Aircraft Accident/Incident Report

Lebec, California 93203
Thursday, February 10, 2005 20:30 PST

During an instrument cross-country flight at night, the pilot encountered a mountain wave with severe turbulence at 9,000 feet mean sea level (msl) and entered an uncontrolled descent to impact with mountainous terrain at an elevation of 2,300 feet msl. The flight was level at 9,000 feet crossing a mountainous area when the pilot reported to air traffic control (ATC) controllers that he was encountering light rime icing conditions. Shortly thereafter, the controller observed the aircraft descending and the pilot asked for a lower altitude. The controller said the flight was already at the minimum altitude that could be assigned. The pilot responded that he was in extreme turbulence. The aircraft then dropped off radar and no further radio transmissions were received. The Area Forecast valid for the flight was predicting multiple layers of broken to overcast ceilings from 6,000 to 12,000 feet, with a freezing level at 10,000 feet. AIRMETS were in effect for mountain obscuration, moderate turbulence, and moderate rime or mixed icing conditions, though the conditions were forecast at elevations above the pilot's planned en route altitude. No SIGMETS were in effect for turbulence. No record was found that the pilot obtained a preflight or in-flight weather briefing from any official aviation weather source. An analysis of the weather conditions found increasing wind velocities from the surface to 7,000 feet over the mountains crossing the airplane's flight path, with several layers of strong vertical wind shear that increased the likelihood of very strong turbulence. In addition, the conditions were favorable for localized mountain wave activity, and supported the potential for moderate to severe turbulence, rotors, and downdrafts to 1,500 feet per minute. An automated weather observation station in the mountains very near the accident site was reporting wind gusts to 45 knots, 10 minutes prior to the accident. The pilot of a Gulfstream G-IV that was descending near the accident location reported encountering strong turbulence with both vertical and lateral components.

the pilot's in-flight loss of control due to the flight's encounter with unforecasted localized mountain wave activity with severe to potentially extreme turbulence, downdrafts, and rotors.