NTSB Identification: ATL03FA065.
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Accident occurred Sunday, March 23, 2003 in Kimble, TN
Probable Cause Approval Date: 06/08/2005
Aircraft: Sikorsky S-61A, registration: N81664
Injuries: 1 Fatal,1 Serious.
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.
According to the co-pilot, the purpose of the flight was to move logs from one-drop area to another using a 150-foot long line attached to the helicopter. On the second airlift, shortly after the logs were laid on the ground, a shudder was felt in the helicopter airframe followed by a slight yaw to the right. On the instrument panel, there was a needle jump in the triple tachometer gauge followed by a very loud bang. The helicopter began a violent left hand descending spin towards the ground. As the helicopter descended, it collided with trees and subsequently the ground. Within seconds the helicopter burst into flames. Examination of the helicopter revealed, the cockpit and the main fuselage of the helicopter were fire damaged. The tail boom was partially fire damaged, and tail rotor blades were bent and connected to the tail rotor transmission. The tail rotor drive shaft was intact and connected to the main rotor transmission. The five main rotor blades were connected to the main rotor head and displayed buckling and deformation damage. Further examination of the main transmission revealed that the left input free wheel unit turned and freewheeled, but made a grinding noise when turned in the freewheel direction. Further examination of the left input free wheel unit revealed the gear housing was worn with numerous roller impressions on the gear housing. The rollers were also rough in surface appearance. The right input free wheel unit operated in both directions without making any detectable noise. Record review of the input free wheel unit assemblies were conducted, and it was reported that the time since overhauled on the components was 960 hours. Sikorsky recommends that free wheel unit times should between overhauls of 350 hours. The FAA-H-8083-21 rotor Flying Handbook notes that pilots need to be familiar with the Height/Velocity diagrams for the particular models of rotorcraft the pilots are flying. Operating at the altitudes and airspeeds shown within the cross-hatched or shaded areas of the Height/Velocity diagram may not allow enough time for the critical transition from powered flight to autorotation. The helicopter operated about 150 feet above the ground inside the shaded area of the Height/ Velocity Curve.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The interruption of rotor system drive power due to the input free wheeling unit malfunctioning , and the pilot's operation of the helicopter in the critical area of the height-velocity curve. Full narrative available
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