What happened
During a night cargo operation transporting mining equipment, a CFR Part 135 flight was cruising at 12,000 feet MSL over remote, mountainous territory when the right engine and propeller experienced an overspeed condition. In response, the pilot feathered the number two engine and declared an emergency. An attempt to divert to an alternate airport located 120 miles away in lower terrain was unsuccessful because the aircraft could not maintain its altitude, as the single-engine service ceiling for the current load was approximately 8,700 feet.
As the pilot increased power to the left engine to maintain flight, the engine began exhibiting abnormal noises, including banging and coughing. The pilot then decided to execute an emergency landing at a nearby, remote military airfield. This specific airfield required a daylight-only, one-way approach and was situated in mountainous terrain. During the visual approach, while the pilot attempted to maintain sight of the runway end identifier lights, the aircraft encountered severe turbulence. In an effort to climb, the pilot applied full throttle to the left engine, but the aircraft lost sight of the lights and struck snow-covered terrain approximately 2 miles west of the airfield. Ground personnel at the time reported that the area was experiencing high winds and blowing snow, which limited visibility.
Findings
- A post-crash inspection of the right engine identified a loss of hydraulic oil within the propeller control system.
- The flight was conducted at 12,000 feet without the use of crew oxygen.
- The crew had exceeded their maximum allowable duty day without receiving adequate rest.