WPR13FA070

The pilot’s failure to regain airplane control following a sudden rapid descent during cruise flight, which resulted in an exceedance of the design stress limits of the aircraft and led to an in-flight structural failure.

The private pilot and passenger departed on the 875-nautical-mile cross-country flight and leveled off at a cruise altitude of 24,000 feet mean seal level, which, based on the radar data, was accomplished with the use of the autopilot. About 1 hour 40 minutes after departure, the pilot contacted air traffic control personnel to request that he would "like to leave frequency for a couple of minutes." No further radio transmissions were made. About 20 seconds after the last transmission, the airplane banked to the right, continued in a spiral while rapidly descending, and subsequently broke apart. At no time during the flight did the pilot indicate that he was experiencing difficulty or request assistance. Just prior to departing from the flight path, the pilot made an entry of the engine parameters in a flight log, which appeared to be consistent with his other entries indicating the airplane was not experiencing any difficulties. Portions of the wings, along with the horizontal stabilizers and elevators, separated during the breakup sequence. Analysis of the fracture surfaces, along with the debris field distribution and radar data, revealed that the rapid descent resulted in an exceedance of the design stress limits of the airplane and led to an in-flight structural failure. The airplane sustained extensive damage after ground impact, and examination of the engine components and surviving primary airframe components did not reveal any mechanical malfunctions or failures that would have precluded normal operation. The airplane was flying on a flight path that the pilot was familiar with over largely unpopulated hilly terrain at the time of the upset. The clouds were well below his cruising altitude, giving the pilot reliable external visual cues should the airplane have experienced a failure of either the flight instruments or autopilot. Further, no turbulence was reported in the area. The airplane was equipped with a supplemental oxygen system, which the pilot likely had his mask plugged into and available in the unstowed position behind his seat; the passenger's mask was stowed under her seat. The airplane's autopilot could be disengaged by the pilot by depressing the appropriate mode switch, pushing the autopilot disengage switch on the control wheel, or turning off the autopilot switch on the control head. All autopilot servos were also equipped with a clutch mechanism that allowed the servo to be manually overridden by the pilot at any time. It is likely that the reason the pilot requested to "leave the frequency" was to leave his seat and attend to something in the airplane. While leaving his seat, it is plausible he inadvertently disconnected the autopilot and was unable to recover by the time he realized the deviation had occurred.

A complete examination report and structures factual report is contained in the public docket for this accident. Fuselage The upper portion cabin area had sustained significant crush damage and the skin was wrinkled and molded around the longerons. The vertical crush deformation was more prominent in the forward section of the airplane with the nose baggage compartment compressed into itself. The empennage vertical deformation was minimal from the baggage door area aft. There was no evidence of any forward motion of the airplane relative to ground at the time of impact. All of the examined fracture surfaces exhibited features consistent with overstress failures with no evidence of fatigue. The cockpit had sustained major crush deformation and it was not possible to determine reliable control positions. The landing gear and flap levers were in the retracted position. The static pressure selector valve was in the "normal source" position. The oxygen knob was aft one inch and bent downward, which would be near the off position. The pilot's oxygen mask was found tucked under the seat compartment in the stowed position and the passenger's mask and hoses were entangled around her in the wreckage. The oxygen microphone toggle switch was selected on. The oxygen mask was plugged in the quick connect on the pilot's side and there was a quick donning storage hook behind the pilot's seat (to his left). The oxygen tank valve was in the off position. The cockpit gauge lights were examined and the filament was stretched on the following lights: navigation, altitude hold, autopilot engaged, navigation function engaged, ILS (instrument landing system) inner marker beacon. The bulbs were broken or missing for the flight director and trim warning, making the illumination at the time of the event not possible to discern. The heading bug on the VOR (very high frequency omnidirectional range) gauge was set at 310-degrees, consistent with the direction of travel. All of the examined fracture surfaces of the left and right wing, vertical and horizontal stabilizer exhibited features consistent with overstress failures with no evidence of fatigue. Passenger Doors The main cabin door was open and all six locking pins were extended consistent with the fully latched position. The upper 16 inches of the door was bent about the center 2 locking pins. The door handle was in the stowed and latched position. Landing Gear The nose and main gears were in the retracted position. Left Engine A post accident examination of the left engine revealed that the firewall structure, engine mounts, exhaust stubs, propeller, starter generator, torque system, and the cowling remained attached, with severe impact damage and deformation. Severe impact damage of the casing precluded a complete disassembly of the turbine and gas generator sections. The engine was mechanically sectioned between the fuel nozzle ports and engine mount collar for separation of the gas generator and power sections. The compressor 1st stage was inspected through the inlet screen. There were no indications of operational distress. The combustion chamber liner displayed severe impact deformation around the upper circumference and there were no indications of pre-impact distress. Inspection of the compressor turbine guide vane ring revealed that the vane airfoils were intact. The compressor turbine shroud displayed light circumferential scoring consistent with contact with the compressor turbine blade tips during impact. The compressor turbine was partially disassembled revealing the downstream side disc outer diameter displayed very light circumferential rubbing due to contact with the power turbine guide vane ring and interstage baffle. The power turbine guide vane ring and interstage baffle were intact. The upstream side disc inner drum and baffle face displayed light circumferential rubbing consistent with contact with the compressor turbine. The power turbine shroud was radially deformed inward due to deformation of the exhaust duct. The shroud face displayed very light circumferential scoring consistent with contact with the power turbine blade tips. The power turbine blades around the upper circumference were deformed and fractured which appeared to be a result of contact with the deformed shroud housing. A fractured blade tip was recovered in the exhaust duct. The blade tip displayed light circumferential rubbing consistent with contact with the shroud. The fracture surface displayed coarse dendritic features characteristic of impact fracture. There were no indications of fatigue or any other progressive fracture mechanism. The reduction gearbox chip detector was clean of debris. The oil and fuel filter were both clean. Right Engine The right engine firewall structure, engine mounts, exhaust stubs, starter generator, torque system, and the cowling remained attached, with severe impact damage and deformation. Severe impact damage of the casing precluded a complete disassembly of the turbine and gas generator sections. The engine was mechanically sectioned between the fuel nozzle ports and engine mount collar for separation of the gas generator and power sections. The compressor 1st stage was inspected through the inlet screen revealing that there were no indications of operational distress. The compressor turbine guide vane ring was intact and displayed no indications of operational distress. The shroud displayed light circumferential scoring consistent with contact with the compressor turbine blade tips. The compressor turbine downstream side disc outer diameter displayed very light circumferential rubbing consistent with contact with the power turbine guide vane ring and interstage baffle. The power turbine housing displayed no indications of distress. The power turbine guide vane ring and interstage baffle were intact. The upstream side disc inner drum and baffle face displayed light circumferential rubbing consistent with contact with the compressor turbine. The power turbine shroud housing was radially deformed inward due deformation of the exhaust duct. The shroud face displayed very light circumferential scoring due to contact with the power turbine blade tips. The power turbine blades around the upper circumference were deformed and fractured which was apparently a result of contact with the deformed shroud housing. A fractured blade tip was recovered in the exhaust duct. The blade tip displayed light circumferential rubbing consistent with contact with the shroud. The fracture surface displayed coarse dendritic features characteristic of impact fracture. There were no indications of fatigue or any other progressive fracture mechanism. The reduction gear box chip detectors on both engines were clean of debris. Propellers Both propellers had very similar damage. Each one had one blade that was straight, one blade that had a very mild forward bend, and one blade that had a very sharp aft bend and a very sharp forward bend. The right propeller had separated from the engine and the left propeller had separated. According to the propeller manufacture, neither propeller appeared to be feathered at the time of impact. Both spinner domes were hydro-formed over the piston and counterweights indicating that the propellers were in a low-operating blade angle position (at or near flight idle position) at the time of impact. There were no discrepancies noted that would preclude normal operation. All damage was consistent with impact damage. The Clark County Corner of Las Vegas, Nevada completed an autopsy on the pilot and passenger. The examiner's pathological diagnosis as cause of death was noted as, "Blunt force injuries." The FAA Civil Aeromedical Institute (CAMI) performed toxicological screenings on the pilot. According to CAMI's report (#201200291001), the toxicological findings were negative for ethanol (alcohol). The anticoagulant warfarin