NTSB Identification: MIA97TA139.
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Accident occurred Wednesday, April 16, 1997 in CALDERWOOD, TN
Probable Cause Approval Date: 01/07/1998
Aircraft: Bell 204-B, registration: N4580Y
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this public aircraft accident report.

The main drive failed at the transmission end resulting in loss of transmission drive. The pilot performed a autorotation landing in trees and the helicopter rolled inverted and descended through the trees coming to rest on a river bank. Metallurgical examination showed the main drive shaft teeth had separated at the transmission end due to wear from frictional overheating which is a result of loss of lubrication. The helicopter manufacturer instructions require that the main drive shaft be inspected by a mechanic for loss of lubrication and signs of overheating during a daily inspection before the first flight of each day. The operator did not have records to show that the daily inspection had been performed prior to the accident flight.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The inadequate maintenance inspection of the helicopter resulting in loss of lubrication from the main drive shaft going undetected until failure of the drive shaft.

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