Some of the early K-Max crashes seem to have involved a recurring problem with the aircraft's sprag clutch. Later on, one or more aircraft suffered a failure of the splines on the shaft that provides turbine speed input to the turbine governor resulting in an engine overspeed. In one case the splines failed due to
"a significant reduction in the flow of lubricating oil resulting from the partial blockage of an oil filter outflow orifice."
Then in 2003 there was the Keller (Washington State) crash (which I think was probably the worst K-Max crash) where the entire rotor assembly disintegrated:
The helicopter was in cruise flight when the pilot radioed "I've got a problem" and "I'm going down." Witnesses observed rotor blades departing from the helicopter. On site examination revealed that all four main rotor blades had departed from their respective rotor hub/blade grip units coming to rest in a circumferential band within several hundred feet of the helicopter's ground impact point. Both main rotor hubs also departed the airframe and were located a short distance north and east of the ground impact site. Post crash examination revealed the presence of extensive corrosion fatigue cracking in the upper half of the blade grip plate associated with blade (s/n) 27B. The crack passed through the 7th blade retention bolt hole from the inboard section of the grip leaving the blade attached at the upper blade grip plate by one bolt inboard of the crack and 5 bolts outboard of the crack. The crack displayed corrosion fatigue striations through most of its surface. The number 7 blade retention bolt bushing was also found to have corrosion fatigue cracking in line with the grip crack. With a crack through the inboard section of the upper blade grip plate occurring, the rigidity of the associated blade (27B) would have been significantly weakened in the vertical axis and dynamic imbalance would have developed leading to successive separation of all four blades. CAUSE: Corrosion fatigue within the blade retention bolt bushing(s) of the main rotor blade grip resulting in fatigue, cracking and ultimate separation of the upper grip plate. The separation of the upper blade grip plate led to a dynamic imbalance within the rotor system and the subsequent loss of all four rotor blades in flight
.