Background
The flight departed with 180 pounds of cargo after deplaning a passenger. Prior to the accident, three company pilots overheard the accident pilot report that he was 13 minutes from his destination.
Accident
The overdue aircraft was located by company airplanes about 700 feet MSL, on the east (downwind) side of an 890 feet msl pass. The airplane impacted on a 330-degree heading and perpendicular to the axis of the canyon/pass, which was oriented east-west, in a flat attitude. No ground scars led to the wreckage.
Investigation
No anomalies were found with the airplane. All blades on the propeller exhibited torsional twisting and leading edge gouging. A westerly wind of seven to nine knots existed, and numerous pilots reported that the mountain passes were not obscured by clouds.
Photographs from the pilot's camera depicted views of the accident canyon, with the pass and accident site above the altitude from which the photographs were taken. These photographs contained the date of the accident.
Numerous depressant and stimulant, over-the-counter cold and asthma medications were found in the pilot's flight bag. Toxicological tests detected several over-the-counter medications used for cold and asthma symptoms with illness effect of distraction or sensory disturbance. As a result, the condition for which the drugs were ingested may have also played a role in the accident.
Probable Cause
The official probable cause stated: Improper in-flight planning/decision by the pilot, and his failure to maintain sufficient altitude over mountainous terrain. Factors related to the accident were downdraft conditions, mountainous/hilly terrain, and the use of over-the-counter medications.