LAX03FA142 · Cessna TR182

The aircraft collided with mountainous terrain on an instrument flight rules (IFR) flight plan and in instrument flight conditions after encountering what a Safety Board weather analysis later established was rime icing conditions, a mountain wave induced downdraft greater than 5,000 feet per minute, and severe turbulence. Over the last 2 hours of the flight, the pilot had experienced difficulty maintaining his assigned altitude and had reported to controllers that he was in moderate turbulence. On one occasion the flight descended below the minimum en route altitude and had difficulty in climbing back to the assigned cruise altitude; however, the pilot continued flying toward his flight planned destination. Just before the accident, the flight's clearance was to maintain 14,000 feet. Recorded radar data indicates that at 1711:28 the airplane had descended to 13,800 feet, and 12 seconds later at the time of the last radar hit, the airplane had descended to 13,200 feet, which was an average descent rate of approximately 3,000 feet/minute. During the uncontrolled descent, the airplane impacted upsloping 5,770-foot mean sea level (msl) terrain in a nose down pitch attitude, about 2 miles from its last recorded radar location. The accident occurred about 53 nautical miles from the pilot's destination airport, and after the pilot had been en route nearly 5 hours. Prior to departure, the pilot called FSS for a weather briefing and told the specialist that he "had been on DUATS" and just needed an overview. FSS personnel informed the pilot that thunderstorms and snow showers were forecast over portions of his planned route including the general area encompassing his destination. No specific mention of turbulence or any mention of mountain wave conditions was provided to the pilot. The NWS's AIRMET that was in effect had not been upgraded to mention the possibility of severe turbulence or the mountain wave condition, despite pilot reports of these types of adverse weather conditions over a wide geographic area. Within a few minutes of the accident, an airport located 23 nautical miles from the crash site reported its surface wind was 19 knots with 32-knot gusts. The visibility was between 0.5 and 1.0 miles in moderate snow and freezing fog; the vertical visibility was between 100 and 900 feet; the temperature was 30 degrees Fahrenheit; and the altimeter setting was 29.78 inches of Mercury. A witness who was located 1 mile from the accident site reported a mixture of snow and rain (sleet) was present and was blowing nearly horizontally at the time, which precluded her from seeing the accident. The pilot's turbocharged and supplemental oxygen-equipped airplane had the capability of cruising at 20,000 feet msl. Flight into known icing conditions was prohibited, and the airplane was neither anti-ice nor deice equipped. Four days prior to the accident, according to the pilot's personal flight record logbook, he indicated having flown the accident airplane into moderate icing conditions.

Verbatim NTSB analysis and probable cause from the NTSB dataset