Accident Details
The aircraft was operating under an instrument flight rules (IFR) flight plan when it collided with mountainous terrain in instrument meteorological conditions. According to radar data, the flight had been en route for nearly 5 hours and was about 53 nautical miles from its destination airport at the time of the accident. The last radar hit at 1711:28 showed the airplane at 13,200 feet mean sea level (msl), descending from 13,800 feet 12 seconds earlier, an average rate of approximately 3,000 feet per minute. The airplane impacted upsloping terrain at 5,770 feet msl in a nose-down attitude, roughly 2 miles from its last recorded radar position.
Weather Conditions
A Safety Board weather analysis determined that the aircraft encountered rime icing conditions, a mountain wave induced downdraft greater than 5,000 feet per minute, and severe turbulence. An airport 23 nautical miles from the crash site reported surface winds of 19 knots gusting to 32 knots, visibility between 0.5 and 1.0 miles in moderate snow and freezing fog, vertical visibility between 100 and 900 feet, temperature 30°F, and altimeter setting 29.78 inches of Mercury. A witness located 1 mile from the accident site reported a mixture of snow and rain (sleet) blowing nearly horizontally, which prevented her from seeing the impact.
Pilot's Actions
Over the final 2 hours of the flight, the pilot experienced difficulty maintaining assigned altitude and reported moderate turbulence to controllers. On one occasion, the flight descended below the minimum en route altitude and struggled to climb back to the assigned cruise altitude, but continued toward the planned destination. Just before the accident, the flight's clearance was to maintain 14,000 feet. The pilot had called a Flight Service Station (FSS) for a weather briefing prior to departure, stating he had already used DUATS and needed an overview. FSS personnel informed him of forecast thunderstorms and snow showers along the route, including the destination area. No specific mention of turbulence or mountain wave conditions was provided.
Meteorological Information
The National Weather Service's AIRMET in effect at the time had not been upgraded to mention severe turbulence or mountain wave conditions, despite pilot reports of such conditions over a wide area. The aircraft was turbocharged and equipped with supplemental oxygen, capable of cruising at 20,000 feet msl, but flight into known icing was prohibited and the airplane lacked anti-ice or deice equipment. Four days prior to the accident, the pilot's personal logbook indicated he had flown the same airplane into moderate icing conditions.