What happened
On October 22, 2012, an AgustaWestland AW139, registration N385RH, was being repositioned between landing pads at the Houma-Terrebonne Airport in Houma, Louisiana. The flight crew had previously completed a preflight and a repositioning flight to the A-5 pad, where they had shut down the aircraft. After being requested to move to another pad, the crew restarted the engines and began a hover taxi.
As the pilot attempted to land the helicopter, the aircraft touched down and immediately began to oscillate violently. The pilot attempted to lower the collective to stabilize the aircraft, but the vibrations intensified. In an effort to regain control, the pilot increased the collective to lift the helicopter off the ground. While airborne, the oscillations continued and became so severe that the pilots' headsets were knocked off their heads. During a left pedal turn intended to avoid another helicopter, the aircraft entered a spin and impacted the grass. The accident resulted in one serious injury and no fatalities.
The investigation
Investigators examined the aircraft's maintenance history and components. The night before the accident, the 'blue' main rotor damper had been replaced. A maintenance test flight conducted following this replacement had revealed a slight vibration and a blade track discrepancy, though there was no written record of the out-of-track issue or a subsequent track and balance procedure.
Mechanical examinations of the flight controls, hydraulic systems, main gearbox, and swashplate revealed no anomalies. While bench testing of the main rotor dampers performed by the manufacturer found that the 'red' and 'white' dampers were out-of-tolerance due to worn valve seats, and the 'blue' damper's friction was beyond allowable limits, investigators found no evidence that these specific issues could account for the sudden, severe vertical vibrations recorded by the flight data recorder.
Analysis of the cockpit voice recorder (CVR) confirmed that the crew experienced intense rattling and several aural warnings, including 'rotor low', 'rotor high', bank angle, and tail too low, immediately preceding the impact.