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ASN Wikibase Occurrence # 39198
Last updated: 24 October 2020
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Type:Silhouette image of generic UH1 model; specific model in this crash may look slightly different
Bell UH-1B
Owner/operator:Horizon Helicopters
Registration: N896W
C/n / msn: 62-4576
Fatalities:Fatalities: 1 / Occupants: 1
Other fatalities:0
Aircraft damage: Substantial
Location:Skykomish, WA -   United States of America
Phase: Unknown
Nature:External load operation
Departure airport:(WA60)
Destination airport:
Investigating agency: NTSB
While performing external load logging operations, the helicopter's main rotor separated from the helicopter and the helicopter crashed. Metallurgical examination of the main rotor mast revealed a fatigue fracture in the upper spirolox groove in the damper support splines. Based on the mast's historical service record, the fractured mast, a Bell part number 204-011-450-001 mast assembly with a part number '204-040-466-9' mast tube (not a valid part number according to Bell), had 4,006.7 hours at the time of the mast separation. An FAA airworthiness directive (AD), AD 97-14-12, applicable to Bell 204B, 205A, 205A-1, 205B, and 212 civil helicopters but not to surplus military UH-1 helicopters, limited the life of part number 204-011-450-001 masts to 6,000 flight hours or a retirement index number (RIN) of 300,000 (the fractured mast had a RIN of 200,335, computed according to the method specified by that AD.) The most recent main rotor mast AD directly applicable to the accident helicopter, AD 89-17-03, limited mast life to 15,000 hours and contained no requirement to establish or track RIN. The 'thin-walled' Bell part number 204-040-366-9 mast tubes installed in part number 204-011-450-001 masts were ordered removed from U.S. Army UH-1 helicopters by a 1984 U.S. Army emergency safety-of-flight message. CAUSE: A fatigue fracture in the main rotor mast, which resulted in an inflight separation of the main rotor from the helicopter. A related factor was insufficient FAA airworthiness standards and requirements for the aircraft.



Revision history:

24-Oct-2008 10:30 ASN archive Added
21-Dec-2016 19:23 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]

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