What happened
On October 25, 1999, a Learjet Model 35, registration N47BA, was traveling from Orlando, Florida, toward Dallas, Texas. During the flight, after receiving clearance to climb to flight level 390, the aircraft disappeared from air traffic control communications while positioned north of Gainesville, Florida.
As the aircraft moved in a northwest direction, it was intercepted by various U.S. Air Force and Air National Guard fighter jets. Pilots from these military units reported that the forward windshields of the Learjet 35 appeared to be obscured by condensation or frost, though they could not see into the passenger cabin. No structural damage or external irregularities were noted by the intercepting pilots.
The aircraft eventually entered an uncontrolled spiral, exiting controlled flight before striking an open field near Aberdeen, South Dakota. The impact resulted in the total destruction of the airplane and six fatalities, including the pilot, first officer, and four passengers.
Findings
The investigation concluded that the crew members became incapacitated due to a failure to receive supplemental oxygen following an unidentified loss of cabin pressure.