History of Flight
On May 23, 2016, about 2156 central daylight time, a Cessna 501, registration N804ST, experienced a loss of cabin pressurization while cruising at flight level 430. The airplane entered an uncontrolled descent. The pilot regained consciousness and control of the airplane at approximately 7,000 feet. The pilot subsequently landed the airplane without further incident at Texarkana Regional Airport-Webb Field (TXK), Texarkana, Arkansas. The airplane sustained substantial damage to both wings due to excessive aerodynamic loads during the descent and recovery. The airline transport pilot and two passengers sustained minor injuries, and one passenger was uninjured. The flight was operated by Eagle II Aero LLC under 14 Code of Federal Regulations Part 91 as an executive/corporate flight on an instrument flight rules flight plan. Night meteorological conditions prevailed. The flight departed from Perryville Municipal Airport (K02) about 2056 and was destined for San Antonio International Airport (SAT).
According to air traffic control (ATC) communications, the airplane made initial contact with Memphis Center at 2056, reporting climbing through 9,200 feet. The airplane responded to ATC transmissions while climbing to and leveling off at flight level 430. At 2154, Fort Worth Center called N804ST without response. After multiple attempts, at 2156, Memphis Center instructed American Airlines flight 93 to attempt contact on guard frequency. N804ST then reported an explosive decompression and stated the airplane was under control. The pilot requested time to "figure this out" and elected to land at TXK. A visual approach and landing to runway 13 was performed without further incident.
The pilot reported in a written statement that within 20 seconds, the airplane lost all cabin pressurization, with cabin altitude rising from 8,000 feet and 8.5 psi differential to ambient altitude and 0 psid. All occupants lost consciousness. The pilot reported not wearing an airplane supplemental oxygen system crew mask during the flight.
Personnel Information
The pilot held an airline transport pilot certificate with multiple type ratings, including CE-500, CE-525, CE-650, among others. He completed single-pilot exemption training for CE-500, CE-550, and CE-560 aircraft on November 15, 2014. His last flight review was May 11, 2016. The pilot reported a total flight time of 15,000 hours, with 5,000 hours in the accident airplane make and model.
Aircraft Information
The Cessna 501 was certified under Part 23 with two Pratt & Whitney Canada JT15D-1A or JT15D-1B turbofan engines, with a maximum operating altitude of 41,000 feet. The accident airplane was modified by supplemental type certificates held by Sierra Industries Ltd., increasing the maximum operating altitude to 43,000 feet. One STC (SA732NW) included installation of the Eagle SP performance system with recontoured wing leading edge, wing tip extension, increased gross weight, and increased fuel capacity. Another STC (ST09559AC) installed Williams International FJ44-2A engines (or FJ44-3A for other models) and increased the maximum operating altitude.
The most recent maintenance was a Phase 5 inspection, documented in a Sierra Industries Ltd. Maintenance Transaction Report dated April 15, 2016, at 7,424.8 aircraft hours. The airplane oxygen system supplies breathing oxygen to the crew at all times and to passengers when required. Passenger masks can be manually dropped or automatically deploy at 14,800 ±200 feet cabin altitude.
Wreckage and Impact Information
Post-accident examination revealed the cockpit supplemental oxygen supply gauge indicated about 17 psi x100, within the green arc. The cockpit supplemental oxygen system switch was in the "Normal" position, though the pilot stated it had been in the "Crew" position and he later changed it. Cabin oxygen masks were not deployed.
Examination of the aft pressure bulkhead check valve revealed that one half of its non-metallic flapper was fractured into several pieces, found on the fuselage floor. The other half remained intact in the valve body. The air conditioning system's primary pressurization duct was separated from its connection with the water separator; the metal worm-gear retaining clamp was found around the duct but not attached. The clamp's retaining screw was in place and the clamp was not fractured. These components were sent to the National Transportation Safety Board Material Laboratory for examination.
Tests and Research
Material laboratory examination identified the check valve flapper halves as fabricated from glass-filled polybutylene terephthalate (PBT) by injection molding. The fractured flapper (flapper 1) exhibited radially-oriented cracks originating near its center, with crack propagation from the downstream side to the upstream side due to bending stresses. Some crack surfaces exhibited yellow discoloration, consistent with slow crack growth over time. The intact flapper (flapper 2) also had a primary crack and a missing hinge portion, with similar yellow discoloration indicative of progressive cracking. The hose clamp was consistent with an SAE J1508 Type F size 48 worm-gear style clamp.
Regulations
Title 14 CFR Part 91.211 requires that at flight altitudes above FL 350, one pilot at the controls must wear and use an oxygen mask that is secured and sealed, supplying oxygen at all times or automatically when cabin altitude exceeds 14,000 feet, with exceptions for two-pilot operations below FL 410.