Disappearance from Radar
The aircraft disappeared from air traffic control (ATC) radar immediately after being cleared to descend from 12,000 feet to 10,000 feet. Radar data showed the aircraft at altitudes between 11,900 and 12,000 feet for approximately the last six minutes before the disappearance. The last Mode C altitude reading, transmitted with discrete transponder code 2402 at 0221:03, was 11,800 feet. The final three radar returns, showing 12,000, 11,900, and 11,800 feet, were consistent with the start of a descent from 12,000 feet. However, due to construction at the radar antenna site, the area where the aircraft disappeared was blocked from radar coverage. Subsequent attempts by the controller to contact the aircraft were unsuccessful. An overcast layer at 7,000 to 7,500 feet was reported at the destination, along with lower clouds and precipitation.
Wreckage Recovery and Examination
The wreckage was not located for approximately two months. Documentation at the accident site indicated an approximate level flight path from broken treetops to the area of ground impact. The aircraft struck a northwest-facing 60-degree mountain slope at approximately the 5,600-foot level. Post-accident examination of the aircraft's Bendix/King KEA130A (United Instruments 5035PB-P57) encoding altimeter revealed needle impressions on the indicator face and pointers consistent with an altitude indication of 12,620 feet. The internal components of the unit were severely damaged, and a functional test was not possible. The aircraft was also equipped with a copilot's altimeter, but only the faceplate was recovered and examined. One impression on the main faceplate showed a needle impression by the 100-foot pointer consistent with 560 feet; the position of the 1,000-foot pointer could not be determined. No evidence of a mechanical failure or malfunction at the time of the accident was found.
Pilot History and Fatigue
The pilot's four-day activity history revealed that he was returning from a demanding three-day work assignment. His recent sleeping schedule was irregular, and he had been awake for approximately 20 hours at the time of the accident.
Probable Cause
The official probable cause was determined to be the pilot's failure to maintain terrain clearance for undetermined reasons. Contributing factors were the mountainous terrain, trees, dark night conditions, clouds, and pilot fatigue.