Synopsis
During an en route cruise flight at an assigned altitude of 11,000 feet in instrument meteorological conditions (IMC), the airplane struck mountainous terrain at 5,300 feet. The aircraft was in wings-level, descending flight at the time of impact.
Flight History
According to recorded military search radar data, the aircraft maintained a steady descent from 11,000 to 5,600 feet over the final 12 minutes of the flight, from 1046 to 1058 Pacific daylight time, at which point radar contact was lost. However, Mode C altitude information received by air traffic control facilities indicated the aircraft remained level at 11,000 feet during this same period.
At 1055:49, after being handed off from SoCal TRACON to Los Angeles Center, the pilot reported, "level at one one thousand." At 1057:28, the pilot queried the controller, asking "what altitude you showing us at." The controller replied that the aircraft's Mode C was not being received. At 1057:37, the pilot transmitted, "o k we'd like to climb to vfr on top, our uh altimeter just went down to uh fifty three hundred." The controller approved the request to climb to VFR conditions on top, and at 1057:54 the pilot responded, "roger we're out." No further communications were received from the aircraft.
Investigation
Post-accident examination revealed that the airplane was equipped with a single instrument static pressure system, comprising two heated static ports. Both the static system and its associated instruments were damaged or destroyed by the impact and subsequent fire, preventing functional testing after the accident.
Probable Cause
The official probable cause of the accident was determined to be total blockage of the instrument static system due to ice.