Accident Overview
The airplane struck a glacier at an elevation of 12,270 feet mean sea level (msl) following an attempt by the pilot to reverse course within a bowl-shaped valley in the Sierra Nevada mountain range. Analysis of the ground scar and crush angle measurements indicated that the aircraft descended in a near vertical nose-down attitude to the point of impact.
Terrain and Weather Conditions
The valley was surrounded by higher peaks and ridges. The accident site was located approximately 1.3 nautical miles southeast of the direct point-to-point course between the departure and destination airports. At the northeast end of the valley, an opening led down slope through lower mountains and valleys toward the departure point. Satellite imagery obtained after the airplane's departure showed a low layer of stratiform clouds over the mountains. The accident site was positioned under the northern edge of these clouds. The cloud tops were measured at 16,000 feet msl, with cloud bases between 10,000 and 14,000 feet msl. The bowl-shaped valley was roughly 2,450 feet in diameter near the tops of the higher peaks and about 1,200 feet in diameter at the elevation of the wreckage.
Turn Performance Analysis
Calculations based on the valley dimensions and typical airspeeds were performed. For a turn radius of 1,225 feet (derived from the upper valley diameter) at an airspeed of 104 knots (best rate of climb), a bank angle of 40 degrees would be required. At 156 knots (maneuvering airspeed), the required bank angle would increase to 65 degrees. Considering a smaller radius of 637.5 feet (based on the valley diameter at the accident site elevation) and 104 knots, the necessary bank angle would be at least 60 degrees. At 156 knots, this bank angle would be approximately 75 degrees. Stall speed data for the airplane indicated that the stall speed would be 85 knots at 40 degrees of bank and 105 knots indicated airspeed at 60 degrees of bank.
Examination Findings
A post-accident examination of the airplane and its engines revealed no evidence of mechanical malfunction or failure. Internal signatures in the propeller hubs showed that both engines were producing power symmetrically at the time of impact.