Accident Sequence
On the day of the accident, the DHC-6 Twin Otter, registration C-GKBM, was operating under contract to the U.S. Forest Service. The aircraft was being repositioned to John Day, Oregon, to pick up passengers. At 11:35 Pacific Daylight Time, the pilot contacted John Day Dispatch and stated an expected arrival time of 12:15. Approximately five minutes later, at about 11:40, the aircraft's right wing struck three trees at the 5,000-foot level of Battle Creek Mountain. The impact separated the wing into three sections. The aircraft then exited over the mountain edge and came to final rest on a ridgeline in a steep canyon to the east. The flight path included a high mountain further east that needed to be crossed before a descent to John Day could commence.
Flight Path and Pilot Condition
Radar track data showed a gradual descent of about 400 feet per minute. The aircraft was off course to the right for approximately five minutes before impacting the trees. The pilot's medical records indicated he had been experiencing various medical problems, including chronic muscular neck pains, back problems, episodes of falling asleep, allergy issues, numbness in the tops of his feet, general fatigue, and leg pain. These records also noted that the pilot did not want these conditions brought to the attention of the FAA-designated medical examiner and did not inform his supervisor about his health issues. The flight track and medical history strongly supported a high probability that the pilot experienced sleep-induced unconsciousness.
Probable Cause
The official investigation determined that the most probable cause of the accident was the pilot's acute in-flight incapacitation due to sleep.