NTSB Identification: LAX00GA297.
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Accident occurred Sunday, August 13, 2000 in Cold Springs, NV
Probable Cause Approval Date: 06/08/2005
Aircraft: Bell 412, registration: N174EH
Injuries: 1 Fatal.
: NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this public aircraft accident report.
While flying along a mountain ridgeline to make a water drop on a wild fire, the helicopter lost power in one engine and collided with terrain as the pilot turned downslope toward a landing area. Ground crews watching the helicopter make its drop run observed smoke emanating from the right engine, then the helicopter made a left descending turn and impacted the downsloping mountainous terrain. A trailing pilot saw the helicopter about 150 feet above the ridgeline, then it made a sudden left descending turn. He did not see the pilot jettison either the water or the bucket. A teardown inspection and metallurgical examination of the No. 1 and No. 2 power sections was conducted. The examination of the No. 1 power section CT disc revealed that the firtree serrations adjacent to the No.s 24 and 25 blade positions were fractured above the blade retaining rivet hole, and that the No.s 27-29 firtree serrations were fractured at the blade roots. During the metallurgical examination, the failure of the CT disc was attributed to cyclic stress rupture due to extended and repeated operation of the engine at, near, or above its temperature/power limits. Dimensional measurements of the blades showed growth and deformation to the disk in the areas of the fractures. There were no material, manufacture, or design deficiencies identified during the metallurgical examination of the CT disc. The examination of the No. 2 power section revealed that the intermediate drive shaft fractured in a counterclockwise direction due to sudden stoppage of the left engine while it was at a high power level. Due to the degree of destruction and lack of dispatch records, the investigation was not able to accurately determine the operating weight of the helicopter at the time of the accident; however, for the 9,500-foot density altitude, it is believed that the helicopter's weight with the water load was at a point that resulted in marginal single engine capability at best. The accident helicopter had been modified with the installation of a water bucket and long line system. The long line and water bucket circuit breakers, and the emergency electrical release, were connected to the nonessential bus. This system was installed on a Form 337 field approval. According to the helicopter manufacturer, the electrical system is designed so that if one generator and/or engine failed both of the nonessential buses would automatically drop offline. Thus the emergency electrical release of the water bucket and long line would have been rendered inoperable in the event of a generator and/or engine failure. An override switch on the electrical panel can restore power to the nonessential buses; however, based on the event timeline reported by the witnesses, it is unlikely that the pilot could have restored power to the nonessential busses in time to prevent a collision with the ground.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: failure of the compressor turbine disc due to cyclic fatigue brought about by repeated operation near or above the engines' temperature/power limits by company personnel over an extended period of time. Factors in the accident were: 1) the high density altitude, mountainous terrain, and the helicopter's resulting marginal single engine performance capability; 2) the design, fabrication, and installation of the emergency external load release system, which had the power supply wired to the nonessential bus that would automatically drop offline during an engine or generator failure; and 3) the pilot's resulting inability to electrically release the water load, bucket, or line while dealing with the engine failure. Full narrative available
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