Incident Overview
A helicopter was returning to pick up a load of water while engaged in firefighting operations. During cruise flight, the pilot radioed "I've got a problem" and "I'm going down." Witnesses then observed rotor blades departing from the helicopter.
On-Site Examination
Examination at the accident site revealed that all four main rotor blades had separated from their respective rotor hub and blade grip units. The blades came to rest in a circumferential band within several hundred feet of the helicopter's ground impact point. Additionally, both main rotor hubs departed the airframe and were located a short distance north and east of the ground impact site.
Post-Crash Examination
Post-crash examination identified extensive corrosion fatigue cracking in the upper half of the blade grip plate associated with blade serial number 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 five bolts outboard of the crack. The crack displayed corrosion fatigue striations through most of its surface. Furthermore, the number 7 blade retention bolt bushing also exhibited corrosion fatigue cracking in line with the grip crack.
Findings
With a crack through the inboard section of the upper blade grip plate, the rigidity of the associated blade (serial number 27B) was significantly weakened in the vertical axis. This condition led to the development of dynamic imbalance, which resulted in the successive separation of all four main rotor blades.