Accident Overview
A helicopter conducting a water drop on a wildfire collided with terrain after losing power in one engine. The accident occurred as the aircraft was flying along a mountain ridgeline to make a drop. Ground crews observed smoke emanating from the right engine before the helicopter executed a left descending turn and impacted downsloping mountainous terrain. A trailing pilot reported seeing the helicopter about 150 feet above the ridgeline before it made a sudden left descending turn; he did not observe the pilot jettison the water or bucket.
Engine Examination
A teardown inspection and metallurgical examination of both power sections were conducted. Examination of the No. 1 power section compressor turbine (CT) disc revealed fractures in the firtree serrations adjacent to blade positions No. 24 and 25, with additional fractures at the blade roots for positions No. 27 through 29. The failure of the CT disc was attributed to cyclic stress rupture resulting from extended and repeated operation of the engine at, near, or above its temperature and power limits. Dimensional measurements of the blades showed growth and deformation of the disk in the fractured areas. No material, manufacture, or design deficiencies were identified.
Examination of the No. 2 power section found that the intermediate drive shaft fractured in a counterclockwise direction due to sudden stoppage of the left engine while it was operating at a high power level.
Weight and Performance Considerations
Due to the degree of destruction and lack of dispatch records, investigators could not accurately determine the helicopter's operating weight at the time of the accident. However, for the 9,500-foot density altitude, it was believed that the helicopter's weight, including the water load, resulted in marginal single-engine capability at best.
Electrical System and Water Bucket Release
The accident helicopter had been modified with a water bucket and long line system. The circuit breakers for the long line, water bucket, and the emergency electrical release were connected to the nonessential bus, installed under a Form 337 field approval. According to the helicopter manufacturer, the electrical system is designed such that if one generator or engine fails, both nonessential buses automatically drop offline. Consequently, the emergency electrical release for the water bucket and long line would become inoperable. Although an override switch on the electrical panel can restore power to the nonessential buses, based on the witness-reported timeline, it was considered unlikely that the pilot could have restored power in time to prevent the collision.