Flight and Weather Conditions
About one hour into the flight, the pilot air-filed an IFR flight plan and requested an altitude of 13,000 feet; air traffic control assigned 16,000 feet. Ten minutes later, the pilot began making repeated requests for lower altitudes, but the controller advised that 16,000 feet was the minimum vectoring altitude for the next 60 miles or more. Weather forecasts indicated cloud bases at 6,000 feet (less than 1,000 feet AGL) and tops at 25,000 feet. AIRMETS predicted mountain obscuration, moderate mixed icing, and moderate to severe turbulence from the 7,000-foot freezing level to 26,000 feet. Winds aloft were from the west at 65 to 70 knots over the mountains. There was no evidence that the pilot obtained a preflight weather briefing.
Emergency Declaration and Collision
Two hours and 22 minutes after departure, the pilot declared an emergency due to an engine failure. Controllers vectored the aircraft toward the nearest airport, which was nine miles behind the aircraft. The aircraft subsequently collided with a mountain at the 10,900-foot level.
Pilot Background and Oxygen System
The pilot had recently moved to the West Coast and had limited experience flying over the Sierra Nevada Mountains. Examination of the cabin revealed that no oxygen tubes were plugged into the regulator panel, and the main oxygen supply tank in the empennage was in the off position.
Fuel and Engine Examination
The fuel selector was found positioned to the left main tank. The right main and fuselage auxiliary tanks showed hydraulic fuel cell bladder rupture and deformation to surrounding structures; the left main tank showed no such damage. Examination of the engine revealed no evidence of preimpact malfunction or failure.
Altitude Exposure and Hypoxia Risk
Review of flight timing showed that the pilot operated at mean sea level altitudes above 12,000 feet for more than 1 hour and 50 minutes, with about one hour spent between 15,000 and 16,000 feet. According to Section 8-1-2 of the Aeronautical Information Manual, pilot performance and judgment can seriously deteriorate within 15 minutes at 15,000 feet due to hypoxia.
No official probable cause was stated in the investigation report.