen happen to disoriented pilots,'' said one source. ''The more experience you have, the more you can cope with it, but experienced pilots can get disoriented themselves.''
Kennedy, the namesake and son of the nation's 35th president, and his wife had been planning to drop off her sister before flying onto Cape Cod for a family wedding the following day. Instead, their plane mysteriously plunged out of the sky.
After an intensive search by the Coast Guard, Navy and National Oceanographic and Atmospheric Administration, the plane - its passengers still strapped inside - was found July 21 about 7 miles off shore.
In the aftermath of the crash, there was a debate about whether pilots without an instrument rating should be allowed to fly at night.
The practice is legal in the United States as long as the pilot has visibility of three miles, but a basic license requires only three hours of night flying - including 10 takeoffs and landings - plus a 100-mile night flight and three hours of instrument training.
In Canada, by contrast, pilots seeking to fly at night must not only have a basic license, but an additional 20 hours of flying - 10 hours at night and 10 hours using instruments.
Investigators initially thought Kennedy had as few as 100 hours in the cockpit. They were later able to determine that he had more than 300.
''It was not irresponsible in any way for him to have booked the flight and tried to fly it,'' said one source. ''For what he knew of the conditions, he was certainly qualified to take the flight.''
A spokesman for the Kennedy family declined comment on the news.
