What happened
During a practice autorotation, the pilot was approximately 10 feet above the ground and began increasing the collective to cushion the touchdown. The flight instructor noted that the main rotor RPM was low and added throttle to attempt a touchdown with power. This action triggered a ground resonance vibration, causing the aircraft to lift off the ground. As the vibration continued, the pilot rolled the throttle off and set the helicopter back on the ground, at which point the aircraft rolled over onto its side.
The investigation
Investigators removed and tested the landing gear dampers and main rotor dampers at the factory. The inspection of the main landing gear dampers revealed that the right rear and left front dampers had charge pressures below acceptable factory specifications, while the right front and left rear dampers were within factory ranges.
Additionally, all three main rotor dampers were found to be adjusted to excessive friction torque values. Specifically, the red blade damper had been assembled incorrectly, featuring a plate stack-up that was improper for any version of the damper assembly. The investigation noted that the helicopter had been in storage for two years prior to being leased in June 1994. The faulty red blade damper had been removed from another helicopter, overhauled, and placed into spare parts inventory before being installed on the red blade on July 20, 1994.