Accident Summary
A CFR Part 135 cargo flight operating under instrument flight rules (IFR) at night encountered an engine problem while cruising at 12,000 feet mean sea level (MSL) over remote, mountainous terrain. Approximately two hours after departure, the right engine and propeller began to overspeed. The captain feathered the right engine, declared an emergency, and initiated a diversion to an alternate destination about 120 miles away in lower terrain. However, the aircraft could not maintain altitude; the single-engine service ceiling with the load was approximately 8,700 feet MSL.
Sequence of Events
As the captain increased power to the left engine, it began producing banging and coughing noises. The captain then elected to perform an emergency landing at a nearby, remote military airfield (A/F) located in mountainous terrain. The airfield had a one-way, daylight-only approach procedure. The captain lowered the landing gear and flaps and began a visual approach, attempting to keep the runway end identifier lights (REIL) in view.
During the approach, the aircraft encountered severe turbulence. The captain applied full throttle to the left engine in an attempt to climb, but the REIL disappeared from view. The aircraft subsequently collided with snow-covered terrain approximately two miles west of the airfield. Ground personnel at the airfield reported high winds, blowing snow, and limited visibility at the time.
Findings
Post-crash examination of the right engine revealed a loss of the propeller control system hydraulic oil. The flight at 12,000 feet was conducted without crew oxygen. Additionally, the crew had exceeded their maximum allowable duty day without adequate crew rest.
Probable Cause
The probable cause was the loss of the right engine propeller control oil, which led to an overspeed of the right engine and propeller, and necessitated a shut-down of the right engine; and failure of the pilot to maintain adequate altitude/distance from terrain during visual approach for a precautionary landing at an alternate airport. Factors relating to the accident were: fluctuation of the left engine power, premature lowering of the airplane flaps, and an encounter with adverse weather conditions (including high winds, severe turbulence, and white-out conditions) during the approach.