Emergency Autorotation and Rollover
A public use helicopter with a pilot and a crewmember/gunner on board experienced an in-flight emergency while at an altitude of approximately 75 feet above ground level. The occupants reported hearing a loud squealing noise emanating from the rear of the helicopter. Concurrently, the rotor RPM began to decay from 3,100 to approximately 2,000 RPM. The pilot immediately initiated an autorotation to manage the descent.
During the landing, the main rotor blades contacted the ground, and the helicopter came to rest on its right side in a rocky dry creek bed. The tension nut for the short shaft was found upstream from the point of impact.
Examination of the Drive System
A post-accident examination revealed that the splines for the drive shaft coupling were damaged, and the plating on the adapter was discolored as a result of excessive heat. The damage sustained by the shaft and adapter was consistent with a loss of grease and subsequent overheating and destruction of the drive splines. The forward rubber boot, which is used to retain lubricant, had separated, resulting in the loss of lubrication. The findings indicate that the loss of lubrication led to the overheating and failure of the drive splines, which likely caused the reported squealing noise and decay in rotor RPM.