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Aviation Accident

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NTSB Identification: SEA99LA171
The docket is stored in the Docket Management System (DMS). Please contact Records Management Division
Accident occurred Tuesday, September 21, 1999 in USK, WA
Probable Cause Approval Date: 05/09/2001
Aircraft: Garlick UH-1H, registration: N32800
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The lower surface of the 90-degree gearbox exhibited fracture surfaces of the six attachment studs. Fretting was observed on the lower surface where the gearbox quill assembly was attached. The fracture surfaces for the six attachment studs each showed one or two relatively flat regions with smooth curving boundaries typical of fatigue. Multiple origin sites were identified on each stud, with observed ratchet marks. All of the origin areas were circumferentially aligned with the center of the gearbox, with origins on both sides of each stud, except for one stud, which had an origin area on only one side. In each case, the larger fatigue region had origins on the counter-clockwise side of the stud (as viewed from above, from the center of the gearbox).

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
  • Fatigue failure of the 90-degree gearbox attachment studs, resulting in separation of the tailrotor and 90-degree gearbox from the vertical fin.