Incident Overview
A helicopter undergoing refresher training encountered ground resonance after landing, resulting in substantial damage. The pilot-undergoing-instruction (PUI) was receiving refresher training. The approach was normal, but the touchdown was harder than normal. After the helicopter was on the ground, the certified flight instructor (CFI) began discussing the touchdown with the PUI. At that point, the helicopter began to vibrate. The CFI did not have time to react before the helicopter self-destructed.
Post-Accident Findings
A post-accident examination of the helicopter's components was conducted. The examination found that in the forward landing gear dampers, the pistons were fully extended and contained fluid in excess of the levels specified in the maintenance instructions. According to the examination, dampers in this condition will not compress or attenuate airframe motion and vibrations, nor will they prevent ground resonance. The last inspection on the helicopter was a 100-hour inspection, which was completed 4.5 hours prior to the accident flight.