What happened
While flying at 75 feet AGL, the pilot and a crewmember/gunner of a public use helicopter heard a loud squealing noise coming from the rear of the aircraft. Following this noise, the rotor RPM began to decay from 3,100 down to approximately 2,000 RPM. In response to the loss of power, the pilot initiated an autorotation.
During the landing sequence, the main rotor blades made contact with the ground. The helicopter came to rest on its right side in a rocky dry creek bed. There were no injuries reported.
The investigation
Investigators located the tension nut for the short shaft upstream from the impact site. An examination of the components revealed that the splines for the drive shaft coupling had sustained damage. Additionally, the plating on the adapter showed discoloration caused by excessive heat.
Further analysis determined that the damage to the shaft and adapter was consistent with loss of lubrication and subsequent overheating, which led to the destruction of the drive splines.