On the morning of 25 October 1999, a small charter jet lifted off from Orlando bound for Dallas. It was an ordinary hop for six ordinary reasons, a golfer, his agents, a course designer, and two pilots, all with places to be. Within minutes, something went quietly and catastrophically wrong, and the aircraft carried its passengers onward in silence for nearly four hours.

The flight of Learjet 35 N47BA has since become one of aviation's most sobering case studies: an airplane flying itself, perfectly and pitilessly, across half a continent while the nation watched. Among those aboard was Payne Stewart, the plus-fours-wearing, US Open-winning showman of American golf, only weeks removed from a Ryder Cup triumph.

What follows is not a mystery so much as a warning, one the National Transportation Safety Board turned into lasting change. It is told here with care, because six people did not come home that day.

A routine departure

At 13:19 UTC, N47BA rolled off the runway at Orlando International on the first leg of a two-day, five-flight trip. The Learjet 35 was a proven business jet, fuelled with enough Jet A for about four hours and 45 minutes aloft. In the cockpit sat 42-year-old captain Michael Kling, a former Air Force tanker and E-3 Sentry pilot, and 27-year-old first officer Stephanie Bellegarrigue, a certified flight instructor. In the cabin were Payne Stewart; his agents Robert Fraley and Van Ardan; and Bruce Borland, a golf-course architect with Jack Nicklaus's design company.

Eight minutes after departure, Jacksonville air traffic control cleared the jet to climb to 39,000 feet. The pilot read the clearance back calmly, three nine zero bravo alpha, as the aircraft passed through 23,000 feet. It was the last word anyone would hear from N47BA.

The silence begins

Six minutes later, a controller called the Learjet and heard nothing. He tried again, and again, five more times over the next four and a half minutes. The jet kept climbing past its assigned altitude, then failed to turn west toward Texas, holding instead a steady northwesterly course as though nothing were wrong. In truth, everything was.

Somewhere in those first minutes the cabin had lost pressurization. At altitude, without supplemental oxygen, the human body has only seconds of useful consciousness. The NTSB later concluded that everyone aboard was quickly overcome by hypoxia, a quiet, painless loss of awareness that would have set in before anyone fully grasped the emergency.

The intercept

As the unresponsive jet tracked across the American South and Midwest, the military moved to take a look. Around 14:54 UTC, an Air Force F-16 test pilot from Eglin Air Force Base closed to within about 2,000 feet. He found no damage. Both engines were running, the anti-collision beacon was flashing, but the cockpit windshields were opaque, as if glazed with ice or condensation from the inside, and he could see no movement at the controls.

Over the hours that followed, more fighters took their turn shadowing N47BA: two F-16s of the Oklahoma Air National Guard, then two more from the North Dakota Air National Guard. Their radio reports carried the same eerie refrain, a dark, still cockpit and control surfaces that never twitched.

Mayday: Air Disaster reconstructs the military intercepts of Payne Stewart's Learjet.

The Pentagon later confirmed the fighters were not armed with air-to-air missiles, and no shoot-down was carried out. In Canada, Prime Minister Jean Chretien wrote in his memoirs that he authorized his air command to bring the jet down if it crossed into Canadian airspace and threatened Winnipeg, a decision he never had to enforce.

The final minutes

The jet's older cockpit voice recorder preserved only the last 30 minutes. At 17:10:41 UTC, its microphones caught the engines winding down as the fuel finally ran out. With no thrust, the autopilot tried to hold altitude until the airspeed bled away toward a stall, the stick shaker fired, and the automation disconnected. The Learjet rolled into an uncontrolled spiral.

Impact came at roughly 17:13 UTC, 12:13 local, after a total flight time of three hours and 54 minutes and some 1,500 miles. The aircraft struck a grassy field just outside Mina, in Edmunds County, South Dakota, near Aberdeen, at nearly supersonic speed. All six aboard were killed. It was, by any measure, unsurvivable.

What the NTSB found, and changed

Because of the extraordinary circumstances, the NTSB conducted a full major investigation. Its finding was as precise as it was unsettling: the crew had been incapacitated after failing to receive supplemental oxygen following a loss of cabin pressure, and the reason for that failure could not be determined. Research the Board cited showed that as little as eight seconds without oxygen after a rapid decompression at altitude can badly impair the mind.

Reconstructed air traffic control audio from the N47BA flight of 25 October 1999.

The report also flagged a pattern of cabin-pressure maintenance issues in the months before the crash. In the years since, the accident sharpened industry and pilot awareness of decompression and hypoxia, reinforcing the hard lesson that when a cabin-altitude warning sounds, the oxygen mask comes first, before any attempt to diagnose the problem.

A short documentary on the South Dakota Learjet crash (Fascinating Horror).

Remembering the six

Payne Stewart was memorialized with a lone bagpiper at that week's Tour Championship, and the following year 21 of his fellow professionals hit balls into the Pacific in a salute at Pebble Beach. The landowner near Mina, in consultation with the families, set aside about an acre for a quiet memorial, a stone from the site inscribed with six names. It is the right note to end on: not the spectacle of a ghost flight, but the people it carried, and the safeguards their loss helped strengthen.

Sources: Wikipedia (1999 South Dakota Learjet crash), NTSB Aircraft Accident Brief NTSB/AAB-00/01, bunkered.co.uk, UPI Archives