NTSB Identification: LAX04LA176.
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Accident occurred Saturday, April 03, 2004 in Bakersfield, CA
Probable Cause Approval Date: 12/28/2004
Aircraft: Bell 47G-4A, registration: N7NB
Injuries: 1 Serious.
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 helicopter experienced a loss of tail rotor control and collided with terrain. During an aerial application flight, the pilot heard a rattling noise from the aft end of the helicopter. He then lost tail rotor authority and entered a flat spin. The pilot attempted to make a run-on landing, but the helicopter continued to spin until it impacted terrain. A Federal Aviation Administration inspector examined the helicopter after the accident. He noted that the cross-over support tube near the aft end of the tail boom was missing. A crack was found on the tail boom left side frame tubes. Deposits of rust in the area suggests that the crack was a pre-existing crack. The inspector thought that this would have added excessive stress to the structure during turning maneuvers and caused the last spline coupling shaft to disconnect from the tail rotor gear box as the tail boom flexed. A 50-hour maintenance inspection was completed the day before the accident.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: a loss of tail rotor drive due to a disconnection of the drive shaft's splined coupling at the tail rotor gear box. The splined coupling to tail rotor gear box disconnection occurred as a result of tail boom flexing due to the combined effects of an uninstalled structural support tube and a crack in left side frame tubes. Inadequate maintenance inspection was a factor. Full narrative available
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