CEN16LA197

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.

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.

Materials Laboratory Examination of Check Valve and Hose Clamp The examination labeled the check valve flapper half pieces recovered from the fuselage floor as flapper 1, and the flapper that was intact in the valve body as flapper 2. Both flappers were fabricated from glass-filled polybutylene terephthalate (PBT). Based on the presence of an injection gate and ejection pin marks, the flappers were fabricated by injection molding. Based on unique artifacts molded into the surfaces of the flappers, both flappers were molded from the same mold cavity. Both flappers had the following number molded onto the downstream side surface, DSP 904-0012-3, and did not exhibit any other identifying marks such as a date code or mold cavity number. Flapper 1 exhibited radially-oriented cracks originating within its approximate center; two of the primary cracks were nearly bilaterally symmetric in shape consistent with possible flow patterns and knitting in the part from the mold filling process. In all instances, the cracks initiated on the downstream side and propagated through the thickness towards the upstream side—driven primarily by bending stresses on the flapper. One fragment from flapper 1 was selected for deeper examination, which showed the presence of networks of fine cracks (also known as surface checks, surface crazing, or craze cracks). A fine network of cracks was present adjacent to the main fracture surface. The primary fracture surface of the polymeric resin phase was smooth and glassy with hackle marks indicative of though-thickness crack propagation from the downstream side to the upstream side. The fracture surface exhibited yellow discoloration in areas along its length indicating that portions were pre-existing. Scanning electron microscope examination also revealed a fine network of secondary cracks along the second primary fracture. The hinge fragments for flapper 1 exhibited cracks that progressed radially outward from the inside surface to the outside surface. The fracture surfaces on two of the hinge cracks exhibited yellow discoloring, consistent with progressive crack development with time (slow crack growth). Flapper 2 exhibited a primary crack on the downstream side. A portion of one of the flapper hinges was also missing. Stereo-zoom microscopic evaluation indicated that a portion of one of the hinges fractured due to the development of a crack that initiated at the inner surface and propagated to the external surface. One of the fractures exhibited yellow discoloring, consistent with progressive crack development with time (slow crack growth). The inside surface length of the hose clamp is about 11.1 inch and corresponds to an approximately 3.5 inch diameter. The hose clamp design is consistent with a worm gear style with a safety collared screw and perforated band manufactured by C. McGunnigle Co. Inc. Kenilworth, NJ, under the Breeze Aero Seal ® trademark. The clamp is consistent with an SAE J1508 Type F size 48 (2 9/16 to 3 1/2 diameter size range). Part 91 Regulations on the Use of Supplemental Oxygen Part 91.211 Supplemental oxygen. (a)General. No person may operate a civil aircraft of U.S. registry – (b) Pressurized cabin aircraft. (1) No person may operate a civil aircraft of U.S. registry with a pressurized cabin - (i) At flight altitudes above FL 250 unless at least a 10-minute supply of supplemental oxygen, in addition to any oxygen required to satisfy paragraph (a) of this section, is available for each occupant of the aircraft for use in the event that a descent is necessitated by loss of cabin pressurization; and (ii) At flight altitudes above FL 350 unless one pilot at the controls of the airplane is wearing and using an oxygen mask that is secured and sealed and that either supplies oxygen at all times or automatically supplies oxygen whenever the cabin pressure altitude of the airplane exceeds 14,000 feet mean sea level, except that the one pilot need not wear and use an oxygen mask while at or below FL 410 if there are two pilots at the controls and each pilot has a quick-donning type of oxygen mask that can be placed on the face with one hand from the ready position within 5 seconds, supplying oxygen and properly secured and sealed. The pilot held an airline transport pilot certificate with the following type ratings: CE-500, CE-525, CE-650, MJ-300, BE-400, G-1159, DC-3, N-P51N, N-T28, T-33. On November 15, 2014, the pilot completed single-pilot exemption training including differences training for CE-500, CE-550, and CE-560 training. The pilot's last flight review was dated May 11, 2016, using a Beechcraft BE400. The pilot reported a total flight time of 15,000 hours, of which 5,000 hours were in the make and mode of the accident airplane. The airplane's type certificate, held by Textron Aviation Inc., shows that the Cessna 501 was certified under Part 23 with two Pratt & Whitney Aircraft of Canada, Ltd. JT15D-1A or JT15D-1B turbofan engines and had a maximum operating altitude of 41,000 feet. The accident airplane was modified from its type design by supplemental type certificates (STCs), held by Sierra Industries Ltd., which increased the maximum operating altitude to 43,000 feet. The two STCs were: SA732NW was for the installation of the Eagle SP performance system, including recontoured wing leading edge, wing tip extension, increased gross weight, increased fuel capacity, and other minor refinements. ST09559AC was for the installation of Williams International Company. L.L.C. FJ44-2A engines in Cessna 501/500 aircraft or Williams International Company, L.L.C. FJ44-3A engines in Cessna Model 550/S550 aircraft. This STC increased the maximum operating altitude to 43,000 feet. The Model 500 Maintenance Manual (Rev 39), Floor Panels – Inspection/Check called for the inspection of the air conditioning system in section D. Inspect Air Conditioning System: (1) Inspect all duct work and tubing for security of installation, evidence of damage or leakage, and (3) Inspect flapper check valves at air mixer assembly for ease of operation or evidence of binding. The most recent maintenance inspection of the airplane was a Phase 5 inspection. A copy of the maintenance checklist used for the inspection, Model 500 Maintenance Manual (Rev 37) had the following entries: "Date:" - no entry made, "Registration Number: N804ST", "Serial Number: 501-0146", and "Total Time: 7424.8". A Sierra Industries Ltd. Maintenance Transaction Report, dated April 15, 2016, stated, "Completed phase inspections in accordance with Cessna 500 maintenance manual chapter 5," at 7,424.8 aircraft hours, 6,411 landings, and no. 1 engine and no. 2 engine total hours were 846.8 hours and 539 cycles. The Hobbs meter following the accident indicated 0411.3 hours. The airplane oxygen system supplies breathing oxygen to the crew at all times and to the passengers when required. The pilot can manually drop the passenger masks by selecting manual drop on the oxygen control valve, or the passenger masks will automatically drop at 14,800 ±200 feet cabin altitude. The pilot can select crew only which will shutoff flow to the passengers. Post-accident examination of the airplane by the FAA coordinator revealed that the cockpit supplemental oxygen supply gauge indicated about 17 "psi x100," which was in the gauge's green arc of 16-18 psi x100. The cockpit supplemental oxygen system switch was in the "Normal" position (which would have provided oxygen to the cabin masks). The cabin oxygen masks were not deployed. The pilot stated to the FAA coordinator that the switch was in the "Crew" position, and he did not know how or why it was in the "Normal" position. The pilot later told the FAA coordinator that he changed the oxygen switch position after the accident. The examination revealed that the aft pressure bulkhead check valve flapper had half of its non-metallic flapper fractured into severa