Accident Details
A helicopter conducting an agricultural spray operation collided with trees following a failure of the engine to transmission drive shaft. After completing a final rinse load, the pilots began a short return flight to a trailer. While maneuvering at about 20 feet above ground level and approximately 5 feet above the treetops, the helicopter experienced an uncommanded yaw and settled into the trees.
Post-Accident Examination
A post-accident examination revealed that the engine to transmission drive shaft (short shaft) had separated at the forward coupling flange, adjacent to the transmission. Investigators removed the rust-covered forward coupling from the gear. The retainer ring, packing seal, and drive shaft coupling seal were not attached to the coupling and were dangling loosely on the drive shaft tube.
Component Condition
Visual examination of the internal coupling showed that the 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 consistent with the shape and dimension of a partial impression of the spring normally inside the coupling. Pieces of the spring were found deformed in the aft coupling housing.
Maintenance Records
There was no evidence of grease in the forward coupling, and the aft coupling contained dirty, black grease. The operator's component card record for the engine to transmission drive shaft indicated that it was last inspected about two months prior to the accident, at which time a mechanic repacked the couplings with grease. The helicopter's applicable Maintenance Manual specified that every 600-hour or 12-month period (whichever occurs first), the main drive shaft component should be inspected.