Cessna 501 N804ST accident near Texarkana, Arkansas

Probable cause

The pilot's failure to use supplemental oxygen as required during high-altitude flight, which resulted in his loss of consciousness following a loss of cabin pressurization. Contributing to the accident was the separation of the air conditioning system's primary pressurization duct and the subsequent failure of the aft pressure bulkhead check valve flapper due to progressive weakening from age.

NTSB analysis

The small jet airplane, which was being operated by a single pilot, experienced a loss of cabin pressurization during cruise flight about 43,000 ft. The pilot and passengers lost consciousness and the airplane entered an uncontrolled descent. The pilot regained consciousness and control of the airplane at 7,000 ft and landed the airplane at an alternate destination without further incident. The airplane sustained substantial damage to both wings due to the excessive aerodynamic loads incurred during the descent/recovery. Given the airplane's cruise altitude, the pilot was required by Federal Aviation Regulations to wear a supplemental oxygen mask; however, he was not doing so at the time of the loss of pressurization.

Postaccident examination of the airplane revealed that the aft pressure bulkhead check valve flapper was fractured. Additionally, the air conditioning system's primary pressurization duct leading to the cabin was separated from its connection with the water separator due to a loose retaining clamp on the duct's metal worm-gear. Material analysis of the fractured flapper valve indicated the presence of progressive cracking and a loss of mechanical properties due to physical aging; exposure to atmospheric pollutants and oxygen can degrade the flapper material.

The airplane lost pressurization when the air conditioning system's primary pressurization duct separated and the pressurized air was discharged into the unpressurized section of the airplane. The subsequent failure of the flapper valve then allowed the pressurized air to escape the cabin.

Text from the NTSB. Open the NTSB docket for CEN16LA197 for the full record.