What happened
During a refresher training flight, a pilot-undergoing-instruction performed a normal approach into the landing phase. However, the touchdown was harder than usual. Once the helicopter was on the ground, the certified flight instructor began discussing the nature of the touchdown with the student. During this discussion, the aircraft entered a state of ground resonance. The vibration became so intense that the instructor had no time to react before the helicopter sustained substantial damage.
The investigation
Post-accident examination of the aircraft components revealed issues with the forward landing gear dampers. Investigators found that the pistons in these dampers were fully extended and contained fluid levels that exceeded the specifications provided in the maintenance instructions. Dampers in this specific condition are unable to compress or attenuate airframe motion and vibrations, which prevents the mitigation of ground resonance.
Findings
- The forward landing gear dampers were improperly maintained with excess fluid levels.
- The extended state of the damper pistons prevented the attenuation of airframe vibrations.
- The aircraft had recently undergone a 100-hour inspection, completed 4.5 hours prior to the accident.