NTSB Identification: SEA97FA196
14 CFR Part 133: Rotorcraft Ext. Load
Accident occurred Wednesday, August 27, 1997 in SKYKOMISH, WA
Probable Cause Approval Date: 07/05/2013
Aircraft: Southern Aero UH-1B, registration: N896W
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 aircraft accident report.
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.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: A fatigue fracture in the main rotor mast, which resulted in an in flight separation of the main rotor from the helicopter. Contributing to the accident was insufficient Federal Aviation Administration airworthiness standards and requirements for the aircraft. Full narrative available
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