BARING, WA, USA
N157LC
OREGON HELICOPTERS UH-1L
WHILE LIFTING A LOG THE CREW HEARD UNUSUAL NOISES & FELT VIBRATIONS. THEY RELEASED THE LOG & FLEW TO A LANDING SITE TO CHECK THE AIRCRAFT. WHILE AT A HIGH HOVER THE AIRCRAFT BEGAN SPINNING AND THE PILOT EXECUTED A HOVERING AUTOROTATION. THE AIRCRAFT LANDED HARD AND ROLLED DOWN A HILL. INVESTIGATION REVEALED A FAILED TAIL ROTOR 42 DEG GEARBOX. THE BOARD'S METALLURGICAL EXAMINATION SHOWED THAT THE GEARBOX'S INPUT SPIRAL BEVEL GEAR HAD FAILED IN FATIGUE. THE ENG N2 TORQUE WAS RED-LINED AT 55 LBS. PILOT STATED THAT A DASH-5 TRANSMISSION WAS INSTALLED RATED FOR 60 LBS, BUT OBSERVED MARKED 55 LB LIMIT. NAVY OPERATORS MANUAL AND FAA TYPE CERTIFICATE BOTH LIST N2 TORQUE LIMIT AS 50 LBS.
FATIGUE FAILURE OF THE TAIL ROTOR 42 DEGREE GEAR BOX DUE TO PILOT INDUCED OVERSTRESS. NAVY DATA AND FAA TYPE CERTIFICATE LIMITATIONS WERE NOT ADHERED TO.
Source: NTSB Aviation Accident Database
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