NTSB Identification: SEA93LA105.
The docket is stored on NTSB microfiche number 50380.
Accident occurred Wednesday, May 12, 1993 in ANATONE, WA
Probable Cause Approval Date: 06/22/1994
Aircraft: KAMAN HH-43F, registration: N846D
Injuries: 1 Fatal.
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 CONVERTED MILITARY HELICOPTER WAS IN A HOVER LIFTING AN EXTERNAL LOAD OF LOGS WHEN THE MAIN ROTOR PYLONS SEPARATED IN FLIGHT, RESULTING IN AN UNCONTROLLED INVERTED DESCENT INTO THE GROUND. ON-SCENE EXAMINATION REVEALED THAT THE TRANSMISSION ROTOR INPUT GEAR SHAFT, P/N K774515-5F, HAD FRACTURED. SAFETY BOARD METALLURGICAL EXAMINATION SHOWED THAT THE FRACTURE WAS CHARACTERISTIC OF FATIGUE FAILURE, AND INDICATED REPETITIVE HIGH LOADS. THE FAA TYPE-CERTIFICATE, BASED ON MILITARY CONFIGURATION, LISTS THE APPROVED ENGINE AS 1100 SHP, AND THE EXTERNAL LOAD LIMIT AS 2,300 LBS. THE FAA HAD APPROVED INSTALLATION OF A 1400 SHP ENGINE, AND OPERATIONS WITH AN INCREASED EXTERNAL LOAD LIMIT OF 4,000 LBS.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: FATIGUE FAILURE OF THE ROTOR TRANSMISSION DRIVE SYSTEM, RESULTING FROM THE IMPROPER CERTIFICATION/APPROVAL OF THE AIRCRAFT MODIFICATION BY THE FEDERAL AVIATION ADMINISTRATION.
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