What happened
On May 23, 2016, a Cessna 501, registration N804ST, was operating an instrument flight rules (IFR) corporate flight from Perryville Municipal Airport, Missouri, to San Antonio International Airport, Texas. While cruising at flight level 430, the aircraft experienced a loss of cabin pressurization.
The pilot reported that within 20 seconds, the cabin altitude rose from 8,000 feet to the ambient altitude, and the cabin pressure differential dropped from 8.5 psi to 0 psi. During this event, the pilot and all passengers lost consciousness. The aircraft entered an uncontrolled descent, and the pilot regained consciousness and control of the airplane at 7,000 feet.
The pilot subsequently performed a visual approach and landed the aircraft at Texarkana Regional Airport-Webb Field (TXK) in Arkansas. The aircraft sustained substantial damage to both wings due to excessive aerodynamic loads during the descent and recovery phase. The flight involved the pilot and three passengers; the pilot and two passengers sustained minor injuries, while one passenger was uninjured.
The investigation
Post-accident examination of the aircraft revealed two primary mechanical failures. First, the primary pressurization duct of the air conditioning system, which leads to the cabin, had separated from its connection to the water separator. The metal worm-gear retaining clamp on this duct was found resting away from its attachment point.
Second, investigators found that the aft pressure bulkhead check valve flapper had fractured. One half of the non-metallic flapper was found in pieces on the fuselage floor. Laboratory analysis of the flapper, which was made of glass-filled polybutylene terephthalate (PBT), showed evidence of progressive cracking and a loss of mechanical properties caused by physical aging.
Additionally, investigators noted that the pilot was not wearing a supplemental oxygen mask during the flight, despite being at an altitude where regulations require its use. While the cockpit oxygen switch was found in the "Normal" position, the pilot stated he had moved it to the "Crew" position after the accident.