Final Report: Ground resonance destroyed helicopter
Enstrom 480, Goshen, Ind., Aug. 5, 2005–The NTSB determined the landing accident was caused by “ground resonance experienced by the pilots.” The student pilot/owner was flying the turbine helicopter, accompanied by a 2,107-hour CFI, when it experienced ground resonance and disintegrated. The helicopter’s tail was sheared from the body and landed 30 feet away, the front seats about 10 feet away. The instrument panel lay in pieces near the fuselage. One occupant suffered serious injury; two had minor injuries.
The CFI stated that the student smoothly lowered the collective and set the helicopter on the ground as he was rolling off the power. The helicopter then “started to bounce” (ground resonance), and the CFI reached down to make sure the collective was down and the power was off. The helicopter continued to “shake violently and ripped itself apart.”
After a similar incident a month before, which caused no damage, the CFI had reported it to Enstrom: “I have flown probably 20 different Enstrom aircraft and am familiar with the Enstrom bounce. That was not what we just experienced.” A subsequent incident, three days later, with the student/owner in command, resulted in “$7,000 worth of damage.” He added, “I notified Enstrom and they basically told me pilot error, pointing the finger at me.”
The helicopter had a new design of elastomeric damper installed 91 hours before the accident. The accident dampers were within manufacturer’s specifications. After the accident, the manufacturer recalled all the new-design elastomeric dampers, stating, “An Enstrom helicopter, equipped with an elastomeric damper, has experienced ground resonance. The aircraft had previously experienced ground resonance, which was attributed to improperly serviced oleo struts. During a later flight, the aircraft encountered ground resonance and was destroyed. During subsequent conversations with the damper manufacturer, it seems that the first encounter may have reduced the damper’s stiffness, thus reducing its ability to prevent ground resonance.”