Background
Approximately 26 minutes after takeoff, while cruising at 21,500 feet, the pilot requested a non-existent route from air traffic control. Seven minutes later, the passenger—who held a former student pilot certificate and had accumulated 73 flight hours in Cessna 150/152 aircraft about five years earlier—reported that the pilot felt lightheaded and was fading, and subsequently lost consciousness.
Events
Air traffic controllers and other pilots on the frequency attempted to assist the passenger. She was advised to administer oxygen to herself and the pilot but was unable to do so. The airplane continued climbing to 34,200 feet, where it departed controlled flight, recovered, and then departed controlled flight several more times before initiating a nose-low descent. Witnesses reported hearing the airplane orbiting multiple times while above a cloud layer, then observed it orbiting beneath the clouds. The aircraft impacted the ground in a descending right-wing-low attitude and came to rest submerged in a pond.
Investigation Findings
Examination of the flight controls, engines, and propellers revealed no evidence of preimpact failure or malfunction. However, a discrepancy with the regulating valve was noted. Two small holes were found in the cabin door seal. The left wing pressurization duct had been replaced approximately eight years earlier, but the right wing pressurization duct had not been replaced; both ducts are classified as on-condition components. No preimpact failure or malfunction was found with the barometric pressure switch, cabin altitude annunciator bulbs, safety valve, solenoid valve, or differential pressure/cabin altitude gauge. Testing for carbon monoxide was negative for both the pilot and passenger.
Probable Cause
The official probable cause was stated as: Inadequate maintenance of the cabin pressurization system, which resulted in inadequate pressurization and incapacitation of the pilot due to hypoxia. Also causal was the pilot's failure to adequately monitor the cabin pressurization system.