What happened
Approximately 26 minutes into the flight, while cruising at 21,500 feet, the pilot contacted air traffic control requesting a route that did not exist. Shortly thereafter, a passenger on board reported that the pilot was experiencing lightheadedness and had lost consciousness. Despite efforts by air traffic controllers and other pilots on the frequency to assist, the passenger was unable to administer oxygen to the crew.
The aircraft reached an altitude of 34,200 feet, during which it experienced several instances of departing controlled flight before entering a nose-low descent. Observers noted the aircraft orbiting both above and below the cloud layer. The flight ended when the aircraft, in a right wing-low attitude, struck the ground and settled in a pond.
Findings
Post-accident inspections of the engines, propellers, and flight controls showed no signs of mechanical failure or malfunction prior to the impact. While testing for carbon monoxide yielded negative results, investigators identified a discrepancy with the regulating valve. Additionally, two small holes were discovered in the cabin door seal.
Investigation into the pressurization ducts revealed that while the left wing duct had been replaced eight years prior, the right wing duct had not been updated. The inadequate maintenance of the cabin pressurization system was identified as a primary factor, leading to insufficient pressure and subsequent pilot hypoxia.