NTSB Identification: LAX06LA161.
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Accident occurred Thursday, May 04, 2006 in Fallbrook, CA
Probable Cause Approval Date: 04/25/2007
Aircraft: Bell 206B3, registration: N2763R
Injuries: 2 Minor.

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 collided with trees following a failure of the engine to transmission drive shaft. After finishing a final rinse load for the agricultural spray operation, the pilots began the short return flight back to a trailer. While maneuvering about 20 feet above ground level (agl), and about 5 feet above the treetops, the helicopter made an uncommanded yaw and settled into the trees. A post accident examination revealed that the engine to transmission drive shaft (short shaft) was separated at the forward coupling flange, adjacent to the transmission. Investigators removed the rust-covered engine to transmission drive shaft forward coupling from the gear. The retainer ring, packing seal, and drive shaft coupling seal were not attached to the coupling and dangling loosely on the drive shaft tube. A visual examination of the internal coupling revealed that grease retainer plate had a shiny polished appearance from the gear sprocket making repetitive contact with its surface. The engine to transmission drive shaft forward gear sprocket teeth were not present, as a result of overheating and persistent contact, which had worn them smooth. On the face of the gear sprocket there was a semicircular indentation that had the shape and dimension consistent with that of a partial impression of the spring that is normally inside the coupling. Pieces of the spring were found deformed in the aft coupling housing. There was no evidence of grease in the forward coupling, and the aft coupling contained grease that was dirty and black. The operator's component card record for the engine to transmission drive shaft indicated that it was last inspected about 2 months prior to the accident at which time a mechanic repacked the couplings with grease. The helicopter's applicable Maintenance Manual indicated that every 600-hour or 12-month period (whichever occurs first) the main drive shaft component should be inspected.

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

Failure of the engine to transmission drive shaft forward coupling due to overheating from lack of lubrication. The cause of the lack of lubrication was a result of improper maintenance. A factor was the trees.

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