Incident Overview
A helicopter pilot reported a normal, smooth landing. Approximately five to six seconds after touchdown, with engine rpm reduced to between 2700 and 2800, a vibration was felt. The pilot increased throttle to 3100 rpm and attempted to initiate a hover. The vibrations became severe, and about one second later, the left skid collapsed, allowing the main rotor to strike the tailboom. The pilot secured the helicopter and evacuated after all motion stopped.
Post-Incident Examination
Laboratory tests of the landing gear dampers revealed that the charge pressure for all four units was below the value specified in the maintenance instructions. The maintenance instructions explicitly warned that improper shock strut pressure levels could result in ground resonance and destruction of the helicopter. Examination of the main rotor head dampers showed no evidence of preexisting cracks or fatigue.
Findings
No additional issues were identified with the main rotor head dampers. The low charge pressures in the landing gear dampers were noted as a deviation from maintenance specifications, with a known potential to lead to ground resonance.