SEVIERVILLE, TN, USA
N49669
Robinson R-44
The pilot reported that about 10 minutes into his sightseeing flight, at 400 to 500 feet of altitude, in cruise flight, he heard a noise similar to a shotgun blast and experienced vibration, uncommanded yaw, and low rotor and engine rpm. He regained control and conducted a successful autorotation. Postcrash examination of the helicopter revealed drive train, framework, and fuel tank shrapnel damage originating from a failed transmission forward main driveshaft yoke. Examination of the failed yoke by the NTSB Materials Laboratory revealed a cracked weld that originated during manufacture.
On June 11, 1999, about 1700 eastern daylight time, a Robinson R-44, N49669, registered to Highway 66 Aviation, LLC, dba Scenic Helicopter Tours, operating as a Title 14 CFR Part 91 sightseeing flight, crashed into a field near Sevierville, Tennessee, after sustaining a main and tail rotor drive failure while in cruise flight. Visual meteorological conditions prevailed and no flight plan was filed. The aircraft sustained substantial damage and the airline transport-rated pilot and two passengers were uninjured. The flight originated about 10 minutes before the flight. According to the pilot, he was in cruise flight at 400 to 500 feet agl, when he heard a loud "pop" and the helicopter vibrated and yawed without a control input. Shortly thereafter, he got a low rotor warning horn and light, both main and tail rotor tachometer indications went to zero, and he commenced an autorotation. He landed in a tobacco field, and applied the rotor brake, which had no effect. Examination by FAA personnel revealed no damage to the external fuselage; however, removal of the rear cowling revealed (1) the forward yoke of the main drive shaft at the transmission end had failed and was in several pieces, (2) the tubular framework to the left and below the main drive shaft had been mangled and was in two pieces, (3) a 4-inch by 2-inch puncture of the left fuel tank was present, (4) the double drive belts had been displaced as was their drive pulleys, (5) the tail rotor drive shaft was sheared and its forward drive shaft yoke had failed, (6) the tail rotor control tube rod ends had failed. The Robinson Helicopter Factory arranged for shipping the wreckage for NTSB disassembly examination at their Torrance, California, facilities. Examination of the helicopter's rotor drive systems revealed that the sequence of failure events began with engine-to-transmission drive shaft excessive end play at the transmission end. The component that supports the drive shaft in that location is the forward main rotor drive yoke, part number C908-1. The component was subsequently removed and shipped to the NTSB Materials Laboratory, Washington, D.C. for further examination. According to the NTSB Materials Laboratory, the yoke failure was an induced fatigue fracture originating from a defective inertia weld at fabrication. Paint found within the crack confirmed the defect existed since manufacture, and probably occurred as a result of a straightening operation. Although the component is subjected to magnetic particle inspection after straightening and before finishing, plating, and painting, this defective component had been allowed to be installed. The report from the NTSB Materials Laboratory is included under, "Materials Laboratory Factual Report".
The induced fatigue fracture of a component in the engine-to-transmission rotor drive system resulting in a disconnection of the drive system during cruise flight caused by inadequate inspection processes by the manufacturer and the subsequent autorotative emergency descent and forced landing.
Source: NTSB Aviation Accident Database
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