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ASN Wikibase Occurrence # 23029
Last updated: 26 November 2021
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Type:Silhouette image of generic B412 model; specific model in this crash may look slightly different
Bell 412
Owner/operator:US Department of Interior
Registration: N174EH
MSN: 33085
Fatalities:Fatalities: 1 / Occupants: 1
Other fatalities:0
Aircraft damage: Written off (damaged beyond repair)
Location:Cold Springs, NV -   United States of America
Phase: En route
Nature:Fire fighting
Departure airport:Cold Springs, NV
Destination airport:
Investigating agency: NTSB
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.
Probable Cause: 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.




Revision history:

27-Sep-2008 01:00 ASN archive Added
14-Aug-2010 13:56 Alpine Flight Updated [Registration, Cn, Operator, Other fatalities, Location, Nature, Departure airport, Destination airport, Source, Damage]
21-Dec-2016 19:14 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
21-Dec-2016 19:16 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
21-Dec-2016 19:20 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
12-Dec-2017 19:01 ASN Update Bot Updated [Nature, Destination airport, Source, Narrative]

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