What happened
While operating in instrument meteorological conditions, an aircraft was cruising at a designated altitude of 11,000 feet MSL when it struck a mountain at 5,300 feet. The impact occurred while the aircraft was in a descending, wings-level flight path.
Radar data from military search sources indicates that during the final 12 minutes of the flight, the aircraft was actually in a continuous descent from 11,000 feet down to 5,600 feet, at which point radar contact was terminated. However, altitude data received via Mode C by Los Angeles Air Traffic Control Center and SoCal TRACON suggested the aircraft remained level at 11,000 feet.
At 10:55 PDT, the pilot reported being level at 11,000 feet during a handoff. Shortly after, at 10:57 PDT, the pilot inquired about the altitude being displayed by controllers, as the controller noted that Mode C data was no longer being received. The pilot then notified the controller of an intention to climb to VFR-on-top conditions, noting that the aircraft's altimeter had dropped to 5,300 feet. Following the controller's approval of the climb, the pilot confirmed the maneuver, but no further communication was established.
Findings
Investigation of the aircraft's static pressure system, which included two heated static ports, was not possible because the impact and subsequent fire destroyed the instruments and the system. The discrepancy between the actual descent recorded by primary radar and the level altitude reported by Mode C suggests a malfunction in the altitude reporting equipment or static system.